Search

Beet necrotic yellow vein virus (BNYVV) generally has a four‐segmented positive‐sense RNA genome (RNAs 1–4), but some European and most Asian strains have an additional segment, RNA5. This study examined the effect of RNA5 and RNA3 on different sugar beet cultivars using a Polymyxa‐mediated inoculation system under field and laboratory conditions. In field tests, the degree of sugar yield served as an index for assessing the virulence of BNYVV strains. Japanese A‐II type isolates without RNA5 caused mostly 15%–90% sugar yield reductions, depending on the susceptibility of sugar beet cultivars, whereas the isolates with RNA5 induced more than 90% yield losses in the seven susceptible cultivars, but small yield losses in one Rz1‐resistant and Rizor cultivars. However, a laboratory‐produced isolate containing RNA5 but lacking RNA3 caused higher yield losses in Rizor than in susceptible plants, and induced scab‐like symptoms on the root surface of both susceptible and resistant plants. In laboratory tests, A‐II type isolates without RNA5 had low viral RNA accumulation levels in roots of Rizor and Rz1‐resistant plants at early stages of infection, but in the presence of RNA5, viral RNA3 accumulation levels increased remarkably. This increased RNA3 accumulation was not observed in roots of the WB42 accession with the Rz2 gene. In contrast, the presence of RNA3 did not affect RNA5 accumulation levels. Collectively, this study demonstrated that RNA5 is involved in the development of scab‐like symptoms and the enhancement of RNA3 accumulation, and suggests these characteristics of RNA5 are associated with Rz1‐resistance breaking.

Beet western yellows virus (BWYV; genus Polerovirus, family Luteoviridae) is one of the most important viruses causing yellowing disease of many field and vegetable crops. This study isolated different poleroviruses from sugar beet, spinach, radish and brassica in Japan, and identified them as BWYV‐JP, Beet leaf yellowing virus (BLYV), Brassica yellows virus (BrYV) and BrYV‐R (radish strain) based on host range and molecular analysis. Among over 100 plant species from 19 families inoculated with the vector Myzus persicae, about half of the species in 13 families were infected with some of these viruses. BLYV shared a similar host range to Beet mild yellowing virus (BMYV). These had a much more limited host range than BWYV‐JP, which resembled BWYV‐USA. The host range of BrYV was similar to that of Turnip yellows virus (TuYV). Phylogenetic analyses at the 5′ portion (replication‐related gene) of the genome showed that BLYV, BMYV, BWYV (‐JP and ‐USA) and Cucurbit aphid‐borne yellows virus (CABYV) formed one large group, whereas BrYV and TuYV were grouped together. BLYV and BWYV were most closely related to each other, and were more closely related to CABYV than to BMYV. However, at the 3′ end (coat protein gene), BLYV and BWYV‐JP formed a distinct group, separated from the BrYV group, which in turn was more closely related to BWYV‐USA, BMYV, TuYV and Beet chlorosis virus, a group originating from outside Asia. Thus, this study presents host range differences and phylogeographical relationships of BWYV‐like poleroviruses that are distributed worldwide.

Pseudomonas syringae pv. aptata is the causal agent of bacterial leaf spot disease of sugar beet (Beta vulgaris). During 2013, 250 samples were collected from leaf lesions with typical symptoms of bacterial leaf spot in commercial fields of sugar beet in Serbia, and 104 isolates of P. syringae pv. aptata were obtained. Identification and characterization was performed using biochemical, molecular and pathogenicity tests. Identification included LOPAT tests and positive reactions using primers Papt2F and Papt1R specific for P. syringae pv. aptata. Repetitive (rep) sequence‐based PCR typing with ERIC, REP and BOX primers revealed high genetic variability among isolates and distinguished 25 groups of different fingerprinting profiles. Pulse‐field gel electrophoresis (PFGE) and multilocus sequence analysis (MLSA) of representative isolates showed higher genetic variability than in rep‐PCR analysis and distinguished three and four major genetic clusters, respectively. A pathogenicity test performed with 25 representative isolates on four cultivars of sugar beet confirmed the occurrence of leaf spot disease and showed correlation between the most aggressive isolates and the genetic clusters obtained in MLSA. All these findings point to the existence of several lines of P. syringae pv. aptata infection in Serbia that are genetically and pathologically different.

Cercospora leaf spot (CLS) poses a high economic risk to sugar beet production due to its potential to greatly reduce yield and quality. For successful integrated management of CLS rapid and accurate identification of the disease is essential. Diagnosis on the basis of typical visual symptoms is often compromised by the inability to differentiate CLS symptoms from similar symptoms caused by other foliar pathogens of varying significance, or from abiotic stress. An automated detection and classification of CLS and other leaf diseases, enabling a reliable basis for decisions in disease control, would be an alternative to visual as well as molecular and serological methods. This paper presents an algorithm based on a RGB-image database captured with smartphone cameras for the identification of sugar beet leaf diseases. This tool combines image acquisition and segmentation on the smartphone and advanced image data processing on a server based on texture features using colour, intensity, and gradient values. The diseases are classified using a support vector machine with radial basis function kernel. The algorithm is suitable for binary-class and multi-class classification approaches, i.e. the separation between diseased and non-diseased, and the differentiation among leaf diseases and non-infected tissue. The classification accuracy for the differentiation of CLS, Ramularia leaf spot, Phoma leaf spot, beet rust, and bacterial blight was 82%, better than that of sugar beet experts classifying diseases also from images. Still, the technology has not been tested by practitioners. This tool can be adapted to other crops and their diseases and may contribute to improve decision making in integrated disease control. This article is protected by copyright. All rights reserved.

Rhizomania is one of the most damaging and widely spread diseases in major sugar beet growing regions of the world. The causal agent, beet necrotic yellow vein virus (BNYVV), is transmitted via the fungus Polymyxa betae, which retains it in the field for years. In this study, an RNA silencing mechanism was employed to induce resistance against rhizomania using intron-hairpin RNA (ihpRNA) constructs. These constructs were based on sequences of the BNYVV 5′-untranslated region of RNA-2 or the flanking sequence encoding P21 coat protein, with different lengths and orientations. Both transient and stable transformation methods produced effective resistance against rhizomania correlated with the transgene presence. Among the constructs, those generating ihpRNA structures with small intronic loops produced the highest frequencies of resistant events. The inheritance of transgenes and resistance was confirmed over generations in stably transformed plants.

Rhizomania, one of the most devastating diseases in sugar beet production, is caused by Beet necrotic yellow vein virus (BNYVV) and transmitted by Polymyxa betae. Previously, disease control was possible by cultivation of sugar beet hybrids carrying a major resistance gene Rz1, which restricts virus accumulation in taproots and suppresses symptom development. Over the last few years, BNYVV strains with four RNA components have arisen, which are able to overcome Rz1-mediated resistance. All strains described so far possess an A67V amino acid exchange within the RNA3-encoded P25 pathogenicity factor. In this study, BNYVV was isolated from Rz1 plants, collected in the United Kingdom, the Netherlands and Germany, displaying patches of strong rhizomania symptoms. Sequencing of the coat protein and P25 gene of three isolates showed 100% nucleotide sequence identity and detected AYPR as the P25 tetrad amino acid composition. The ability of this strain to accumulate to higher levels in young plants of Rz1 resistant but not in Rz1 + Rz2 resistant genotypes was initially demonstrated in a greenhouse assay in natural field soil from the Netherlands. This strain was loaded into a virus-free P. betae population and compared to reference strains. The AYPR strain retained its resistance-breaking ability in the Rz1 genotypes and displayed replication at a higher rate compared to the Rz1-resistance-breaking P type. The strain origin is unclear and it remains speculative whether the occurrence at different geographic locations is the result of independent selection or displacement of infested soil.

Fusarium spp. are responsible for significant yield losses in sugar beet (Beta vulgaris) with Fusarium oxysporum f. sp. betae most often reported as the primary causal agent. Recently, a new species, F. secorum, was reported to cause disease in sugar beet but little is known on the range of virulence within F. secorum or how this compares to the virulence and phylogenetic relationships previously reported for Fusarium pathogens of sugar beet. To initiate this study, partial translation elongation factor 1‐α (TEF1) sequences from seven isolates of F. secorum were obtained and the data were added to a previously published phylogenetic tree that includes F. oxysporum f. sp. betae. Unexpectedly, the F. secorum strains nested into a distinct group that included isolates previously reported as F. oxysporum f. sp. betae. These results prompted an expanded phylogenetic analysis of TEF1 sequences from genomes of publicly available Fusarium spp., resulting in the additional discovery that some isolates previously reported as F. oxysporum f. sp. betae are F. commune, a species that is not known to be a sugar beet pathogen. Inoculation of sugar beet with differing genetic backgrounds demonstrated that all Fusarium strains have a significant range in virulence depending on cultivar. Taken together, the data suggest that F. secorum is more widespread than previously thought. Consequently, future screening for disease resistance should rely on isolates representing the full diversity of the Fusarium population that impacts sugar beet.

The broad-host-range necrotizing fungal pathogen Rhizoctonia solani is responsible for economically significant diseases to crops as diverse as wheat, maize, barley, canola, sugar beet, potato, soyabean, bean, lupin and alfalfa. Germplasm screens in many of the crop hosts have not identified strong genetic resistance which, together with the lack of effective control, mean the pathogen remains a substantial problem for agriculture in many parts of the world. Following the establishment of a robust inoculation assay, a germplasm collection of the model legume Medicago truncatula was screened with various legume-infecting isolates of R. solani. While some significant differences in susceptibility/resistance were detected between some lines, in the majority of cases M. truncatula was susceptible to R. solani. Comparison of a legume- and cereal-infecting AG8 isolate with a legume-specific AG11 isolate revealed no difference in pathogenicity between the two isolates when infecting M. truncatula. The most significant differences in susceptibility occurred with an AG6 isolate, which caused root canker. This included significant differences between the moderate resistance of the M. truncatula reference genotype A17 and the high susceptibility of line A20. The analysis of a recombinant inbred line population derived from A17 and A20 revealed a single locus contributing to the resistance in A17. Interestingly, the locus only affected the development of post-emergent (late) symptoms, such as necrosis of cotyledons at 11 days after inoculation and root- and above-ground-weights, but not pre-emergent seedling damping off. These findings pave the way for further studies to dissect the genetic and molecular mechanisms of resistance.

The effects of biofumigation using a Brassica juncea (mustard) cover crop on the dynamics of rhizoctonia root rot of sugar beet were recorded in two field trials in 2007 and 2008, and analysed using epidemiological modelling. Differences between partial biofumigation, involving the pulling up of mustard plants, and complete biofumigation, involving the crushing and incorporation of mustard residues into the soil, were compared with bare soil treatment. An epidemiological model was used that includes rates of transmission of primary and secondary infection, pre-emergence damping off, and expression of wilting symptoms (above-ground disease) due to infected roots (below-ground disease). The model indicated that biofumigation reduces the transmission of primary infections but affects secondary infections in a variable pattern between field trials. Likewise, the proportion of infected plants expressing wilting was significantly reduced, by 28%, in the partial and complete biofumigation treatments compared with bare soil in the trial of 2007 but not in 2008. It is suggested that the effects of biofumigation on secondary infection and the expression of disease are more variable than those on primary infection, and that this is probably due to an interplay between pathogen, antagonists, host, and environmental factors. These interactions may or may not offset the benefits afforded by a reduction in primary infection and account for the overall variable success of biofumigation to control disease.

The population structure of Sclerotium rolfsii from autumn-sown sugar beet crops in Mediterranean-type climate regions of Chile, Italy, Portugal and Spain was determined by analyses of mycelial compatibility groups (MCGs) and pathogenicity to 11 economically important plant species. Twelve MCGs (i–xii) were identified among 459 S. rolfsii isolates. MCG iii was the most prevalent group in all countries except Italy. MCG i, the most abundant group (64AE7% of isolates) was identified in Portugal and Spain. The remaining MCGs were restricted to various regions within one country (ii, vi, ix) or different countries (v), or to specific localities (iv, vii, viii, x, xi, xii). MCGs iv, vii and x each comprised one isolate. Fields extensively sampled in southern Spain were infected with one to three MCGs. Plant species differed in susceptibility to MCG tester isolates with a MCG by species interaction. Cluster analyses allowed selection into five MCG groupings and grouped plant species into species-groups 1 (broccoli, chickpea, sunflower, tomato) and 2 (cotton, pepper, sugar beet, watermelon). MCG groupings 1 (i, ix), 2 (ii, iii, vi, viii) and 5 (x, xii) were moderately virulent to species-group 1 and mildly virulent to species-group 2. MCG groupings 3 (iv, v, xi) and 4 (vii) were mildly virulent to both species-groups. Across MCG groups, species were rated highly susceptible (chickpea, sunflower), susceptible (cotton, pepper, tomato, watermelon), moderately resistant (broccoli, melon, sugar beet) and resistant (corn, wheat). Establishing the MCG population structure and virulence variability among S. rolfsii isolates should help in the management of sclerotium root rot diseases.

Cercospora leaf spot, caused by the fungus Cercospora beticola, is a major fungal sugar beet disease worldwide and the cause of significant yield losses. The disease is most successfully countered by the introduction of genetic tolerance into elite sugar beet hybrids. To this end, breeding programmes require high quality biological assays allowing discrimination of minor differences between plants within a segregating population. This study describes the successful implementation of image analysis software in the bioassays for quantification of necrotic lesions at different stages of C. beticola infection, allowing selection on minor phenotypic differences during the sugar beet breeding process for C. beticola resistance. In addition, a real-time PCR assay was developed for the quantification of C. beticola pathogen biomass in infected beet canopy. The use of both techniques, even in an early stage of infection, fine-tunes current bioassays, allowing more accurate and efficient selection of resistant breeding material.

Fusarium yellows, caused by the soil-borne fungus Fusarium oxysporum f. sp. betae (Fob), can lead to significant yield losses in sugar beet. This fungus is variable in pathogenicity, morphology, host range and symptom production, and is not a well characterized pathogen on sugar beet. From 1998 to 2003, 86 isolates of F. oxysporum and 20 other Fusarium species from sugar beet, along with four F. oxysporum isolates from dry bean and five from spinach, were obtained from diseased plants and characterized for pathogenicity to sugar beet. A group of sugar beet Fusarium isolates from different geographic areas (including nonpathogenic and pathogenic F. oxysporum, F. solani, F. proliferatum and F. avenaceum), F. oxysporum from dry bean and spinach, and Fusarium DNA from Europe were chosen for phylogenetic analysis. Sequence data from b- tubulin, EF1a and ITS DNA were used to examine whetherFusarium diversity is related to geographic origin and pathogenicity. Parsimony and Bayesian MCMC analyses of individual and combined datasets revealed no clades based on geographic origin and a single clade consisting exclusively of pathogens. The presence of FOB and nonpathogenic isolates in clades predominately made up of Fusarium species from sugar beet and other hosts indicates that F. oxysporum f. sp. betae is not monophyletic.

The susceptibility of intercrop species (Raphanus sativus, Brassica juncea, B. rapa, Sinapis alba and Phacelia tanacetifolia) to the sugar beet pathogen Rhizoctonia solani was investigated in vitro, in the greenhouse and in the field with artificial inoculation. Disease severity in subsequently cultivated sugar beet was monitored in the field. Differences in susceptibility between species were found to be consistent in all experimental systems. All intercrop species were susceptible to R. solani. Brassica rapa and R. sativus were less susceptible than P. tanacetifolia. Compared to fallow, the cultivation of B. rapa and R. sativus reduced disease severity in subsequently grown sugar beet (median ratings of up to 3·0 and 3·5, respectively, depending on environmental conditions). This resulted in higher white sugar yield compared to fallow (up to 210% and 157% for B. rapa and R. sativus, respectively). This study demonstrates that in vitro and greenhouse resistance tests are suitable systems to predict the effects of intercrop species susceptibility in the field on disease severity and white sugar yield in subsequently grown sugar beet. Intercrop breeding programmes might profit from fast and efficient screening tests to provide Rhizoctonia-resistant intercrops as an additional control measure against R. solani in sugar beet.

The plasmodiophoromycete Polymyxa betae is an obligate root parasite that transmits Beet necrotic yellow vein virus (BNYVV), the cause of sugar beet rhizomania disease. Currently, control of this disease is achieved through the use of cultivars with monogenic (Rz1) partial resistance to the virus. To improve the level and durability of this resistance, sources of resistance to the virus vector, P. betae, were sought. Over 100 accessions of the wild sea beet (Beta vulgaris ssp. maritima) from European coastal regions were evaluated for resistance in controlled environment tests. Quantification of P. betae biomass in seedling roots was achieved using recombinant antibodies raised to a glutathione-s-transferase expressed by the parasite in vivo. Several putative sources of resistance were identified and selected plants from these were hybridized with a male-sterile sugar beet breeding line possessing partial virus resistance (Rz1). Evaluation of F1 hybrid populations identified five in which P. betae resistance had been successfully transferred from accessions originating from Mediterranean, Adriatic and Baltic coasts. A resistant individual from one of these populations was backcrossed to the sugar beet parent to produce a BC1 population segregating for P. betae resistance. This population was also tested for resistance to BNYVV. Amplified fragment length polymorphism and single-nucleotide polymorphism markers were used to map resistance quantitative trait loci (QTL) to linkage groups representing specific chromosomes. QTL for resistance to both P. betae and BNYVV were co-localized on chromosome IV in the BC1 population, indicating resistance to rhizomania conditioned by vector resistance. This resistance QTL (Pb1) was shown in the F1 population to reduce P. betae levels through interaction with a second QTL (Pb2) found on chromosome IX, a relationship confirmed by general linear model analysis. In the BC1 population, vector-derived resistance from wild sea beet combined additively with the Rz1 virus resistance gene from sugar beet to reduce BNYVV levels. With partial virus resistance already deployed in a number of high-yielding sugar beet cultivars, the simple Pb1/Pb2 two-gene system represents a valuable additional target for plant breeders.

Five hundred and eight Pythium isolates were obtained from diseased sugar-beet seedlings in the field from beet field soils in Ireland. Pythium sylvaticum and P. intermedium were the dominant species, together representing 86% of isolates, and P. salpingophorum, P. ultimum, P. mamillatum, P. oligandrum and Pythium‘group HS’were the other species identified. The cultural characteristics of these species corresponded generally with previous descriptions, with the exception of P. sylvaticum and P. mamillatum, which varied in some respects. In glasshouse pathogenicity tests, P. sylvaticum, P. intermedium, P. salpingophorum, P. ultimum, P. mamillatum and one unidentified Pythium isolate caused pre-emergence and early post-emergence damping-off of sugar-beet seedlings. Isolates off. oligandrum and Pythium‘group HS’were weakly pathogenic or not pathogenic to beet seedlings.

From 1996 to 1999, single-lesion isolates of Cercospora beticola, the cause of sugar beet leaf-spot disease, were obtained from sugar beet fields with different histories of use of the sterol-demethylation-inhibiting (DMI) fungicides, flutriafol and difenoconazole in Amyndeon, Imathia and Serres in northern Greece. Fields were sampled twice per year, before and after the within-season exposure of fungal populations to applications of DMI fungicides. The sensitivity to flutriafol and difenoconazole of 150 isolates of C. beticola per sampling site and sampling period was determined. In areas with a history of prolonged DMI use, sensitivity to both fungicides decreased gradually over the study period, suggesting directional selection on a polygenically controlled character. The size of the shift in a particular year seemed to depend on which fungicide was applied during that year in the fields, suggesting only partial cross-resistance between flutriafol and difenoconazole. Within the Serres population there was a significant correlation (r = 0·57) between sensitivities to flutriafol and difenoconazole. Decreased sensitivity was the result of the selection of less sensitive strains during the spraying periods. The frequency of such strains significantly increased through the years and finally reached values of 32% for both flutriafol and difenoconazole in the Serres population and 20 and 16%, respectively, in the Imathia population, at the end of 1999. The high frequency of less sensitive strains in the Serres population was related to a slight decline in DMI field performance in this area during that year. However, the frequency of the less sensitive strains was not stable. The sensitivity assays carried out early in each season, before the DMI spray applications, showed that the mean sensitivities of the populations were significantly higher (P

In a survey conducted during October 1995, single-lesion isolates of the sugar beet leaf-spot fungus, Cercospora beticola, were tested for sensitivity to the sterol demethylation inhibiting fungicides (DMIs) flutriafol and bitertanol. The isolates were collected from fields in three different areas of northern Greece. Fields at Serres and Imathia had been sprayed with DMIs for about 15 years to control sugar beet leaf-spot. At the third site, Amyndeon, DMI fungicides had not been used. From each area 150 isolates were tested. ED50 values were calculated for individual isolates by regressing the relative inhibition of colony growth against the natural logarithm of the fungicide concentration. The mean ED50 values for flutriafol for the Serres, Imathia and Amyndeon populations were 1·07, 0·73 and 0·5 m gm L 21 , respectively (significantly different at Pa 0·05). For bitertanol the mean ED50 values for the Serres and Imathia populations were 0·72 and 0·81 m gm L 21 , respectively, which were not significantly different at Pa 0·05. The mean ED50 value of the Amyndeon population was 0·48 m gm L 21 , which was significantly lower than those of the other two populations (P , 0·05). A cross-resistance relationship was found to exist between the two triazole fungicides tested when log transformed ED50 values of 60 isolates were subjected to a linear regression analysis (ra 0·81).

Polymyxa betae is the fungal vector of beet necrotic yellow vein virus (BNYVV), which is the causal agent of the sugar beet disease rhizomania. The within-season dynamics of the fungus are a crucial factor in the occurrence and severity of rhizomania. Late infection of the host by viruliferous fungi enables host resistance to the virus to develop and hence limits crop damage. A previously published mechanistic model for the dynamics of Polymyxa betae is extended in this paper to incorporate the effect of temperature on the germination of resting spores, and on the latent periods between infection and the production of secondary zoospores and new resting spores. It is shown that, for UK temperature conditions, the effect of sowing date on infection is greater than that of year-to-year variations in temperature associated with a single representative sowing date. The variation in inoculum build-up predicted when temperature data from a range of soil types were used in the model agreed with field observations, where higher levels of infection are observed on sandy soils than on black fen peat soils. The difference was most distinct when daily maximum soil temperature values were used to drive the model rather than rolling 24-hour average values.

The effect of beet yellows virus (genus Closterovirus, BYV) on sugar beet growth was studied in a series of field and glasshouse experiments. Infection reduced total plant weight by 20%, primarily through a 25% reduction in storage root growth. Sugar extraction efficiency was depressed by an increase in root impurities. BYV had little effect on above-ground yield or total crop cover but did decrease green cover significantly. Infection did not reduce water extraction depth in field experiments despite decreasing lateral root growth in the glasshouse. The growth reduction in infected plants resulted from both a decrease in net photosynthesis and an increase in the proportion of light intercepted by yellow leaves. Damage to the photosynthetic mechanism at least partly caused the reduction in net photosynthesis.

Application of Pythium oligandrum to a soil-based compost as a mycelial suspension (5 × 102 CFU g−1 of dry compost) and oospore alginate pellets (105 oospores/g of dry compost) controlled pre- and postemergence damping-off of sugar beet caused by Pythium ultimum to a level similar to metalaxyl seed treatment. Oospore seed treatments and aqueous suspensions of oospores applied to compost failed to control disease. Problems in the use of P. oligandrum oospore inocula for the control of damping-off were highlighted. It was shown that treatment of oospores with cellulase (20 g L−1) increased germination approximately three-fold in comparison to untreated spores. Untreated and cellulase pretreated oospores were subsequently evaluated as seed treatments for their ability to control damping-off of sugar beet. The highest rate of pretreated oospores (104 oospores/seed) gave levels of emergence and establishment in infested compost that were not significantly different from the uninfested controls, whereas seed treatment with untreated oospores gave no significant reduction in disease. In a trial carried out in a controlled environment to assess the effect of pH (4.5–8.0), P. oligandrum (104 cellulase pretreated oospores/seed) was shown to control pre- and postemergence damping-off of sugar beet at pH 7.0 and 7.5 only.

The host range of beet necrotic yellow vein virus (BNYVV) and Polymyxa betae was determined by growing plants in naturally infested soils from rhizomania outbreaks in England. Apart from Beta vulgaris, plant species infected by BNYVV were included in the families Chenopodiaceae (Atriplex patula, Chenopodium bonus-henricus, C. hybridum, C. polyspermum and Spinacia oleracea), Amaranthaceae (Amaranthus retroflexus) and Caryophyllaceae (Silene alba, S. vulgaris, S. noctiflora and Stellaria graminea). Only P. betae isolates from B. vulgaris, C. polyspermum and S. oleracea were found to be able to transmit BNYVV back to sugar beet. When a range of weed plants from infected fields were tested, none were found to be infected by BNYVV. Therefore, it seems likely that the weed hosts play only a minor role in the spread of rhizomania disease compared to that of sugar beet, other Beta vulgaris crop types or spinach.

A mathematical equation was developed that describes the inoculum densities of microsclerotia of Verticillium dahliae in the soil over a long time span. The equation was based on measurable parameters and ecologically meaningful principles. In the model, the number of systemic infections of plant roots during crop growth was related to soil inoculum density. In turn, formation of microsclerotia in debris and reduction of the amount of crop growth were related to the number of systemic infections. Finally, a gradual release and mortality of microsclerotia in the soil were included to calculate subsequent inoculum densities in the soil. Fitting the function to experimental data of potato cvs Element, Ostara, Mirka and Astarte, flax, pea, barley, sugar beet, onion and faba bean gave a very high correlation between observed and predicted soil inoculum densities. The clear differences in inoculum production among potato cultivars and other crops were expressed in quantitative terms. The highest inoculum density after incorporation of the debris of a susceptible crop was estimated to occur at 2.3 thermal time units of 3600 degree days (base 0°C). Ten per cent of the initial input of inoculum was still present after 4.5 thermal time units. The model was used to predict the dynamics of soil inoculum densities for V. dahliae under various cropping frequency schemes and performed satisfactorily.

Two independent bioassays demonstrated an antifungal effect of a basic sugar beet chitinase on Cercospora beticola, the causal agent of leaf spot disease in sugar beet (Beta vulgaris). In one assay, the growth of submerged spore cultures of C. beticola in microtitre wells was followed by measuring the increase in absorbance at 620 nm. Addition of chitinase to the culture resulted in a delay in germination and a slower initial growth rate. A more detailed picture of the action of the chitinase on the fungal cell wall was provided by an autoradiographic study. An intense labelling was observed at the apex of fungal hyphae grown in medium containing [ 3 H]N-acetylglucosamine, through incorporation of the radioactive chitin monomer into newly synthesized chitin in the cell wall. After fixation of the fungal specimen, the radioactive labelling could be removed by treatment with purified chitinase, i.e. nascent chitin chains were hydrolysed by the enzyme. When the fungal culture was subjected to a chase phase prior to fixation, the radioactive depositions were less accessible to hydrolysis by the chitinase. HPLC analysis of the radioactive hydrolysis products released from the apex of the fungal hyphae showed that the main products were small chito-oligosaccharides, mainly dimers, trimers and tetramers of chitin

Twenty common arable weed species were inoculated using Myzus persicae to transmit beet yellows virus (BYV), beet mild yellowing virus (BMYV), and an isolate of beet western yellows viruses (BWYV) that did not infect beet. The viruses were detected by enzyme-linked immunosorbent assay (ELISA), in which monoclonal antibodies distinguished between BMYV and BWYV, and by aphid transmissions to indicator host plants. Spergula arvensis, Stellaria media, Lamium purpureum and Papaver rhoeas were susceptible to all three viruses whereas Senecio vulgaris, Capsella bursa-pastoris, Anagallis arvensis and Chrysanthemum segetum were susceptible to both BMYV and BWYV, and Matricaria perforata, Raphanus raphanistrum, Veronica persica, Urtica urens and Viola arvensis were susceptible to BWYV only. Atriplex patula, Chenopodium album and Portulaca oleracea were susceptible to BYV only. Myosotis arvensis, Silene alba, Poa annua and Solanum nigrum were not susceptible to any of the viruses. Portulaca oleracea was shown for the first time to be a host of BYV, and C. segetum a host of BMYV and BWYV. In spring 1991, 8% of weeds sampled in a field of autumn-sown oilseed rape contained BWYV. Tests on weeds collected from an area of ‘set-side’ adjacent to sugar beet showed that 3% contained BMYV and 3% BWYV. No sampled weeds were infected with BYV. The role of weeds in the epidemiology of sugar beet virus yellows is discussed.

In a survey of fungi causing seedling diseases of sugar beet using a soil bioassay, Aphanomyces cochlioides and Pythium spp. were found to occur in 39% and 31%, respectively, of 341 sugar-beet fields selected in a stratified random sample in England. The frequency of A. cochlioides-infested soils varied widely in the different sugar-beet growing areas of the country. Soil pH was the single factor most strongly associated with the distribution of the pathogen, but regression models applied to combinations of factors indicated that soil texture and the interval between sugar-beet crops were also relevant to its frequency. It was detected less often in soils of high pH (greater than or equal to 7.5), heavy texture and where the interval between sugar-beet crops exceeded 5 years. No significant associations were found between the proportion of soils with Pythium spp. and soil or cropping factors.

The relationships of two host-adapted pathotypes of Verticillium dahliae have been examined at the molecular level using restriction fragment length polymorphisms. Isolates obtained from and adapted to Mentha×piperita (peppermint), which were presumed to be haploid, formed a distinct subspecific group (referred to as M) related to the previously described non-host-adapted subspecific group A of V. dahliae. The limited molecular variation found among the four group M isolates was not related to geographic origin. Isolates from several cruciferous hosts (and one from Beta vulgaris (sugar beet)), which are thought to be natural, stable diploids, formed another distinct group (referred to as D) that was markedly different from all previously described subspecific groupings in both V. dahliae and V. alboatrum. This group of isolates might better be regarded as a separate species. Again, only limited variation was found within the D group. Polymorphisms revealed by two probes distinguished two isolates derived from Brassica rapa (Chinese cabbage) from the six other isolates (four from Brassica napus (oilseed rape) and one each from Raphanus raphanistrum (wild radish) and Beta vulgaris).

Seven-day-old seedlings of three different sugar-beet varieties (Carla, Rizor and Desiree) were inoculated mechanically with the Ahlum isolate of beet soil-borne virus (BSBV) in crude sap from infected leaves of Chenopodium quinoa. Control plants were mock-inoculated with sap from healthy C. quinoa. After potting up, the plants were arranged in a randomized block design on nine neighbouring benches in a glasshouse. Plants were sampled after 11 weeks, the fresh weight of the tap roots determined and the presence of virus checked by ELISA. Considerable variation in tap-root growth was observed between benches. Overall, BSBV inoculation reduced tap-root weight of Rizor and Carla by c. 20%. Problems arising in attempts to assess yield loss due to BSBV in naturally infected sugar beet are discussed.

The effect of sowing date on the infection of sugar-beet seedlings by Polymyxa betae was examined in a small-plot experiment on a naturally infested site. Seed was sown on seven occasions at weekly intervals from late March to early May. From each sowing, plant samples were taken at approximately weekly intervals over a period of 7 weeks. The extent of root infection by P. betae and the dry weight of plants was determined at each sampling date, and the progress of infection and rate of plant growth were examined against time and thermal time. Infection occurred sooner after sowing and the subsequent rate of fungal development was more rapid in late-sown than in early-sown plants. Early sowing allowed germination and growth of sugar beet at temperatures too low for fungal infection. The growth of late-sown plants appeared to be reduced by P. betae infection. The implications of these findings for the development of rhizomania disease are discussed.

In a national survey of the incidence of powdery mildew (Erysiphe betae) in sugar beet in Britain, between 1980 and 1989, in which between 335 and 717 fields were inspected at the end of August each year, the average crop area infected ranged from 2% in 1986 to 68% in 1989. The effect of various climatic factors that might be expected to influence powdery mildew development over this period was examined, using meteorological records from Broom's Barn Experimental Station, Using simple linear regression analyses, a significant association was found between disease incidence and both air temperature and rainfall incidence in the period April-August, However, air temperature and frost incidence in some winter months were also closely associated with the incidence of powdery mildew in the following crop. Combinations of these variables were examined in stepwise multiple-regression analyses, The model of best fit, incorporating winter frost incidence with summer temperature and rainfall incidence, accounted for 95% of the variation. Model validation procedures demonstrated the superiority of this model over those involving single variables. Nevertheless, for forecasting purposes, the best correlation was found between the crop area infected at the end of August each year and the number of days with a ground frost in the preceding February and March. This single climatic variable accounted for 82% of the variation and its potential use to forecast powdery mildew incidence in the national sugar-beet crop is discussed.

Rhizoctonia solani and R. cerealis were isolated from diseased sugar-beet seedlings in Ireland. Isolates of R. solani were assigned to anastomosis groups AG-2, AG-4, AG-5 and an unidentified group that did not anastomose with recognized tester isolates. Cultures of AG-2 were similar to those of AG-5 on oatmeal agar (OA) and potato-dextrose-marmite agar (PDMA). Cultures of AG-4, the unidentified group and R. cerealis were morphologically distinct from one another, AG-2 and AG-5. The optimum temperature for growth of AG-2 was 225 C, with optimum growth of AG-4, AG-5 and the unidentified group at 275-C. R. cerealis grew slower than all groups of R. solani, with optimum growth at 225°C. Hyphae of R. cerealis were significantly narrower than those of the groups of R. solani studied. In glasshouse pathogenicity tests, some AG-2 and all AG-4, AG-5 and isolates from the unidentified group caused damping-off of beet seedlings. In controlled environments of 10-25°C, an AG-2 isolate was the most aggressive at 10 C whilst AG-4, AG-5 and the unidentified group caused most disease at or above 15°C. R. cerealis was also pathogenic to beet seedlings, causing damping-off at 10 and 15 C.