Search

Septoria brown spot (Septoria glycines) and Cercospora leaf blight (Cercospora kikuchii) are late season foliar diseases (LSFD) that co-occur every year in the soybean growing regions of Argentina. Repeated use of commercial, formulated mixes of strobilurin-triazole fungicides to control LSFD has prompted the need for tactics to increase the efficacy of these fungicide groups, thereby reduce number of applications and help prolong their life use. The objective of this study was to determine the effect of copper phosphite (CuPhi), tank-mixed with a strobilurin-triazole fungicide, in the fungicide efficacy in controlling LSFD and protecting soybean yield. Field experiments were conducted during the 2014/2015 and 2016/2017 growing seasons in six different locations in the Pampas region of Argentina. At each location, treatments consisted of: i) one foliar application of a fungicide formulated as a mix of picoxystrobin and cyproconazole (60 and 24 g a.i. ha−1, respectively), ii) one foliar application of the fungicide tank mixed with a CuPhi formulation, and iii) an untreated control. Treatment application timing in each trial was defined by an LSFD scoring system previously developed by the authors. All treatments were disposed to the experimental units in a randomized complete block design with four replications. Severity of LSFD was estimated visually as the percentage of diseased leaf area in 50 plants randomly selected in each plot at R6-R7. Grain yield (kg ha−1) was determined at physiological maturity in all trials. Severity and yield analysis using a linear mixed-effect model indicated that addition of CuPhi to the fungicide tank mix significantly reduced LSFD severity and protected yield (P

Three on-farm verification trials focusing on the different planting dates against major peanut diseases was conducted. The study generally aimed to determine the disease reaction of three peanut varieties namely, Farmers’ variety, NSIC Pn18 and NSIC Pn15 against Cercospora leaf spot (CLS) and rust and determine their potential yield and its productivity using various planting dates (October, November and December). Regardless of variety, the highest disease incidence of CLS and rust was obtained in the month of October while the lowest infection of CLS and rust was obtained from the month of December. The highest yield of Farmers’ variety, NSIC Pn 18 and NSIC Pn15 were obtained when plants were planted during the month of October despite the fact that during this time there was a high incidence of CLS and rust. Among the varieties used, the NSIC Pn 15 consistently and significantly produced the highest yield when grown from October to December. In addition, at least 27.6% yield increase in NSIC Pn 15 was obtained when planted between October to December while a yield increase up to 22% was achieved when NSIC Pn18 was used.

Four experimental trials conducted to determine the response of different rates of carrageenan (PGP) on the reaction (resistance and susceptibility) to major and yield of Pagasa 7 and Pagasa 19 mungbean varieties to increase crop productivity. The experimental design of the field area is a Randomized Complete Block Design (RCBD) where each treatment replicated four times. Mungbean varieties treated by at least 100ppm PGP consistently obtained the lowest infections caused by Cercospora Leaf Spot (CLS) and rust with significantly reduced infections by 15.3-21.5%. The PGP-treated mungbean plants consistently and significantly produced the highest yield of 1.28 and 1.38 t/ha equivalent to 88-102.9% yield increase over the Control, however the MBCR of 31.3 were from 150 ppm PGP.

Many diseases affect plants, causing physiological dysfunctions and structural deviation from normal. The effects of plant diseases on yield and quality of crops have been documented. Little, however, is known about the pathology and agronomy of Mango ginger (Curcuma amada Roxb.), an under-cultivated crop. The present study was carried out to assess disease incidence and severity of associated fungal diseases of Mango ginger; assess rhizome yield loss due to disease; isolate and identify fungal pathogens associated; evaluate growth parameters of Mango ginger. Mango ginger plants began showing symptoms of leaf spot and leaf blight at first week and at fifth week after emergence, respectively; at twenty-fourth week some rhizomes were visually observed to be affected with rot. Cercospora curcumae, Phyllosticta zingiberi and Colletotrichum capsici; C. gloeosporioides, Alternaria alternata and Rhizoctonia solani; R. solani, Fusarium solani and Pythium aphanidermatum were frequently found associated with leaf spot, leaf blight and rhizome rot, respectively. Correlation coefficient of: Spot Disease Incidence with plant height was −0.04 and −0.05 for 2016 and 2017, respectively; number of leaves in progression with age was significant at p

Vegetables that are propagated by seeds are considered to be commercially and nutritionally valuable. However, there is a need to overcome the present challenge of developing alternative technologies for organic horticulture. This paper evaluates the potential use of the aqueous extracts of clove, cinnamon, and coriander and the sodium hypochlorite in the germination of lettuce (Lactuca sativa) seeds and in the control of Cercospora disease. Two experimental groups were thus designed: 1) the effects of the different aqueous extracts on germination and initial development of lettuce seeds were observed and; 2) the effects of these extracts on the mycelial growth of the fungus Cercospora longissima (isolated from lettuce plants) were evaluated. The treatments used in the first group were control (with water only), 5% (v/v) sodium hypochlorite (NaOCl) and 5% (w/v) aqueous extracts of clove, cinnamon, and coriander. The pretreatment of seeds was performed just before placing them for germination by immersing in these solutions. In the second set of the experiment, the inhibition of mycelial growth of the Cercospora longissima, isolated from lettuce plants presenting cercosporiosis symptoms, was evaluated. In the seeds treated with clove and cinnamon extracts, the germination speed index was negatively affected, while those treated with coriander extract showed an increase in the fresh mass of the aerial part and in the number of leaves of lettuce seedlings. In the second experimental group, it was observed that the growth of Cercospora longissima was inhibited completely by the cinnamon extract and by sodium hypochlorite.

Gray leaf spot, caused by the fungus Cercospora zeae-maydis, is an important foliar disease of corn (Zea mays) in the United States (U.S.). Application of foliar products that contain quinone outside inhibitor (QoI) and demethylation inhibitor (DMI) fungicides is one way that corn farmers in the U.S. manage this disease. Since DMI and QoI fungicides pose medium to high risks of selecting for resistant fungal pathogens, it is important that fungicides from different classes be registered for use on corn to manage gray leaf spot. The fungicide pydiflumetofen (adepidyn; Syngenta Crop Protection, Greensboro, NC) is a new succinate dehydrogenase inhibitor (SDHI) that currently is being registered for use on corn in the U.S. The objective of this study was to establish the baseline sensitivity of C. zeae-maydis isolates collected from different areas of the U.S. to pydiflumetofen using in vitro assays. Effective concentrations of pydiflumetofen in which C. zeae-maydis isolates were inhibited by 50% (EC50 values) ranged from 0.0008 to 0.0101 μg/ml, with mean and median values of 0.0040 and 0.0038 μg/ml, respectively. These results can be used as part of a fungicide resistance monitoring program for C. zeae-maydis in the U.S.

ICRISAT safely exported 182,605 seed samples to 127 countries during 1998–2019. The maximum number of exported samples were of sorghum (51,309) followed by chickpea (48,513) and pearl millet (39,771). A total of 2749 seed samples (chickpea 1301, groundnut 300, minor millets 50, pearl millet 338, pigeonpea 395 and sorghum 365) were rejected due to presence of seed-borne pathogens of quarantine significance. Thirty-one pathogenic fungal species were detected in the blotter test and seven pathogens detected in seed washing and sedimentation test. Maximum fungal/oomycetes species were detected in sorghum seed samples (18) followed by chickpea (16), pigeonpea (15), groundnut (10), pearl millet (9) and minor millets (6). Acremonium sp., Botrytis sp., Cercospora sp., Colletotrichum sp., Exserohilum turcicum, Fusarium sp., Phoma medicaginis, Phomopsis sp., Pyricularia grisea, Sclerotium rolfsii, Peronosclerospora sp., Sporisorium sorghi, Puccinia purpurea, Moesziomyces penicillariae, Claviceps fusiformis and Sphacelotheca sp. were detected in the seed samples.

Narrow brown leaf spot (NBLS) of rice ( Oryza sativa L.) is caused by Cercospora janseana (Racib). O. Const. (Synonyms: Cercospora oryzae Miyake, Passalora janseana Racib. U.). Experimental studies were conducted in Louisiana to study NBLS progression in resistant to very susceptible cultivars at different planting dates. The very susceptible cultivars CL131 and Cheniere, susceptible CL111, moderately susceptible CL151, and resistant cultivars, Della and Presidio were planted in mid-April and mid-May plantings. Weekly disease assessments began at 45 days after sowing (DAS) and the final disease assessments were done at 109 DAS. Results revealed that AUDPC and final NBLS severity were greater in very susceptible and susceptible cultivars as compared to resistant cultivars. The AUDPC for mid-May plantings were greater for all cultivars. AUDPC increased with increasing susceptibility rating in the mid-April planting. Early onset of NBLS was observed in very susceptible and susceptible cultivars regardless of planting date. Significant interactions were detected between cultivar and planting date for AUDPC and final disease but were non-significant for apparent infection rate. Apparent NBLS infection rate was greatest in the highly susceptible Cheniere and CL131 in the mid-May planting. Apparent infection rates were less in the susceptible and moderately susceptible cultivars and least for the resistant cultivars in both plantings.

Cercospora leaf spot caused by Cercospora beticola is a significant threat to the production of sugar and table beet worldwide. A de novo genome assembly of C. beticola was used to develop eight polymorphic and reproducible microsatellite markers for population genetic analyses. These markers were used, along with five previously described microsatellite loci to genotype two C. beticola populations from table beet fields in New York, USA. High allelic and genotypic diversity and low population differentiation were found between fields. Linkage disequilibrium of loci after clone-correction of datasets was attributed to the presence of two distinct clonal lineages within the populations. Linkage equilibrium of loci in one of the clusters supported the presence of sexual reproduction. The draft de novo genome assembly will help elucidate the reproductive system of C. beticola through investigating evidence of recombination in the C. beticola genome.

Field research was conducted to examine traditional and alternative fungicide options for controlling leaf diseases of carrots caused by powdery mildew and Cercospora . Wet conditions favoured leaf blights caused by Cercospora and dry favoured powdery mildew. Pyraclostrobin, azoxystrobin, and tebuconazole controlled both Cercospora leaf blight and powdery mildew whereas sulphur on its own controlled only powdery mildew compared to untreated plots. Spray oils did not improve powdery mildew control, but when combined with sulphur, improved Cercospora leaf blight control. A baseline sensitivity study indicated a powdery mildew isolate from South Australia, sourced from carrots sprayed with a fungicide containing azoxystrobin over several seasons, was less sensitive to azoxystrobin than an isolate from New South Wales which had not been exposed to the fungicide.

An efficient Agrobacterium tumefaciens-mediated transformation (ATMT) approach was developed for the plant pathogenic fungus, Cercospora zeae-maydis, which is the causative agent of gray leaf spot in maize. The transformation was evaluated with five parameters to test the efficiencies of transformation. Results showed that spore germination time, co-cultivation temperature and time were the significant influencing factors in all parameters. Randomly selected transformants were confirmed and the transformants were found to be mitotically stable, with single-copy T-DNA integration in the genome. T-DNA flanking sequences were cloned by thermal asymmetric interlaced PCR. Thus, the ATMT approach is an efficient tool for insertional mutagenesis of C. zeae-maydis.

As the interest in Unmanned Aerial Systems (UAS) has increased, so has the interest in the application of these systems for use in agriculture. A variety of sensors, including Multi-Spectral, Near-Infrared, Thermal, and True-Color have the potential to benefit farmers when mounted to a UAS. But as this is an emerging field, there is little data available to demonstrate their use for early detection of plant diseases in crop production. In 2016, a preliminary study was launched to examine the potential of using aerial imagery from UAS to detect diseases in soybean crops. Two irrigated fields in Alabama were selected: Experiment 1, a 50-hectare field, and Experiment 2, a 5-hectare field. Each trial consisted of replicated plots using two foliar fungicide treatments and an untreated control. Aerial imagery (multi-spectral and true-color) was collected on a biweekly basis during this study. Using multi-spectral imagery, both the Normalized Difference Vegetative Index (NDVI) and Normalized Difference Red Edge Index (NDRE) were generated and compared to direct observations in the field. Disease severity of soybean rust, charcoal rot and Cercospora leaf blight were monitored on a biweekly basis and correlated to the UAS imagery. Preliminary results indicated plant stress can be detected using UAS imagery. In Experiment 1, stress associated with charcoal rot was visible in the NDRE imagery. This was of interest because at the time of flight, while it was noted that plants were yellowing, the root and stem disease itself had not been identified by direct observation. In Experiment 2, soybean rust was observed by direct observation and in both the NDRE and NDVI imagery. Soybean rust did have a negative impact on yield in Experiment 2, however severe drought conditions may have negated the yield loss likely caused by the development of charcoal rot in Experiment 1.

The performance of a new formulation of fungicide, azoxystrobin and flutriafol (AZXFLT), was evaluated in the control of fungal leaf spots in corn crops in Brazil. Two experiments were realized in the 2nd harvest (late growing season) in 2015 and 2016, and one in the 1st harvest (normal growing season) in 2016/2017. The area under the disease progress curve (AUDPC) was evaluated for the following leaf diseases that occurred in each experiment: northern leaf blight (Exserohilum turcicum), southern corn rust (Puccinia polysora), gray leaf spot (Cercospora zeae-maydis), maize white spot (Pantoea ananatis) and Diplodia leaf streak (Stenocarpella macrospora). The productivity and economic viability of each treatment were estimated. The experimental design was a randomized block with 4 replications. The AZXFLT formulation was evaluated at a dose of 62.5 + 62.5 g active ingredient (a.i.) ha−1 in two applications and 75 + 75 g a.i. ha−1 in one and two applications in the February 2015 and November 2016 planted experiments. In the February 2016 planted experiment, AZXFLT (75 + 75 g a.i. ha−1 in two applications) was evaluated separately and in combination with mancozeb, chlorothalonil, copper hydroxide, copper phosphite, difenoconazole and propiconazole. All fungicides reduced the AUDPC of all diseases and provided an increase in productivity, with positive financial returns when compared with treatment without fungicide. There was a greater emphasis on treatments with AZXFLT in two applications of 75 + 75 g a.i. ha−1. The addition of protective fungicides contributed to increased productivity and economic viability.

Cercospora leaf spot (CLS) is one of the most serious diseases of sugar beet worldwide, and if uncontrolled, causes nearly complete defoliation and loss of revenue for beet growers. The beet sugar industry continuously seeks CLS-resistant sugar beet cultivars as one strategy to combat this disease. Normally human experts manually observe and rate the resistance of a large variety of sugar beet plants over a period of a few months. Unfortunately, this procedure is laborious and the labels vary from one expert to another resulting in disagreements on the level of resistance. Therefore, we propose a novel computer vision system, CLS Rater, to automatically and accurately rate plant images in the real field to the "USDA scale" of 0-10. Given a set of plant images captured by a tractor-mounted camera, CLS Rater extracts multi-scale superpixels, where in each scale a novel Histogram of Importances feature encodes both the within-superpixel local and across-superpixel global appearance variations. These features at different superpixel scales are then fused for learning a regressor that estimates the rating for each plant image. We further address the issue of the noisy labels by experts in the field, and propose a method to enhance the performance of the CLS Rater by automatically calibrating the experts ratings to ensure consistency. We test our system on the field data collected from two years over a two-month period for each year, under different lighting and weather conditions. Experimental results show that both the CLS Rater and the enhanced CLS Rater to be highly consistent with the rating errors of 0.65 and 0.59 respectively, which demonstrates a higher consistency than the rating standard deviation of 1.31 by human experts. HighlightsWe design a practical system to automatically rate plant diseases in a real field.We propose a novel HoI feature over the multi-scale superpixels representation.We address the problem of noisy labels by proposing a label enhancement module.We collect a Real-World Sugar Beet Database over two years and make it public.

The goal of the study was to construct a reliable molecular and biochemical tool for the selection of resistant sugar beet cultivars to minimize the loss in productivity caused by Cercospora leaf spot disease. Three PCR marker systems: inter simple sequence repeats (ISSR), and sequence-related amplified polymorphisms (SRAP), and random amplified polymorphic DNA (RAPD) were used to assess the genetic diversity among eight cultivars of sugar beet and to detect the molecular markers linked to resistance to Cercospora beticola. Molecular tools successfully differentiated resistant from susceptible cultivars and detected 10 specific DNA markers in the resistant cultivars. Euclidean distance analysis based on physiological traits grouped the eight cultivars into two main clusters. Protein and isozyme patterns of peroxidase, polyphenol oxidase, chitinase, super oxide dismutase, glutamate oxalo-acetate transaminase and esterase were validated as quick means to select resistant cultivars of sugar beet. Specific protein and isozyme bands were lacking or were induced in cultivars based on their resistance phenotype. Our Results show that molecular markers and isozyme patterns are powerful tools for distinguishing resistant sugar beet cultivars resistant against Cercospora leaf spot.

Variety resistance is a principal element of integrated pest management and part of the integrated control of Cercospora leaf spot (CLS) in sugar beet. Currently, varieties with a high level of resistance to CLS yield lower than susceptible ones and consequently lack acceptance in commercial farming in Germany. In a comparative analysis of three different trial series, yield performance of selected sugar beet varieties with varying susceptibility to CLS was examined with regard to CLS-disease severity of the trial site. In disease-free environments, the current resistant varieties yielded 5–7% less than susceptible varieties, indicating progress in sugar beet breeding and a reduction in yield-penalty by half within the last 20 years. In environments with strong infestation, the resistant variety achieved a white sugar yield (WSY) comparable to varieties with generally higher yield potential. The observed changes in variety-ranking, especially between environments with low to medium and strong CLS-infestation, suggested that superiority of some varieties depended on disease pressure in the respective growing area. It is concluded that disease severity of CLS should be taken into account for the characterisation of varieties in order to improve variety-recommendations and to allow conclusions about the superiority of varieties in regions with varying disease pressure. An analysis of the effect of different control strategies agreed with prior studies that WSY is higher in treatments with fungicide applications according to the summary threshold system than in the non-treated control. Preventive fungicide applications appeared to be unnecessary for German growing areas as they did not increase WSY. Moreover, a reduced strategy related to the summary threshold system may allow a reduction in fungicide use as it achieved a WSY comparable to the threshold strategy but with omission of the last application.

Novel species of Cercospora and Pseudocercospora are described from Australian native plant species. These taxa are Cercospora ischaemi sp. nov. on Ischaemum australe (Poaceae); Pseudocercospora airliensis sp. nov. on Polyalthia nitidissima (Annonaceae); Pseudocercospora proiphydis sp. nov. on Proiphys amboinensis (Amaryllidaceae); and Pseudocercospora jagerae sp. nov. on Jagera pseudorhus var. pseudorhus (Sapindaceae). These species were characterised by morphology and an analysis of partial nucleotide sequence data for the three gene loci, ITS, LSU and EF-1α. Recent divergence of closely related Australian species of Pseudocercospora on native plants is proposed.

Cercospora beticola , causal agent of Cercospora leaf spot (CLS) of sugar beet, is primarily controlled by fungicides. Benzimidazole and demethylation inhibiting fungicides, including carbendazim and flutriafol, have been widely used in Serbia. Since these fungicide groups have a site-specific mode of action, there is a high risk for developing resistance in target organisms, which is the most important limiting factor in Cercospora leaf spot chemical control. A rapid identification of flutriafol and carbendazim resistance can help researchers in examining the potential of different fungicide resistance management practices, as well as in selection of fungicides for use in the areas where resistance has occurred. One hundred single-conidia isolates were collected from 70 representative locations of the sugar beet production region in Serbia. Evaluation of the isolates' sensitivity was based on the reduction of mycelial growth on medium amended with 1.25 μg mL −1 flutriafol and 5 μg mL −1 carbendazim. Resistance to flutriafol and carbendazim was detected in 16% and 96% of the tested isolates, respectively. All isolates resistant to flutriafol were resistant to carbendazim as well, which is the first report of a double resistance to fungicides in C. beticola . Detection of the isolates resistant to flutriafol and carbendazim using Cleaved Amplified Polymorphic Sequence (CAPS) markers confirmed the results of the in vitro tests. The efficacy of carbendazim, flutriafol, azoxystrobin, and tetraconazole at commercially recommended doses was evaluated in field trials where sugar beet plants in plots were inoculated with a mixture of isolates either sensitive and/or resistant to flutriafol and carbendazim. Carbendazim and flutriafol efficacy was very low in plots inoculated with isolates resistant to these fungicides. Presented results will contribute to development of a pathogen population sensitivity monitoring strategy that could be used for an effective CLS management in the region.

Gray leaf spot (GLS), caused by Cercospora zeae-maydis, is an important foliar disease of maize (Zea mays L.) worldwide, resistance to which is controlled by multiple quantitative trait loci (QTL). To gain insights into the genetic architecture underlying the resistance to this disease, an association mapping population consisting of 161 inbred lines was evaluated for resistance to GLS in a plant pathology nursery at Shenyang in 2010 and 2011. Subsequently, a genome-wide association study, using 41,101 single-nucleotide polymorphisms (SNPs), identified 51 SNPs significantly (P

Field surveys were conducted in two major safflower growing states of India for two consecutive cropping seasons (2015-16 and 2016–17). A total of 872 fields spanning 22 districts of Karnataka and Maharashtra were examined to determine the occurrence and distribution of various fungal diseases on safflower. Safflower fields were found to be infested by four fungal diseases, namely Alternaria leaf spot, Fusarium wilt, Cercospora and Ramularia leaf spot. Among these, Alternaria leaf spot and Fusarium wilt were most widespread and found in all the districts investigated, while Ramularia leaf spot was confined to the districts of Karnataka. The disease prevalence, incidence and severity varied among districts, cropping seasons, with sowing periods and cropping patterns. The first survey recorded higher disease in both states. In both cropping seasons, Karnataka showed higher disease parameters compared to Maharashtra. District Dharwad (Karnataka) exhibited the maximum mean disease incidence for both Alternaria leaf spot and Fusarium wilt (55.41% and 41.42%) among all the districts. The disease prevalence was less in fields with late as compared to early sown crops. Intercropping with chickpea, cotton, sorghum alone and chickpea and sorghum together decreased disease prevalence in comparison to monocrop fields of safflower. This is the first comprehensive survey reporting the distribution of fungal diseases on safflower in India. The study encourages agriculturalists to adopt cultural practices such as late sowing and intercropping with chickpea and sorghum as an effective management strategy for reducing disease in safflower.

Field experiments were conducted during the rainy seasons of 2009 and 2010 for the management of the major diseases of mungbean, namely, wet root rot ( Rhizoctonia solani ), cercospora leaf spots ( Cercospora canescens and Pseudocercospora cruenta ) and yellow mosaic (Mungbean Yellow Mosaic Virus) by using different combinations of an insecticide, fungicide, and bio-formulation as seed treatment, with or without foliar sprays. A combination of seed treatment with thiamethoxam (Cruiser™) at 4 g kg −1 , carboxin (Vitavax™) at 2 g kg −1 and Pusa 5SD ( Trichoderma virens ) at 4 g kg −1 followed by simultaneous foliar sprays of thiamethoxam (Actara™) 0.02% and carbendazim (Bavistin™) 0.05% at 21 and 35 days after sowing resulted in the highest seed germination and grain yield in mungbean with the lowest intensities of cercospora leaf spots and mungbean yellow mosaic, and moderate incidence of wet root rot. The lowest whitefly population was also observed in this treatment during all stages of the crop. The treatment combinations having Pusa 5SD as seed treatment provided the lowest wet root rot incidence. Two sprays were superior to single spray for all variables recorded, but in combination with seed treatment, single spray was found to be more cost effective as it obtained the highest return per rupee of input. Use of T. virens based bio-formulation Pusa 5SD with insecticide thiamethoxam has been effectively demonstrated for the first time along with fungicides Bavistin and Vitavax for the management of the major diseases of mungbean.

The potential for improved management of Cercospora leaf spot (CLS), caused by Cercospora beticola , using the herbicide glyphosate in glyphosate-resistant sugar beet varieties was investigated. Controlled field experiments were conducted in 2008 and 2009 to determine if glyphosate and glyphosate–fungicide combinations improved the management of CLS in four commercial varieties of glyphosate-resistant sugar beet. Variety and fungicide main effects were significant for CLS development. However, regardless of the herbicide program, glyphosate or a conventional herbicide program, CLS development was not affected. Therefore, results from of this research indicate that glyphosate and glyphosate–fungicide combinations do not significantly contribute to CLS management.

Spectral vegetation indices (SVIs) have been shown to be useful for an indirect detection of plant diseases. However, these indices have not been evaluated to detect or to differentiate between plant diseases on crop plants. The aim of this study was to develop specific spectral disease indices (SDIs) for the detection of diseases in crops. Sugar beet plants and the three leaf diseases Cercospora leaf spot, sugar beet rust and powdery mildew were used as model system. Hyperspectral signatures of healthy and diseased sugar beet leaves were assessed with a non-imaging spectroradiometer at different developing stages and disease severities of pathogens. Significant and most relevant wavelengths and two band normalized differences from 450 to 950 nm, describing the impact of a disease on sugar beet leaves were extracted from the data-set using the RELIEF-F algorithm. To develop hyperspectral indices for the detection of sugar beet diseases the best weighted combination of a single wavelength and a normalized wavelength difference was exhaustively searched testing all possible combinations. The optimized disease indices were tested for their ability to detect and to classify healthy and diseased sugar beet leaves. With a high accuracy and sensitivity healthy sugar beet leaves and leaves, infected with Cercospora leaf spot, sugar beet rust and powdery mildew were classified (balanced classification accuracy: 89%, 92%, 87%, 85%, respectively). Spectral disease indices were also successfully applied on hyperspectral imaging data and on non-imaging data from a sugar beet field. Specific disease indices will improve disease detection, identification and monitoring in precision agriculture applications.

Gray leaf spot is one of the most important yield-limiting diseases of maize (Zea mays L.) and it is more severe in reduced tillage cropping where over-seasoning soilborne inoculum from infected crop debris provides primary inoculum for infection of the newly planted maize. The objective of this study was to investigate the effects of intercropped beans on movement of soil surface inoculum and its overall impact on gray leaf spot severity and yield of maize. Conventional tillage with minimum soil surface inoculum was included for comparison. Trials were conducted in two cropping seasons using resistant (‘UH6303’), moderately resistant (‘Staha’) and susceptible (‘Pannar’) maize varieties. Pooled data for susceptible maize during two growing seasons indicated that the area under disease progress curve (AUDPC) in the inter-cropping treatments was reduced by 12%, 15% and 23% in no-till, minimum, and conventional tillage treatments, respectively, compared to non-intercropping (P ≤ 0.05). The rate of disease increase (r) and the number of spores trapped during the season had similar trends. A moderately resistant cultivar displayed results similar to the susceptible cultivar, whereas treatment effects on resistant genotype were not significant. Yield was 21% higher in the intercropped compared to non-intercropped maize. Spore counts of Cercospora zeae-maydis trapped in the inter-cropping treatment were lower than in non-intercropped maize (P ≤ 0.05). It is concluded that the bean canopy in maize/bean intercrops created a barrier to movement of soilborne spores into the maize canopy, thus minimizing the number of leaves infected by wind-borne, and rain splash. The concomitant use of multiple management options (conventional tillage + inter-cropping + resistant variety) provided superior reduction of gray leaf spot severity and increased yield of maize.

Water hyacinth is a beautiful monocotyledon plant that has been dispersed all over the world by humans. The plant has been present in Mexico since 1907, and many water bodies have become infested with it since then. In 2001, we initiated a survey in Yuriria lagoon in southern Guanajuato state to isolate fungi able to biocontrol the plant. We isolated 25 morphologically distinct fungal cultures, of which two were identified as members of the genus Cercospora. Cercospora species are among the most prevalent and destructive of plant pathogens and can be found on leaves, pedicels, stems, fruits, and bracts. Only two species of Cercospora, Cercospora piaropi, and Cercospora rodmanii, have been described on water hyacinth; however, the classification of these species has been controversial. Several molecular approaches have been used for Cercospora identification, and some candidate genes have been identified for use in Cercospora species determination. Although the nrRNA genes alone do not show sufficient resolution for species determination, histone H3, translation elongation factor1-α, β-tubulin, actin, and calmodulin have been shown in previous studies to have an adequate number of nucleotide changes to allow species identification. In the present study, we used partial sequences of the histone H3, actin, and calmodulin genes to identify our two isolates as C. rodmanii. Our two strains are not specific to water hyacinth, as they are also pathogenic to beet and sugar beet. Similar host ranges were found for C. rodmanii strains isolated from Tabasco in México, Zambia, and Brazil, however, the specificity for water hyacinth persists in Cercospora piaropi Tharp and C. rodmanii Conway, the latter being the most pathogenic.

The mutation G143S has been associated with high-level strobilurin resistance in laboratory mutant strains of Cercospora beticola, one of the most destructive pathogens in sugar beet plants. By using allele specific primers (PASA-PCR) and agarose gel visualization, a molecular diagnostic was developed for the detection of the G143S resistance mutation. This assay is simple and applicable in low tech laboratory settings, with high reliability when a relatively large proportion of mutated mitochondrial alleles are present in the resistant strains. To achieve detection of resistant alleles at low frequencies, a more sensitive Real Time PCR based assay capable of discriminating resistant (S143) genotypes in frequencies as low as 1:10,000 resistant:sensitive alleles was developed. Both diagnostics were successfully validated in laboratory strains. Subsequently, a large number of C. beticola isolates from QoI-treated sugar beet experimental fields in Greece were screened for resistance to Qo fungicides using these diagnostics and classic bioassays. No proportion of the 143S resistant allele was detected in all field isolates tested, which was in agreement with the phenotypes revealed by the biotests confirming that the efficacy of QoIs against C. beticola has been sustained in Greece 7 years after their introduction.

Automatic methods for an early detection of plant diseases are vital for precision crop protection. The main contribution of this paper is a procedure for the early detection and differentiation of sugar beet diseases based on Support Vector Machines and spectral vegetation indices. The aim was (I) to discriminate diseased from non-diseased sugar beet leaves, (II) to differentiate between the diseases Cercospora leaf spot, leaf rust and powdery mildew, and (III) to identify diseases even before specific symptoms became visible. Hyperspectral data were recorded from healthy leaves and leaves inoculated with the pathogens Cercospora beticola, Uromyces betae or Erysiphe betae causing Cercospora leaf spot, sugar beet rust and powdery mildew, respectively for a period of 21 days after inoculation. Nine spectral vegetation indices, related to physiological parameters were used as features for an automatic classification. Early differentiation between healthy and inoculated plants as well as among specific diseases can be achieved by a Support Vector Machine with a radial basis function as kernel. The discrimination between healthy sugar beet leaves and diseased leaves resulted in classification accuracies up to 97%. The multiple classification between healthy leaves and leaves with symptoms of the three diseases still achieved an accuracy higher than 86%. Furthermore the potential of presymptomatic detection of the plant diseases was demonstrated. Depending on the type and stage of disease the classification accuracy was between 65% and 90%.

Two approaches to supervised control of Cercospora leaf spot (CLS) in sugar beet are outlined, and their yield improving capabilities tested in field trials. One approach is based on action thresholds, which use disease severity assessments to indicate the need for a fungicide application. The second is based on measurements of within-crop temperature and relative humidity, to indicate the need for a fungicide application. Disease incidence (% diseased plants) in fungicide treated plots increased in a similar way to unsprayed plots. Incidence increased for all treatments before disease severity, assessed using the Agronomica scale, increased. There was little effect of fungicide treatments on model parameters for disease incidence and severity curves. However, fungicide treatments reduced disease severity by up to 50% and resulted in significantly higher relative sugar yields than the unsprayed treatment (set at 100%). Relative sugar yields from applying the action threshold for two early sprays (115%) and for both weather-based treatments (119% and 121%) did not differ significantly from the calendar sprayed treatment (119%), saving up to two fungicide applications. Fungicide treatments based on weather conditions delayed the increase in disease incidence, and were applied 1 week before treatments based on the first action threshold. Considering the significantly higher financial returns in 2001 and the positive experience with the weather-based system in 2003 and 2004, we conclude that sugar beet growers in the Netherlands should move towards using supervised control approaches for CLS in sugar beet although some further testing of the weather-based system will be necessary.

Field studies were conducted in 2001 and 2002 near Yoakum, Texas, to determine the effect of advisory-based fungicide sprays on foliar and soil-borne disease development and peanut yield compared with fungicide sprays based on a calendar schedule. Chlorothalonil applied on a regular schedule seven times during the growing season was compared with a chlorothalonil/tebucanozole combination and advisory sprays of azoxystrobin, pyraclostrobin, and tebucanozole. Three advisory sprays were applied in both years with chlorothalonil applied at 40 and 110 days after application (DAP) and azoxystrobin, pyraclostrobin, or tebucanozole at 88 DAP in 2001, while in 2002, azoxystrobin, pyraclostrobin, or tebucanozole were applied 39 and 77 DAP followed by chlorothalonil applied 101 DAP. The chlorothalonil/tebuconazole combination consisted of chlorothalonil applied 59 and 74 DAP, followed by tebuconazole applied 74, 89, 104, and 118 DAP with a final chlorothalonil application 132 DAP. All fungicide sprays reduced foliar and soil-borne disease development when compared with the untreated check. No difference in foliar or soil-borne disease development or peanut yield was noted between chlorothalonil alone or tebucanozole applied on a calendar spray schedule and azoxystrobin, pyraclostrobin, or tebucanozole applied by advisory sprays. Peanut yield response reflected the improved disease control with fungicides. All fungicides improved peanut yield over the untreated check by at least 25% in 2001 and more than 70% in 2002.

Cercospora leaf spot, caused by the fungus, Cercospora beticola , continues to be a devastating foliar disease of sugar beet ( Beta vulgaris ), in Minnesota and North Dakota. Commercial sugar beet varieties grown in Minnesota and North Dakota generally have only moderate resistance and require fungicide applications to obtain adequate levels of protection against C. beticola . Trials were conducted in 1999 at Foxhome and Crookston, Minnesota and in 2000 at Breckenridge and Crookston, Minnesota to determine the efficacy of labeled and experimental fungicides for controlling Cercospora leaf spot. Natural inocula were relied on for infection, and disease pressure was high at all sites in both years. Except for azoxystrobin applied alone at Foxhome, and azoxystrobin, and fentin hydroxide, applied alone, and fenbuconazole applied with an adjuvant at Breckenridge, the fungicide treatments provided better Cercospora leaf spot control, and resulted in higher recoverable sucrose yields than non-treated controls. Tetraconazole and pyraclostrobin, when applied alone, consistently provided effective Cercospora leaf spot control and resulted in high sucrose yield.

A pool of 11 randomly selected, uncharacterized Bacillus pumilus isolates from sugar beet were evaluated using a high-throughput screen that utilized laboratory-based tests for 2 pathogenesis-related proteins, chitinase and β-1,3-glucanase, and biphasic hydrogen peroxide production. The screen was followed by a glasshouse test for induction of systemic acquired resistance for control of Cercospora leaf spot in sugar beet. These isolates were compared to the known biological control agent, Bacillus mycoides isolate Bac J, and a chemical inducer of resistance, acibenzolar-S-methyl. All laboratory-based screens identified B. pumilus isolates 203-6 and 203-7, that reduced Cercospora leaf spot symptoms by approximately 70%, even when spatially separated from the casual agent, Cercospora beticola. This level of control was similar to B. mycoides isolate Bac J and acibenzolar-S-methyl. In all cases, systemic resistance elicitation was marked by an increase in 2 pathogenesis-related proteins, chitinase and β-1,3-glucanase, and was preceded by biphasic hydrogen peroxide production, also found in incompatible plant–pathogen interactions in which systemic resistance is induced. A combination of glycol chitin and aniline blue plate assays correctly identified all in planta inducers of systemic resistance as measured by control of Cercospora leaf spot in classical challenge assays for systemic acquired resistance without the inclusion of false positive identifications, reducing the workload in subsequent disease challenge assays by nearly 70%.

This study examines the spatial and temporal variation of populations of the biological control agent (BCA) BacB, a Bacillus subtilis isolate, in the field and growth chamber in the presence of the fungus, Cercospora beticola , the causal agent of Cercospora leaf spot of sugarbeet. The use of the selective BCA support substrate β-glucan, applied at 0, 0.5, and 1.0% of the spray solution, did not influence differences in total population numbers (spores + vegetative cells) of a spontaneous rifampicin resistant isolate of BacB (Rif+) over a 14 day spray period. BacB Rif+, applied as a spore formulation, declined from 10,000 CFU/cm 2 on day 0.5–100 CFU/cm 2 on day 14 at the three levels of β-glucan tested. Distribution of BacB Rif+ populations was modeled on a leaf scale, with and without β-glucan. Higher populations of vegetative cells were more likely at 14 days with 1% β-glucan than with 0% β-glucan. BacB populations were more aggregated without β-glucan than with the nutrient substrate. There was no correlation between BacB density and Cercospora leaf spot disease severity, indicating that neither antibiosis nor parasitism is likely an important mechanism of disease control.

Bacillus spp. have been used to control a number of leaf spot and post harvest diseases. Their capacity to form endospores facilitates long-term storage and relatively easy commercialization. This study focuses on optimizing a Bacillus subtilis isolate, BacB, for the control of sugar beet Cercospora leaf spot, caused by Cercospora beticola Sacc., by examining application timing, biocontrol agent (BCA) concentration, use of the selective nutrient substrate β-glucan, and the form of the BCA at time of application. A method for germinating endospores prior to spraying, without active aeration, is described. Examining the effects of varying β-glucan concentrations and levels of BacB at application demonstrated a complex interaction between β-glucan, BCA population, and disease control. In the 1998 field season, disease severity was significantly decreased, as compared to the control, at an application rate of 1×106 CFU/ml, or higher, with 0% β-glucan. In 1999, there was less disease pressure, and all treatments reduced disease severity. Growth chamber experiments demonstrated that applying the bacteria as vegetative cells instead of spores or applying the BCA 1–5 days before infection could significantly increase disease control. Laboratory experiments demonstrated the ability to induce germination and vegetative growth of BacB from a spore formulation, without shaking or fermentation equipment. This shows promise for optimizing Bacillus sp. for biological control. In field trials the vegetative cells did not perform better than the spore application, though the potential for β-glucan to increase disease was demonstrated.

Among the seven cassava diseases observed in a survey in the cassava-growing regions of Benin and Ghana, cassava mosaic virus disease (CMD) occurred most frequently in all ecozones, with a higher incidence in Ghana (82.2%) than in Benin (48.1%). Stepwise regression and canonical correlation analysis revealed that CMD was positively correlated with increasing numbers of weeds and with cultivar mixtures. Cassava bacterial blight (CBB) was particularly severe in all ecozones of Benin, but rare in Ghana. Cassava in the savanna zones of Benin was heavily diseased with an incidence of 34.5–84.1% and a high percentage of plants showing systemic symptoms, while CBB was not observed in the rainforest (RF) zone and only sporadically in the coastal savanna zone of Ghana. High incidence and severity of CBB was associated with increasing plant age and clay soils, whereas the disease was less when fields were intercropped or planted to cultivar mixtures. Anthracnose and leaf diseases caused by Cercospora spp. were of minor importance. The severity of diseases caused by Cercospora spp. was positively correlated with profusely branching varieties. Incidence of Cercospora caribaea was associated with increasing rainfall and seemed to be suppressed in cultivar mixtures. Integrated control methods are proposed for the diseases, especially CBB.

Gray leaf spot, caused by the fungus Cercospora zeae-maydis , is the most destructive foliar disease of maize in the United States. However, little is known about the biochemical and molecular events of pathogenesis. C. zeae-maydis produces cercosporin, a phytotoxin shown to be a virulence factor in diseases caused by other Cercospora species. To identify genes involved in cercosporin biosynthesis and ultimately determine the role of cercosporin in disease development, we constructed a cDNA subtraction library of C. zeae-maydis by suppression subtractive hybridization. Poly (A) + RNA isolated from cultures grown in a cercosporin-suppressing medium was subtracted from poly (A) + RNA isolated from cultures grown in a cercosporin-inducing medium, resulting in transcripts that are specific to the cercosporin-producing culture. Analyses of 768 sequences in this cDNA subtraction library revealed 197 cDNAs with high similarity to genes in the GenBank and Saccharomyces Genome Database, and these genes were grouped into nine categories based on predicted functions of the encoded proteins. Northern analysis of seven selected clones with predicted functions in fatty acid metabolism (fatty acid synthase, oleate Δ-12 desaturase, and linoleate diol synthase) and secondary metabolism [cytochrome P450 oxidoreductase, cytochrome P450 monooxygenase, dihydrogeodin (phenol) oxidase, and coproporphyrinogen oxidase] indicated that those genes were expressed in cercosporin-inducing conditions. Analysis of expression kinetics confirmed that those genes are expressed concomitantly with cercosporin accumulation. Published by Elsevier Science Ltd.

An experiment to control the cassava green spider mite (CGM) Mononychellus tanajoa Bondar, an exotic pest of cassava, using an exotic predator Typhlodromalus aripo in cassava fields in the transition forest in Nigeria revealed a significant effect on the severity of cassava diseases. High population densities of T. aripo/ cassava leaf in cassava fields were associated with low disease severity of cassava bacterial blight, cassava anthracnose disease and African cassava mosaic disease. Also, the population densities of the T. aripo did not have a significant effect on the severities of cercospora leaf blight, brown leaf spot and white leaf spot. Controlling the CGM using T. aripo therefore reduced the severity of 3 most important diseases of cassava in the transitional forest zone.

A two-year survey (1998, 1999) was conducted in northern Malawi to estimate the occurrence of coffee diseases, with emphasis on coffee berry disease (CBD) caused by Colletotrichum kahawae , in relation to management practices on smallholder plantations. A total of 113 farms was visited in five extension planning areas (EPAs) which grow coffee, and the prevalence, incidence and severity of the diseases was recorded. Disease prevalence was clearly affected by agro-environmental factors, with no CBD recorded in the SE Mzimba EPA and low levels in the Phoka Hills EPA, but a mean of 95% of farms were affected in the Misuku Hills EPA over the 2 years. In contrast, coffee rust was found in all areas and was usually more prevalent than CBD. Cercospora leaf spot ( Cercospora coffeicola ) and fusarium berry disease ( Fusarium stilboides ) were also recorded in most EPAs. Disease levels were higher in 1999 than in 1998, and the mean incidence of CBD per affected farm in 1999 was 40%. This ranged, however, from 10 to 56% depending on the EPA. The worst affected farm had 97% of bushes infected with CBD. There was also a difference in disease levels on the two most commonly-grown cultivars. The mean incidence of CBD for cv. Agaro was 23% but only 11% for cv. Geisha in 1998 compared to 59 and 19%, respectively in 1999. Management practices were correlated with levels of disease in 1999, and pruned or intercropped plantations had lower disease incidence.

Partial rDNA sequences were obtained from 26 isolates representing species of Cercospora, Passalora, Paracercospora, Pseudocercospora, Ramulispora, Pseudocercosporella and Mycocentrospora. The combined internal transcribed spacers (ITS) including the 5.8S rRNA gene and 5′ end of the 25S gene (primer pairs F63/R635) on rDNA were amplified using PCR and sequenced directly. The ITS regions including the 5.8S varied in length from 502 to 595 bp. The F63/R635 region varied from 508 to 519 bp among isolates sequenced. Reconstructed phylogenies inferred from both regions had highly similar topologies for the taxa examined. Phylogenetic analysis of the sequences resulted in four well-supported clades corresponding to Cercospora, Paracercospora/Pseudocercospora, Passalora and Ramulispora, with bootstrap values greater than 92% for each clade. Based on the results of the analysis, a new combination for Pseudocercosporella aestiva is proposed in Ramulispora, and Paracercospora is reduced to synonymy with Pseudocercospora.

The new species Cercosporella pergulariae sp. nov., Pseudocercospora catalpicola sp. nov., Sirosporium rhamnigenum sp. nov., and Spiropes desmodiicola sp. nov. are described and Cercospora commelinicola Chupp is validated. Cercostigmina curta (Syd.) comb, nov., Eriocercosporella vitis-heterophyllae (Henn.) comb, nov., Mycovellosiella lactucae (Henn.) comb, nov., M. trichostemmatis (Henn.) comb, nov., M. tylophorae (Hansf.) comb, nov., Passalora caespitosa (Ellis & Everh.) comb, nov., P. chionanthi (Ellis & Everh.) comb, nov., Prathigada condensata (Ellis & Kellerm.) comb, nov., Pseudocercospora carrii (Barthol.) comb, nov., P. glaucescens (G. Winter) comb, nov., P. pamelae-ellisiae (G.P. Agarwal & N.D. Sharma) comb, nov., Pseudophaeoramularia angolensis (T. Cavalho &O. Mendes) comb, nov., Sporidesmium seminale (Ellis & Everh.) comb, nov., and Stenella praelonga (Syd.) comb. nov. are introduced. Cercospora litseae Henn. is reduced to synonym with Mycovellosiella litseae Meenu et al., and a second collection of Cercosporella indica from Vietnam is recorded.

In response to certain stress agents, infection with Cercospora petroselini and treatment of this infection with Cuprosan, changes in the quantity and quality of the volatile oils produced from Mentha piperita L. and Petroselinum Crispum Mill become apparent. The treated samples showed increase in the concentration of Cu ions in the leaves. Some components were produced under the effect of these stress agents (infection and treatment). Psoralen was isolated from samples treated with Cu salts. Corylidin, Angladin and Pereflorin B were isolated from Parsley herb under the influence of Cercospora petroselini infection. These compounds were examined for antimicrobial activity. Corylidin, Angladin and Psoralen, inhibited growth of Pseudomonas putida , E. Coli and Rhizobium meloloti (gm−ve bacteria) , while Pereflorin B inhibited Streptococcus lactis and Bacillus subtilis (gm+ve bacteria). ©

Alternaria zinniae, a fungus causing leaf necrosis in Xanthium occidentale, a widespread noxious weed of Australian summer crops and pastures, produces toxic metabolites when grown in liquid culture. Extensive application of NMR techniques, HR, EIMS and X-ray analysis permitted the identification of brefeldin A (4H-cyclopent[f]oxacyclotridecin-4-one-1,6,7,8,9,1 1a,12,13,14,14a-decahydro-1,13-dihydroxy-6-methyl) and alpha,beta-dehydrocurvularin (4,5,6,7-tetrahydro-11,13-dihydroxy-4-methyl-2H-3-benzoxacyclododecin-2,10 (1H)-dione), two bioactive metabolites produced by a number of fungal species belonging to the genera Alternaria, Ascochyta Penicillium, Curvularia, Cercospora, and Phyllosticta, This is the first report on the isolation of these two phytotoxins from A. zinniae. The interesting semi-selective toxicity to X. occidentale is described here, and the possible use of fungus and toxins in integrated weed management programs is discussed. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.

The following foliicolous fungi on Tabebuia species are described and illustrated: Anhelia tabebuiae sp. nov. and Dictyonella tabebuiae sp. nov. (ascomycetes), Fumagospora tabebuiae sp. nov., Polychaeton tabebuiae sp. nov. and Septoria tabebuiae-impetiginosae sp. nov. (coelomycetes), Cercospora tabebuiae-impetiginosae sp. nov. and Pseudocercospora tabebuiae-roseo-albae sp. nov. (hyphomycetes). Uncinula peruviana was found and described on T. impetiginosa as a first record for Distrito Federal, Brazil.

Grey leaf spot, Cercospora zeae-maydis , has, in a relatively short period, caused significant annual grain yield losses in the maize industry of KwaZulu-Natal, South Africa. No commercial hybrids are resistant to the disease, and because alternative measures of control, such as crop rotations and tillage practices, have limited effects, fungicides are the main option available for control of the disease. Benomyl was initially registered for the control of grey leaf spot (GLS), but, because of the possible development of fungicide-resistant strains of C. zeae-maydis to the benzimidazole chemical group of fungicides, alternative fungicides, with different modes of action, were investigated. This study was initiated to establish which fungicides and fungicide mixtures, with different modes of action, would control grey leaf spot effectively, and delay the possible development of pathogen resistance. Fungicides belonging to the triazole chemical group, and combinations of fungicides of the benzimidazole and triazole group, were highly effective. These combinations provided effective disease control, longer duration control and higher grain yield responses. The combination of the two fungicide groups, with their different modes of action, not only provide excellent control of GLS, but also offer the benefit of slowing down the development of pathogen resistance to fungicides. Lower than recommended rates of fungicides tested resulted in higher disease and lower grain yields. Recommended rates for difenoconazole and carbendazim were optimum for the control of GLS. However, recommended rates tested for benomyl, flutriafol and flusilazole/carbendazim were less than optimum for the control of GLS.

The subcellular distribution of a basic class IV chitinase (Ch4) in Cercospora beticola -infected sugar beets has been studied by immunohistological analysis. Ch4 was primarily deposited in extracellular globuli proximal to the necrosis. High levels of Ch4 were observed in the vicinity of the necrotic lesions, low levels were found at distant sites from the necrosis. This plant-pathogen interaction thus gives rise to a local induction of Ch4, whereas no apparent systemic induction of Ch4 was found. After infection with Cercospora , a tolerant sugar beet cultivar accumulated Ch4 faster than a susceptible one, and the protein was spatially distributed over a comparatively larger area in the tolerant variety. In the tolerant beet cultivar symptoms appear a few days later, and the number of necroses are reduced and smaler than in the sensitive cultivar. In uninfected sugar beet leaves, Ch4 is present in low amounts in epidermis, stomata and xylem cells. A specific autofluorescence and a high endogenous β-glucuronidase activity in Cercospora were used to demonstrate that this pathogen did not penetrate the tissue beyond the necrosis.

The species Cercospora heliotropiicola, C. taurica and C. heliotropii were revised. Microscopical examination of type material and additional collections of these taxa revealed a similarity between C. heliotropiicola and C. apii and showed that C. heliotropii is not correctly placed in Cercospora, Phaeoramularia being a more appropriate genus for it. C. taurica was found to be a true Cercospora. The type specimen of C. heliotropii-bocconi could not be located so it was not possible to resolve the status of this species. Cercosporidium deightonii is introduced as a new species. Descriptions and illustrations of the investigated species are given.

Four isomers of 3′-hydroxy-γ-ionylideneacetic acid were isolated from Cercospora cruenta cultures. The ratio of isomers was found to be 44:24:21:11 for (2Z,4E)-(1′S,3′S)-, (2Z,4E)-(1′S,3′R)-, (2E,4E)-(1′S,3′S)- and (2E,4E)-(1′S,3′R)-3′-hydroxy-γ-acid, respectively. Feeding experiments with [14C] labelled (2Z,4E)-(1′S,3′S)-3′-hydroxy-γ-acid showed high incorporation into 1′,3′-dihydroxy-γ-acid and 1′-hydroxy-3′-oxo-γ-acid, but not into 4′-hydroxy-γ-acids. This result suggested that 3′-hydroxy-γ-acids were not involved in the ABA biosynthetic pathway. The biological activities of these new metabolites were compared with those of known ABA biosynthetic intermediates.

This contribution is the second of a series aimed at revising South African records of cercosporoid fungi. The present study describes six species, proposes one new combination, and reports one new record for South Africa. Two species of Ramularia, R. grewiae-occidentalis and R. zinniae are described from Grewia occidentalis and Zinnia peruviana respectively. Mycovellosiella dichondrae is described from Dichondra repens, and M. berkheyae-maritimae from Berkheya maritima, while Fulvia berkheyae is transferred to Mycovellosiella. Pseudocercospora diospyri-lycioides and Paracercospora scolopiae are described from Diospyros lycioides and Scolopia zeyheri respectively, and Cercosporella hypoestis noted as a new record on Hypoestes forskalii.

Nine of the 12 species of Cercospora isolated from various plants produced cercosporin. Some Cercospora spp. also produced a yellow fraction and C. arachidicola produced orange and yellow-orange fractions but not cercosporin. Neither cercosporin nor the orange fraction were host specific and caused similar damage on Swiss chard, tobacco and groundnuts. The amount of damage and toxin concentration were positively correlated. The orange fraction caused considerably less damage in the dark than in the light and young leaves were more resistant to damage than older leaves. The yellow-orange fraction was only weakly phytotoxic.

Sorghum is an important subsistence crop in Tanzania. Surveys were carried out between the 1986 and 1990 crop seasons to determine the prevalence of diseases and parasitic weeds in the major sorghum-growing areas of Tanzania. Twenty diseases of sorghum and three parastic weeds were observed. Grain moulds (several fungi), grey leaf spot (Cercospora sorghi), anthracnose (Colletotrichum graminicola), rust (Puccinia purpurea), leaf blight (Exserohilum turcicum), ladder leaf spot (Cercospora fusimaculans), sooty stripe (Ramulispora sorghi) and zonate leaf spot (Gleocercospora sorghi), were economically important diseases. Striga asiatica was the most common parasitic weed in Tanzania. Ilonga in the low-altitude (⩽ 1000 m) zone and Hombolo in the mid-altitude (> 1000 m) zone were identified as locations with a high frequency of diseases and striga incidence. These locations were recommended for resistance screening against most of the leaf diseases, grain moulds and S. asiatica.