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Spongospora subterranea f. sp. subterranea is an important pathogen of potato responsible for major losses in most potato growing regions of the world. Infection is initiated by bi-flagellated motile zoospores released from long-lived resting spores. Zoospore chemotaxis to the host plant root is widely believed to be stimulated by host root exudate compounds, although direct evidence is lacking. This study refined the traditional chemotaxis capillary assay. With this we provided the first empirical evidence of S. subterranea zoospore chemotaxis. Individual potato root exudate metabolites were either taxis neutral, inhibitory, or attractant to the zoospores. L-glutamine was the strongest chemoattractant, while spermine was the most inhibitory. Zoospore motility and chemotaxis were constrained by strongly acidic or alkaline solutions of pH < 5.3 and > 8.5, respectively. Beyond pH, ionic constituents of the test solution affected zoospore motility as Sorensen’s phosphate buffer stalled zoospore motility, but HEPES buffer at the same concentration and pH had little or no negative motility effect. Zoospore motility, as characterized by several parameters, influenced chemotaxis. Among the parameters measured, total distance travelled was the best predictor of zoospore chemotaxis. The characterization of environmental and ecological effects on zoospore motility and chemotaxis highlights useful targets for S. subterranea disease control through manipulation of zoospore taxis or selection of host resistance traits.

This study examined the natural and experimental host range and aphid and graft transmission of the tentative polerovirus phasey bean mild yellows virus (PBMYV). Eleven complete coding sequences from PBMYV isolates were determined from a range of hosts and locations. We found two genetically distinct variants of PBMYV. PBMYV-1 was the originally described variant, and PBMYV-2 had a large putative recombination in open reading frame 5 such that PBMYV-1 and PBMYV-2 shared only 65-66% amino acid sequence identity in the P5 protein. The virus was transmitted by a clonal colony of cowpea aphids (Aphis craccivora) and by grafting with infected scions but was not transmitted by a clonal colony of green peach aphids (Myzus persicae). PBMYV was found in natural infections in 11 host species with a range of symptoms and severity, including seven important grain legume crops from across a wide geographic area in Australia. PBMYV was common and widespread in the tropical weed phasey bean (Macroptilium lathyroides), but it is likely that there are other major alternative hosts for the virus in temperate regions of Australia. The experimental host range of PBMYV included the Fabaceae hosts chickpea (Cicer arietinum), faba bean (Vicia faba), pea (Pisum sativum), and phasey bean, but transmissions failed to infect several other members of the families Asteraceae, Cucurbitaceae, Fabaceae and Solanaceae. PBMYV was commonly found in grain legume crops in eastern and western Australia, sometimes at greater than 90% incidence. This new knowledge about PBMYV warrants further assessments of its economic impact on important grain legume crops.

Opium poppy, belonging to the family Papaveraceae, is grown for its alkaloid compounds of pharmaceutical value. Downy mildew caused by Peronospora somniferi and Pe. meconopsidis, substantially impacts crop production. The present study was conducted to identify the host range of Pe. somniferi and Pe. meconopsidis within selected weed and ornamental members of the Papaveraceae family. Nine Papaver spp., Meconopsis cambrica and a nonhost control (tomato, Solanum lycopersicum) were challenged with both pathogens under controlled glasshouse or laboratory conditions using infested soil or foliar applied sporangia as inocula. Peronospora somniferi and Pe. meconopsidis induced disease symptoms, including sporulation, in at least one trial for all tested species except for Pa. atlanticum and tomato. Species‐specific PCR testing of foliage of challenged plants confirmed infections by both pathogen species of symptomatic plants, identifying these as hosts. Positive PCR tests were also obtained from Pa. atlanticum plants for both pathogens. However, in the absence of pathogen sporulation structures as further evidence of infection, the host status of Pa. atlanticum remains inconclusive. Testing of seeds collected from Pe. somniferi and Pe. meconopsidis infected plants of Pa. somniferum, Pa. dubium, Pa. rhoeas and Pa. nudicaule showed presence of both pathogens, indicating likely ability for seed transmission in these species. We identified new hosts of these pathogens and discuss potential implications of these alternative hosts in pathogen survival, dissemination and epidemic initiation.

Tuber greening is one of the major causes of quality loss in the potato industry, however, the impact of nitrogen in-field fertilisation on this visual defect remains unknown. Two field experiments, one in Tasmania with final N treatment rates of 40, 100 and 190 kg N ha-1 for Nicola and 35, 95 and 185 Kg N ha-1 for Maranca, and another in South Australia with N rates of 100, 200 and 300 kg ha-1 for Maranca, showed a positive linear relationship (R2 = 0.99, 0.86 and 0.90, respectively) between tuber N content and tuber greening, as determined by chlorophyll content. However, increased N fertilisation rates did not directly translate to increased tuber N content, with responses appearing to be variety specific. We conclude that tuber N content is a risk factor associated with tuber greening, but the manipulation of tuber N content through N fertilisation rate is not straightforward and may depend on variety and location.

A review of the records of plant viruses, viroids and phytoplasmas in Tasmania has found evidence for 51 viruses and one viroid. Based on insufficient evidence, 22 viruses and reports of at least three phytoplasmas are unconfirmed. Most of these unconfirmed pathogens are probably present but further work is required to confirm this. Forty two of the listed viruses are aphid transmitted, including 15 potyviruses, 8 carlaviruses and 8 luteoviruses. Only one leafhopper and one whitefly transmitted virus are reported. One new viroid and eighteen new virus records are included. As well as infecting weeds, agricultural and horticultural species, some of the viruses have invaded Tasmania’s native flora.

Light-induced tuber greening is one of the most important quality defects of potato. Although varietal and maturity factors are known to affect greening resistance, physiological mechanisms of resistance are poorly understood. We proposed that physiological and biochemical factors within the tuber periderm provide resistance and hypothesised that resistance is primarily related to suberin content. We investigated differences in the tuber periderm between genotypes and tuber maturities that varied in greening propensity. We examined suberin and light-induced pigment accumulation, and phellem cell development and studied greening propensity in mutant and chemically treated tubers with enhanced suberisation. Resistance to greening was strongly linked to increased suberin in the periderm, which varied with variety and tuber maturity. Furthermore, greening was reduced in mutant and chemically treated tubers with enhanced suberisation. Increases in phellem cell layers and light-induced carotenoids and anthocyanins were identified as secondary resistance factors. Our work represents the first physiological mechanism of varietal and tuber maturity resistance to greening, expanding the known functionality of suberin and providing for the first time a biomarker that will aid producers and breeders in selection and improvement of potato varieties for greening resistance.

The pathogen Spongospora subterranea infects potato roots and developing tubers resulting in tuber yield and quality losses. Currently, there are no fully effective treatments for disease control. Host resistance is an important tool in disease management and understanding the molecular mechanisms of defence responses in roots of potato plants is required for the breeding of novel resistant cultivars. Here, we integrated transcriptomic and proteomic datasets to uncover these mechanisms underlying S. subterranea resistance in potato roots. This multi-omics approach identified upregulation of glutathione metabolism at the levels of RNA and protein in the resistant cultivar but not in the susceptible cultivar. Upregulation of the lignin metabolic process, which is an important component of plant defence, was also specific to the resistant cultivar at the transcriptome level. In addition, the inositol phosphate pathway was upregulated in the susceptible cultivar but downregulated in the resistant cultivar in response to S. subterranea infection. We provide large-scale multi-omics data of Spongospora-potato interaction and suggest an important role of glutathione metabolism in disease resistance.

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Simple Summary Infections of potato tubers and roots by Spongospora subterranea result in powdery scab and root diseases. Losses due to infections with S. subterranea are substantial in most potato-growing regions of the world with no fully effective treatments available. Understanding the gene regulation of pathogens in their host is dependent on multidimensional datasets. In this study, we profiled the transcriptome and proteome of S. subterranea within the susceptible and resistant host. Enzyme activity and nucleic acid metabolism appear to be important to the virulence of S. subterranea. Our results provide a good resource for future functional studies of powdery scab and might be useful in S. subterranea inoculum management. Abstract Spongospora subterranea is an obligate biotrophic pathogen, causing substantial economic loss to potato industries globally. Currently, there are no fully effective management strategies for the control of potato diseases caused by S. subterranea. To further our understanding of S. subterranea biology during infection, we characterized the transcriptome and proteome of the pathogen during the invasion of roots of a susceptible and a resistant potato cultivar. A total of 7650 transcripts from S. subterranea were identified in the transcriptome analysis in which 1377 transcripts were differentially expressed between two cultivars. In proteome analysis, we identified 117 proteins with 42 proteins significantly changed in comparisons between resistant and susceptible cultivars. The functional annotation of transcriptome data indicated that the gene ontology terms related to the transportation and actin processes were induced in the resistant cultivar. The downregulation of enzyme activity and nucleic acid metabolism in the resistant cultivar suggests a probable influence of these processes in the virulence of S. subterranea. The protein analysis results indicated that the majority of differentially expressed proteins were related to the metabolic processes and transporter activity. The present study provides a comprehensive molecular insight into the multiple layers of gene regulation that contribute to S. subterranea infection and development in planta and illuminates the role of host immunity in affecting pathogen responses.

For soilborne pathogens, germination of the resting or dormant propagule that enables persistence within the soil environment is a key point in pathogenesis. Spongospora subterranea is an obligate soilborne protozoan that infects the roots and tubers of potato causing root and powdery scab disease for which there are currently no effective controls. A better understanding of the molecular basis of resting spore germination of S. subterranea could be important for development of novel disease interventions. However, as an obligate biotroph and soil dwelling organism, the application of new omics techniques for the study of the pre-infection process in S. subterranea has been problematic. Here, RNA sequencing was used to analyse the reprogramming of S. subterranea resting spores during the transition to zoospores in an in-vitro model. More than 63 million mean high-quality reads per sample were generated from the resting and germinating spores. By using a combination of reference-based and de novo transcriptome assembly, 6,664 unigenes were identified. The identified unigenes were subsequently annotated based on known proteins using BLAST search. Of 5,448 annotated genes, 570 genes were identified to be differentially expressed during the germination of S. subterranea resting spores, with most of the significant genes belonging to transcription and translation, amino acids biosynthesis, transport, energy metabolic processes, fatty acid metabolism, stress response and DNA repair. The datasets generated in this study provide a basic knowledge of the physiological processes associated with spore germination and will facilitate functional predictions of novel genes in S. subterranea and other plasmodiophorids. We introduce several candidate genes related to the germination of an obligate biotrophic soilborne pathogen which could be applied to the development of antimicrobial agents for soil inoculum management.

Thrips tabaci (Lindeman) is an important polyphagous pest, and vector of Tomato spotted wilt virus responsible for sporadic, but devastating epidemics in potato. T. tabaci shows significant preferential differences between potato cultivars that may be important for field resistance. To investigate the role of visual cues in host choice we tested colour preference using two-choice assays. Thrips tabaci showed a significant preference for mid-green over red, blue, and white coloured cards, a preference for both light-green and mid-green over dark-green, and light-green over yellow, but no preferential difference between mid-green and yellow, and between light-green and mid-green. Analysis of the spectral reflectance of potato cultivars differing in thrips preference, revealed significant differences within the 400–700 nm wavelengths. In most cases cultivars preferred by T. tabaci had lighter green foliage and higher reflectance at 552 nm, and thus colour preference may be important for host selection. Oviposition choice of T. tabaci for potato cultivars was determined from counts of larvae and unhatched eggs from leaf disks under choice and no-choice conditions. In contrast to the colour choice experiments, onion thrips showed oviposition preference for cultivars with darker green foliage and lower reflectance at 552 nm.

The soil-borne and obligate plant-associated nature of S. subterranea has hindered a detailed study of this pathogen and in particular, the regulatory pathways driving the germination of S. subterranea remain unknown. To better understand the mechanisms that control the transition from dormancy to germination, protein profiles between dormant and germination stimulant-treated resting spores were compared using label-free quantitative proteomics. Among the ~680 proteins identified 20 proteins were found to be differentially expressed during the germination of S. subterranea resting spores. Elongation factor Tu, histones (H2A and H15), proteasome and DJ-1_PfpI, involved in transcription and translation, were upregulated during the germination of resting spores. Downregulation of both actin and beta-tubulin proteins occurred in the germinating spores, indicating that the changes in the cell wall cytoskeleton may be necessary for the morphological changes during the germination of the resting spore in S. subterranea. Our findings provide new approaches for the study of these and similar recalcitrant micro-organisms provide the first insights into the basic protein components of S. subterranea spores. A better understanding of S. subterranea biology may lead to the development of novel approaches for the management of persistent soil inoculum.

Attempts at management of diseases caused by protozoan plant parasitic Phytomyxea have often been ineffective. The dormant life stage is characterised by long-lived highly robust resting spores that are largely impervious to chemical treatment and environmental stress. This review explores some life stage weaknesses and highlights possible control measures associated with resting spore germination and zoospore taxis. With phytomyxid pathogens of agricultural importance, zoospore release from resting spores is stimulated by plant root exudates. On germination, the zoospores are attracted to host roots by chemoattractant components of root exudates. Both the relatively metabolically inactive resting spore and motile zoospore need to sense the chemical environment to determine the suitability of these germination stimulants or attractants respectively, before they can initiate an appropriate response. Blocking such sensing could inhibit resting spore germination or zoospore taxis. Conversely, the short life span and the vulnerability of zoospores to the environment require them to infect their host within a few hours after release. Identifying a mechanism or conditions that could synchronise resting spore germination in the absence of host plants could lead to diminished pathogen populations in the field.

Forty-seven potato virus A (PVA) isolates from Europe, Australia, and South America’s Andean region were subjected to high-throughput sequencing, and 46 complete genomes from Europe (n = 9), Australia (n = 2), and the Andes (n = 35) obtained. These and 17 other genomes gave alignments of 63 open reading frames 9,180 nucleotides long; 9 were recombinants. The nonrecombinants formed three tightly clustered, almost equidistant phylogroups; A comprised 14 Peruvian potato isolates; W comprised 37 from potato in Peru, Argentina, and elsewhere in the world; and T contained three from tamarillo in New Zealand. When five isolates were inoculated to a potato cultivar differential, three strain groups (= pathotypes) unrelated to phylogenetic groupings were recognized. No temporal signal was detected among the dated nonrecombinant sequences, but PVA and potato virus Y (PVY) are from related lineages and ecologically similar; therefore, “relative dating” was obtained using a single maximum-likelihood phylogeny of PVA and PVY sequences and PVY’s well-supported 157 CE “time to most common recent ancestor”. The PVA datings obtained were supported by several independent historical coincidences. The PVA and PVY populations apparently arose in the Andes approximately 18 centuries ago, and were taken to Europe during the Columbian Exchange, radiating there after the mid-19th century potato late blight pandemic. PVA’s phylogroup A population diverged more recently in the Andean region, probably after new cultivars were bred locally using newly introduced Solanum tuberosum subsp. tuberosum as a parent. Such cultivars became widely grown, and apparently generated the A × W phylogroup recombinants. Phylogroup A, and its interphylogroup recombinants, might pose a biosecurity risk. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Downy mildew is a serious threat to opium poppy production globally. In recent years, two pathogen species, Peronospora somniferi and Peronospora meconopsidis, which induce distinct symptoms, have been confirmed in Australia. In order to manage the spread of these pathogens, identifying the sources of inoculum is essential. In this study, we assessed pathogen presence associated with poppy seed. We developed PCR and qPCR assays targeting the coxI and coxII gene regions, for the detection, differentiation, and quantification of P. somniferi and P. meconopsidis in poppy seed. These results were complemented and compared with direct seed histological examination and a seed washing combined with viability staining for oospore detection. The majority of seed lots from all harvest years contained detectable P. meconopsidis, the earliest (1987) predating the first official record of the disease in Tasmania (1996). In contrast, only seed lots harvested in 2012 or later contained P. somniferi, evidence of its more recent introduction. P. meconopsidis contamination was estimated to be as high as 33.04 pg DNA/g of seed and P. somniferi as high as 35.17 pg DNA/g of seed. Incidence of pathogen contamination of seeds, estimated via a group testing protocol, ranged from 0 to 9% (P. meconopsidis) or 0 to 11% (P. somniferi). Mycelia were predominately found external to the seed coat. Seed washing and viability staining demonstrated that putatively viable oospores were present in the majority of seed lots. Transmission testing confirmed both pathogens can be successfully transmitted from infested seed to infected seedling. PCR and qPCR pathogen assays were found to be reliable and offer a routine test for determining pathogen inoculum in poppy seeds.

Spongospora subterranea, which causes powdery scab of potato, infects a diverse range of plant species. Crop rotation as a powdery scab management tool will be compromised if pathogen hosts exist between potato crops. Opium poppy (Papaver somniferum) and pyrethrum (Tanacetum cinerariifolium) are important crops within intensive vegetable production rotations in NW Tasmania. Measurements of S. subterranea soil inoculum within a commercial field showed pathogen amounts were substantially elevated following an opium poppy crop, which suggested host status. In glasshouse testing, opium poppy and pyrethrum were confirmed as hosts of S. subterranea, with opium poppy the more susceptible of the two. Both species were less susceptible than tomato, a known host. Observations of early growth suggested inoculation impacts on all three plant species, although at 16 (tomato and opium poppy) or 26 (pyrethrum) weeks postinoculation, only tomato had significantly reduced shoot and root development. The role of rotation crops in inoculum persistence and the possible role of S. subterranea as a minor pathogen of nonpotato crops are discussed.

Tomato plants are highly susceptible to root infection by Spongospora subterranea and are commonly used as bioassay hosts. The impacts of root infection with S. subterranea on plant productivity and yield have been debated. Recent experiments with potato, the major economic host of S. subterranea, have indicated significantly reduced plant growth and potato yield following heavy infection. However, there have been very few similar studies that have examined the possible impacts of S. subterranea infection on tomato plant growth. Three tomato cultivars, "Grape," "Roma" and "Truss," were challenged with S. subterranea inoculum in hydroponic culture. Moderate to severe zoosporangial infections were observed with minor but statistically significant differences in susceptibility among the three tomato cultivars. Zoosporangial root infection in the absence of root gall formation resulted in significantly diminished shoot lengths and plant fresh weights in pathogen challenge tests conducted both in hydroponic culture and glasshouse-grown plants in potting mix. Root lengths were reduced, but the differences were statistically significant in a single trial only. The findings from this study demonstrate that, as with potato, root infection by S. subterranea can result in reduced tomato plant growth and that root gall production associated with root infection was not necessary for this retardation of growth response. This further suggests that possible yield impacts in other crop species that are hosts for S. subterranea root infection are worthy of examination.

Greening is a major cause of quality loss in potato tubers. As underground stems, potato tubers are non-photosynthetic plant organs that lack photosynthetic machinery. However, after light exposure, amyloplasts convert to chloroplasts in tuber peripheral cell layers, which cause the accumulation of the green photosynthetic pigment, chlorophyll. Tuber greening can be impacted by genetic, cultural, physiological and environmental factors including planting depth, tuber physiological age, temperature, atmospheric oxygen levels, and lighting conditions. Numerous studies have been devoted to understand and control this costly defect for the potato industry. This review brings together the available knowledge on light-induced greening, from causes to solutions and suggestions on further research with a focus on identifying the underlying mechanisms of tuber greening.

Hoagland’s solution (HS), a defined nutrient supplement for plants, has been previously reported to stimulate zoospore release from resting spores of the potato pathogen Spongospora subterranea f. sp. subterranea. This study obtained direct empirical evidence for an increase in zoospore release with HS treatment, and identified Fe-EDTA as the stimulant component of HS. Stimulation of resting spores by HS and Fe-EDTA resulted in greater and earlier zoospore release compared to a distilled water control, and in the presence of a susceptible tomato host plant resulted in enhanced root infection. Given the labile nature of S. subterranea zoospores, it was postulated that stimulation of premature release of zoospores from the dormant resting spores in absence of susceptible hosts could reduce soil inoculum levels. In two glasshouse trials in the absence of host plants, both Fe-EDTA and HS soil treatments reduced S. subterranea soil inoculum levels, providing proof of concept for the ‘germinate to exterminate’ approach to inoculum management.

A series of experiments was conducted under glasshouse and hydroponic conditions to determine the specific window period for common scab disease infection in potato tubers. The study was performed in a glasshouse system where separate tubers from the root zone were inoculated at different intervals during plant growth along with a novel hydroponic system to inoculate individual tubers at specific times of development growth allowing non-destructive observations of common scab symptoms developing. The window of tuber susceptibility to common scab disease infection was shown to vary with the season or conditions under which the plants were grown. Different internodes on tubers were found susceptible to infection at different times during tuber development. Basal internodes, which are the first sections of the tuber to expand, were susceptible to infection in the beginning of tuber development, whereas apical internodes only became susceptible later in tuber growth when the basal internodes were no longer susceptible.Journal of Nepal Agricultural Research Council Vol.3 2017: 19-26

The soil-borne potato pathogen Spongospora subterranea persists in soil as sporosori, which are aggregates of resting spores. Resting spores may germinate in the presence of plant or environmental stimuli, but direct evidence for resting spore dormancy is limited. A soilless tomato bait plant bioassay and microscopic examination were used to examine features of S. subterranea resting spore dormancy and infectivity. Dried sporosori inocula prepared from tuber lesions and root galls were infective after both short- and long-term storage (1 week to 5 years for tuber lesions and 1 week to 1 year for root galls) with both young and mature root galls inocula showing infectivity. This demonstrated that a proportion of all S. subterranea resting spores regardless of maturity exhibit characteristics of stimuli-responsive dormancy, germinating under the stimulatory conditions of the bait host plant bioassay. However, evidence for constitutive dormancy within the resting spore population was also provided as incubation of sporosorus inoculum in a germination-stimulating environment did not fully exhaust germination potential even after 2.4 years. We conclude that S. subterranea sporosori contain both exogenous (stimuli-responsive) and constitutively dormant resting spores, which enables successful host infection by germination in response to plant stimuli and long-term persistence in the soil.

The incidence of Groundnut rosette assistor virus (GRAV) in farmers’ fields and sequence diversity of groundnut rosette disease (GRD) agents were assessed in the three northern groundnut production regions of Ghana. GRAV incidence was high (69.5 to 75.0%) but not significantly different between the regions. Nucleotide sequencing of GRAV coat protein (CP) gene revealed 99–100% identity among the Ghanaian isolates and 97–100% similarity to GRAV sequences from Nigeria and Malawi for both nucleotide and predicted amino acids. Nucleotide sequence identities of partial ORF3 and 4 of Groundnut rosette virus (GRV) among the Ghanaian isolates were more variable (89–100%). Ghanaian GRV isolates were more closely related in nucleotide sequence identity to Nigerian isolates (95–98%) than Malawian isolates (88–90%). Similarly, nucleotide identity within Ghanaian GRV-sat RNA’s were close (94–100%), but distinct from Nigerian (82–87%) and Malawian (82–86%) GRV-sat RNAs. Ghanaian isolates of all three agents of GRD showed no obvious isolate diversity patterns based on the regions from where they were collected. We present the first report on the distribution and genetic diversity of GRD agents in Ghana.

Downy mildew of opium poppy is the single biggest disease constraint afflicting the Australian poppy industry. Within the pathosystem, the transmission of infections via infested seed is of major concern. Both downy mildew pathogens of poppy; Peronospora meconopsidis and P. somniferi, are known contaminants of commercial seed stocks. Using seed naturally infested with these pathogens, the effect of physio-chemical seed treatments on seedling health and disease transmission were evaluated. Individual seed treatments were tested to determine optimal treatment parameters for each; including incubation time, temperature and treatment concentration. Optimised physiochemical treatments were then compared. The most effective treatment methods were seed washes in acidified electrolytic water (400 ppm hypochlorous acid for 5 min) and hypochlorite solution (2% NaOCI for 5 min). In seed to seedling transmission assays, these two treatments reduced transmission of P. somniferi by 88.8% and 74.61%, and P. meconopsidis by 93.3% and 100%, respectively. These methods are recommended for seed treatment of commercial opium poppy seed to assist in the control of the downy mildew diseases.

Phytophagous mites and thrips are global pests affecting a wide range of agricultural crops (Mouden et al., 2017; Agut et al., 2018). Among the arthropods, they are phylogenetically distant, but both classes harbor species ranging from highly specialized to extremely polyphagous (Rioja et al., 2017; Wu et al., 2018). Through convergent evolution, mites and thrips evolved stylets to facilitate feeding from mesophyll or epidermal cells (Bensoussan et al., 2016; Rioja et al., 2017; Wu et al., 2018). Despite large crops losses (Agut et al., 2018; Steenbergen et al., 2018) that are expected to become more severe with global warming (Ximenez-Embun et al., 2017; Urbaneja-Bernat et al., 2019), the interactions between mites/thrips and their host plants have been understudied. Hence, understanding how plants defend themselves against these pests is essential for developing crop protection strategies. This Research Topic provides an update on recent advances in the plant molecular and physiological mechanisms associated with phytophagous mite/thrips-plant interactions, and provides an overview of different approaches for improving crop resistance sustainably, either through repellence, feeding disruption or prevention of feeding damage. Here, we highlight some of the major points arising from these reports.

Wasabi (Wasabia japonica (Miquel) Matsum.), a native perennial of Japan and Shakhalin Island used to produce a condiment for Japanese dishes, is under commercial development in Tasmania, Australia. Plants propagated within shade houses showed systemic necrotic flecks and veinal necrosis in leaves and sunken necrotic stem lesions similar to those, reported in Japan (1), caused by cucumber mosaic cucumovirus (CMV). Necrosis progressed rapidly, resulting in death of plants transferred to or mechanically inoculated in a glasshouse (15 to 30°C) under full light. Disease progression in plants maintained in shade houses (5 to 20°C) was slower and symptoms less severe. Presence of CMV in symptomatic plants was confirmed by enzyme-linked immunosorbent assay (ELISA), using antiserum from Agdia (Elkhart, IN), and by sap transmission tests to healthy wasabi and Chenopodium quinoa plants. Incidence of CMV among plants in the shade houses was estimated at 10%. A survey of a commercial planting of 400 to 500 plants in November 1997, using ELISA, showed an incidence of 2.6% CMV-infected plants. They were concentrated at the field margins, suggesting that the inoculum originated from external sources rather than from transplants. This pathogen could have significant impact on the longevity and production of this crop in Australia. Reference: (1) S. Adachi. 1987. Wasabi cultivation. Shizuoka Pref. Agric. Exp. Sta. Pub., Shizuoka, Japan.

The hop (Humulus lupulus L.) is a dioecious climbing plant, cultivated for its resins, which are produced in the cone, used primarily for the bittering of beer. In Australia, hops are grown in the states of Victoria and Tasmania. In late summer 2001, necrotic lesions were observed on the tips of bracts and bracteoles of developing cones at three hop farms in Tasmania. The necrotic area varied between 1 and 25% of the bracts and bracteoles, and in some cases progressed throughout the entire hop cone. Pieces of infected hop cones were surface sterilized for one minute in 2% sodium hypochlorite, plated on 2% water agar, and incubated at 22 ± 2°C. The most frequently isolated fungi (total number of isolations = 60) were transferred to 2% water agar and potato dextrose agar. In 90% of cases, the isolated fungus was Alternaria alternata (Fr.:Fr.) Keissl, as identified by M. Priest, NSW Agriculture, Australia. The pathogenicity of A. alternata was determined on detached, freshly picked cones of hop cultivar “Nugget.” Cones (n = 25) were inoculated with a conidial suspension of the fungus (1,000 spores per ml) and incubated at room temperature in a sealed container on plastic mesh over tissue wetted with sterile distilled water. Symptoms first appeared three days after inoculation as necrotic tips of bracts and bracteoles, and within 10 days the entire cone had become necrotic. Symptoms were more severe in vitro compared to in the field. This was probably due to the maintenance of detached cones under constant high relative humidity. Disease symptoms did not appear on cones inoculated with sterile distilled water. The pathogen was reisolated from inoculated cones, completing Koch's postulates. The pathogenicity of A. alternata to hop cones was reported in the United Kingdom in 1988. To our knowledge, this is the first report of A. alternata on hop cones in Australia. References: (1) P. Darby. Trans. Br. Mycol. Soc. 90:650–653, 1988.

The incidences of Hop latent virus (HpLV), Hop mosaic virus (HpMV), and American hop latent virus (AHLV), members of the genus Carlavirus, and Prunus necrotic ringspot virus and Apple mosaic virus, members of the genus Ilarvirus, were assessed for two hop cultivars, Horizon and Nugget, in Washington State. The spatial distribution of plants infected by the carlaviruses was assessed in two Horizon gardens in 2000 and one Nugget garden in 1993, 1994, and 1995. In the first Horizon garden (garden 1) and the Nugget garden, plants were separated by 2.1 m within and between rows. In these gardens, cultivation and the wide plant spacing discouraged contact between plants in either direction. In the second Horizon garden (garden 2), plants were separated by 4.3 m between rows and 1.0 m within rows. In all gardens, mechanical operations operated predominantly along rows; however, the closer plant spacing within rows in garden 2 permitted contact between adjacent plants within rows. In both Horizon gardens, the distribution of plants infected with HpMV was aggregated within rows. However, the distribution of plants infected with HpLV and AHLV was strongly influenced by contact between plants. In the Nugget garden, the distribution of plants infected by all three carlaviruses was autocorrelated within rows by 1995.

Hop (Humulus lupulus L.) is grown primarily for the alpha and beta acids produced in the strobile (cone) and used for bittering beer. In late summer (March) 2001, necrotic lesions covering the tips of cones of cvs. Agate, Nugget, and Willamette at hop farms in Tasmania, Australia, were observed. The necrotic lesions encompassed the proximal tips and affected between 5 and 60% of the cone; however, all bracts in the whorl were always affected. Diseased cones were observed in all seven gardens included in the survey. The incidence of plants with cone tip blight in ‘Nugget’ ranged from 5 to 30% in three gardens, in ‘Agate’ ranged from 3 to 10% in three gardens, and in the only ‘Willamette’ garden 30% of cones were affected. Pieces of infected hop cones (N = 55) were surface-treated for 1 min in 2% sodium hypochlorite, placed on 2% water agar, and incubated at 22 ± 2°C. Fusarium crookwellense Burgess, Nelson, & Toussoun was isolated from 95% of the cones (1). F. crookwellense was identified on carnation leaf agar by L. Burgess, University of Sydney, Australia. Koch's postulates were fulfilled by inoculating detached mature hop cones of cvs. Nugget and Willamette (N = 20 for each cultivar) with an atomized conidial suspension (3.5 × 105 spores of a single F. crookwellense isolate per milliliter) until runoff and incubated at 20 ± 2°C in a sealed container on plastic mesh over tissue wetted with sterile distilled water. Symptoms first appeared 5 days after inoculation and were identical to those found in the field. No disease symptoms were observed on cones subjected only to sterile distilled water. The pathogen was reisolated from diseased tissue on inoculated cones, completing Koch's postulates. Similar disease symptoms on hop cones have been described in Oregon and were associated with infection by F. sambucinum and F. avenaceum (C. Ocamb, personal communication). To our knowledge, this is the first report of the infection of hop cones by F. crookwellense. Reference: (1) L. W. Burgess et al. Laboratory Manual for Fusarium Research, 3rd ed. University of Sydney, Australia, 1994.

The efficacy of a range of fungicides were assessed under in vitro conditions in four field trials over three seasons for the management of ray blight of pyrethrum, caused by the fungus Phoma ligulicola in Tasmania. Fungicides belonging to the demethylation inhibitor group, including prochloraz, tebuconazole, difenoconazole, and cyproconazole, produced the most significant reduction in mycelial growth in vitro compared with unamended controls. Significant reductions in germination of conidia resulted from low concentrations of azoxystrobin. In the field, azox-ystrobin (as Amistar) at a rate of 300 g of product/ha significantly decreased disease intensity and increased pyrethrin yield (kg/ha) in all seasons. Applications of either azoxystrobin or difenoconazole (as Score) at a rate of 250 ml of product/ha in early spring were the most effective, whereas no significant benefit was demonstrated from applications in autumn. Over all seasons, azoxystrobin applications resulted in double the weight of flowers produced compared with nontreated areas, whereas increases in pyrethrin content within the flowers were season and location specific. Application of azoxystrobin and chlorothalonil (as Bravo 720) at a rate of 1.4 liters of product/ha resulted in a 45 to 48% increase in flowers, reduced defoliation, and increased stem diameter. The weight of individual flowers and flower maturity was not significantly affected by fungicide application. These data provide the basis for cost-effective management of this disease.

Pethybridge, S. J., Esker, P., Dixon, P., Hay, F., Groom, T., Wilson, C., and Nutter, F. W., Jr. 2007. Quantifying loss caused by ray blight disease in Tasmanian pyrethrum fields. Plant Dis. 91:1116-1121. The efficacy of newly implemented fungicide recommendations on reducing the intensity of ray blight disease caused by Phoma ligulicola to achieve site-specific attainable yield potentials in Tasmanian pyrethrum fields was quantified over two seasons in 46 and 51 fields during the 2003 and 2004 growing seasons, respectively. Disease intensity and yield in two plots (10 × 24 m), one following the commercial fungicide protocol recommendations and the second receiving no fungicide, were assessed in each pyrethrum field. The commercial fungicide protocol consisted of one application of azoxystrobin at 150 g a.i./ha, followed by two applications of a tank mixture of difenoconazole at 125 g a.i./ha and chlorothalonil at 1,008 liters a.i./ha at 14- to 21-day intervals. This program resulted in significant decreases in defoliation severity and the incidence of stems and flowers with ray blight, and increases in the height of stems and number of flowers produced per stem in October and November. In plots receiving the commercial fungicide protocol, the dry weight of flowers was increased by 76 and 68% in 2003 and 2004, respectively. Moreover, pyrethrin yield increased by 81 and 78% when the commercial fungicide protocol was used compared with the nontreated plots. Tobit regression was used to examine the relationships and thresholds among disease intensity measures (defoliation severity, stem severity, and incidence of flowers with ray blight) assessed just prior to harvest. This regression utilized a leftcensored regression model to define subminimal thresholds, as none of the disease intensity measures could be less than 0. Defoliation severity had a threshold of 35.3% before stem severity linearly increased and a threshold of 38.2% before the incidence of flowers with ray blight linearly increased. Finally, the threshold for stem severity was 13.7% before the incidence of flowers with ray blight linearly increased. These thresholds can be used to assist growers in making disease management decisions with the objective of minimizing loss of flowers by maintaining defoliation severity below the critical point at which the incidence of flowers with ray blight begins to linearly increase.

Foliar disease due to ray blight (Phoma ligulicola) in pyrethrum was quantified at three locations over 2 years in Tasmania, Australia. To obtain a range of ray blight disease intensities, replicated plots were treated with fungicides that varied in efficacy to control ray blight. Visual disease assessments and measurement of canopy reflectance were made at least once during spring (September through December). Visual assessments involved removal of flowering stems at ground level from which measurements of defoliation severity and the incidence of stems with ray blight were obtained. Reflectance of sunlight from pyrethrum canopies was measured at 485, 560, 660, 830, and 1,650 nm using a handheld multispectral radiometer. Measurements from these wavelengths also were used to calculate all possible reflectance ratios, as well as four vegetative indices. Relationships between wavelength bands, reflectance ratios, vegetative indices, and disease intensity measures were described by linear regression analyses. Several wavelength bands, ratios, and vegetative indices were significantly related in a linear fashion to visual measures of disease intensity. The most consistent relationships, with high R2 and low coefficients of variation values, varied with crop growth stage over time. The ratio 830/560 was identified as the best predictor of stem height, defoliation severity, and number of flowers produced on each stem in October. However, reflectance within the near-infrared range (830 nm) and the difference vegetative index was superior in November. The use of radiometric assessment of disease was noninvasive and provided savings in disease assessment time, which is critical where visual assessment is difficult and requires destructive sampling, as with pyrethrum.

Twelve cultivars of groundnut were screened in field trials for resistance to groundnut rosette disease (GRD), caused by coinfection with Groundnut rosette assistor virus (GRAV), Groundnut rosette virus (GRV), and its satellite RNA in the coastal savannah of Ghana. ‘Oboshie’ groundnut was rated as highly resistant; ‘Bremaowuo’, ‘Nkatefufuo’, and ‘Behenase’ as resistant; and ‘Nkosuor’, ‘Kumawu’, and ‘Otuhia’ as moderately resistant. GRAV infection rates of 11.8 to 61.8% (dry season) and 13.9 to 100% (wet season) were found, which included symptomless plants, suggesting that some lacked coinfection with GRV and its satellite. Chlorotic ringspot and line-pattern symptoms were observed, suggesting infection with Groundnut ringspot virus (GRSV). Virus identity was confirmed by enzyme-linked immunosorbent assay, reverse-transcription polymerase chain reaction, and amplicon sequencing. This is the first report of GRSV in Ghana. GRSV infection rates were 0.0 to 69.5% (dry season) and 26.1 to 69.5% (wet season). Mixed infections of GRAV and GRSV were common in all cultivars except Nkosuor and Bremaowuo in the dry season. Most cultivars graft inoculated with GRD showed significantly reduced height, leaf area, chlorophyll content, dry haulm weight, and seed yield compared with healthy plants. The sources of resistance to GRD and possibly GRAV and GRSV identified in this study could be exploited in groundnut breeding programs.

Brevipalpus oncidii Baker was detected for the first time in Australia naturally present as a pest on Brassia verrucosa Bateman ex Lindl. orchids. Although the majority of Brevipalpus species are believed to be predominantly thelytokous, males in the population described in this study were observed to be sexually functional. In repeated transmission studies, this B. oncidii population failed to successfully acquire an isolate of Orchid fleck virus (OFV) and transmit to cymbidium hosts. Mites fed on OFV-infected cymbidium leaves initially tested positive for OFV, but this was lost within 48 hours in absence of the OFV host suggesting failure to internalize the virus within the mite.http://www.zoobank.org/urn:lsid:zoobank.org:pub:1A8E42C2-9A9E-4066-94DC-E9E9683D8C68

Plant diseases are responsible for considerable losses in many crops. Most plant diseases cause a reduction in yield of the infected plants with diminished quantity and/or quality of harvested plant products, but the extent of damage and loss can vary greatly. To most vegetable growers, in crop or post-harvest disease invariably rates as the greatest impact on their profitability. Disease "steals" both quantity and quality from production, and necessitates additional cost and time inputs in disease mitigation. Estimation of these losses can be difficult, and in this paper I argue the full costs of disease are often underestimated. Measuring direct yield losses associated with plant disease is generally difficult to determine as disease symptoms may be obscure or difficult to distinguish from other diseases or abiotic factors; disease may be have a variable impact on plants depending upon its stage of development and crop maturity; disease may be aggregated within field requiring careful and intensive sampling strategies; there may be a lack of suitable populations of healthy and diseased populations to provide adequate comparisons; and there may be compensatory effects from healthy plants following removal of plants by disease. Loss estimates are invariably highly varied. The costs of disease are of course not restricted to the economic impact of reduced yields or diminished quality. Financial losses from increased production costs, additional costs associated with disease mitigation, and lost markets can be substantial. Furthermore, significant social and environmental costs can also result from plant disease. In this paper I examine holistically the range of "costs" associated with plant disease. Using select potato diseases as case studies I highlight some of the obvious and hidden costs of disease and suggest a more thorough examination of the impact of individual diseases is required in prioritising investment in disease management.