Showing papers in "Plant Pathology in 2011"

Climate change, plant diseases and food security: an overview

Abstract: Global food production must increase by 50% to meet the projected demand of the world’s population by 2050. Meeting this difficult challenge will be made even harder if climate change melts portions of the Himalayan glaciers to affect 25% of world cereal production in Asia by influencing water availability. Pest and disease management has played its role in doubling food production in the last 40 years, but pathogens still claim 10–16% of the global harvest. We consider the effect of climate change on the many complex biological interactions affecting pests and pathogen impacts and how they might be manipulated to mitigate these effects. Integrated solutions and international co-ordination in their implementation are considered essential. Providing a background on key constraints to food security, this overview uses fusarium head blight as a case study to illustrate key influences of climate change on production and quality of wheat, outlines key links between plant diseases, climate change and food security, and highlights key disease management issues to be addressed in improving food security in a changing climate.

Climate change and forest diseases

Abstract: As climate changes, the effects of forest diseases on forest ecosystems will change. We review knowledge of relationships between climate variables and several forest diseases, as well as current evidence of how climate, host and pathogen interactions are responding or might respond to climate change. Many forests can be managed to both adapt to climate change and minimize the undesirable effects of expected increases in tree mortality. We discuss four types of forest and disease management tactics – monitoring, forecasting, planning and mitigation – and provide case studies of yellow-cedar decline and sudden aspen decline to illustrate how forest diseases might be managed in the face of climate change. The uncertainties inherent to climate change effects can be diminished by conducting research, assessing risks, and linking results to forest policy, planning and decision making.

Dickeya species: an emerging problem for potato production in Europe

Abstract: Dickeya species (formerly Erwinia chrysanthemi) cause diseases on numerous crop and ornamental plants world-wide. Dickeya spp. (probably D. dianthicola) were first reported on potato in the Netherlands in the 1970s and have since been detected in many other European countries. However, since 2004–5 a new pathogen, with the proposed name ‘D. solani’, has been spreading across Europe via trade in seed tubers and is causing increasing economic losses. Although disease symptoms are often indistinguishable from those of the more established blackleg pathogen Pectobacterium spp., Dickeya spp. can initiate disease from lower inoculum levels, have a greater ability to spread through the plant’s vascular tissue, are considerably more aggressive, and have higher optimal temperatures for disease development (the latter potentially leading to increased disease problems as Europe’s climate warms). However, they also appear to be less hardy than Pectobacterium spp. in soil and other environments outside the plant. Scotland is currently the only country in Europe to enforce zero tolerance for Dickeya spp. in its potato crop in an attempt to keep its seed tuber industry free from disease. However, there are a number of other ways to control the disease, including seed tuber certification, on-farm methods and the use of diagnostics. For diagnostics, new genomics-based approaches are now being employed to develop D. dianthicola- and ‘D. solani’-specific PCR-based tests for rapid detection and identification. It is hoped that these diagnostics, together with other aspects of ongoing research, will provide invaluable tools and information for controlling this serious threat to potato production

Control of blackleg and tuber soft rot of potato caused by Pectobacterium and Dickeya species: a review.

Abstract: This paper briefly reviews research on the causative agents of blackleg and soft rot diseases of potato, namely Pectobacterium and Dickeya species, and the disease syndrome, including epidemiological and aetiological aspects. It critically evaluates control methods used in practice based on the avoidance of the contamination of plants, in particular the use of seed testing programmes and the application of hygienic procedures during crop production. It considers the perspective of breeding and genetic modification to introduce resistance. It also evaluates the application of physical and chemical tuber treatments to reduce inoculum load and examines the possibility of biocontrol using antagonistic bacteria and bacteriophages.

Possible climate-change effects on mycotoxin contamination of food crops pre- and postharvest

Abstract: This paper examines the available information on the potential for climate-change impacts on mycotoxigenic fungi and mycotoxin contamination of food crops pre- and postharvest. It considers the effect of changes in temperature/water availability on mycotoxin contamination, especially incidences where aflatoxin B1 and ochratoxin A production has been influenced. The potential of using preharvest models to predict risk from deoxynivalenol (DON) in wheat, fumonisin B1 in maize and aflatoxins in maize and peanuts in different continents are considered in the context of potential for adaptation to include climate-change scenarios. Available information suggests that slightly elevated CO2 concentrations and interactions with temperature and water availability may stimulate growth of some mycotoxigenic species, especially under water stress. The accumulated knowledge on interacting conditions of water/temperature effects on optimum and boundary conditions for growth and mycotoxin production has been used to predict the effects that +3 and +5°C increases under water stress would have on growth/mycotoxin production by mycotoxigenic species. Various spatial scales, from toxin gene expression to regional approaches using geostatistics, are examined for their use in understanding the impact that climate change may have on food contamination in developing and developed countries. The potential for using an integrated systems approach to link gene expression data, phenotypic toxin production under different interacting abiotic conditions is discussed using Fusarium species and DON as examples. Such approaches may be beneficial for more accurate predictions of risk from mycotoxins on a regional basis and also the potential for new emerging toxin threats.

Climate change and diseases of food crops

Abstract: Despite complex regional patterns of projected climate change, significant decreases in food crop yields have been predicted using the ‘worst case’ CO2 emission scenario (A1FI) of the Intergovernmental Panel on Climate Change. Overall, climate change is predicted to have a progressively negative effect on the yield of food crops, particularly in the absence of efforts to mitigate global CO2 emissions. As with all species, plant pathogens will have varying responses to climate change. Whilst the life cycle of some pathogens will be limited by increasing temperatures, e.g. Puccinia striiformis f.sp. tritici, other climatic factors such as increasing atmospheric CO2, may provide more favourable conditions for pathogens such as Fusarium pseudograminearum. Based on published literature and unpublished work in progress, we have reviewed the qualitative effects of climate change on pathogens that cause disease of four major food crops: wheat, rice, soybean and potato. The limited data show that the influence will be positive, negative or neutral, depending on the host–pathogen interaction. Quantitative analysis of climate change on pathogens of these crops is largely lacking, either from field or laboratory studies or from modelling-based assessments. Systematic quantitative analysis of these effects will be necessary in developing future disease management plans, such as plant breeding, altered planting schedules, chemical and biological control methods and increased monitoring for new disease threats.

Influence of atmospheric and climatic change on plant–pathogen interactions

Abstract: Atmospheric change studies conducted in free air concentration enrichment (FACE) systems and open-topped chambers have increased understanding of how factors, such as rising CO2 and O3 levels, impact the development of plant disease epidemics Using these systems, plant scientists have been able to study host/pathogen systems under real-world conditions where variations in multiple environmental parameters impact disease outcomes Results from these studies are useful for evaluating earlier predictions on plant responses to climate-change parameters and the resulting impacts on plant disease epidemics Some of these predictions have been verified, whilst others have yet to be tested Significant interactions among climate-change parameters are highlighting the importance of conducting studies under real-world conditions The development of molecular and gene expression tools is allowing the fine scale mechanisms responsible for the observed reactions to be determined, and should increase the ability to predict plant disease outcomes under future climatic conditions

Soil organisms and global climate change

Abstract: Climate changes will influence soil organisms both directly (warming) and indirectly (warming and elevated CO2) via changes in quantity and quality of plant-mediated soil C inputs. Elevated atmospheric CO2 commonly stimulates flow of organic C into the soil system, increases root production and exudation, but decreases litter quality. There is little evidence that atmospheric CO2 enrichment will increase total soil organic matter content because greater C flow into soil stimulates the soil food web, often leading to equivalent increases in soil CO2 efflux. Effects of warming on C allocation belowground, on the other hand, will depend largely on the temperature optima of different plant species. Warming is likely to increase the rate of soil organic matter decomposition by stimulating soil heterotrophic respiration, although some degree of acclimatization to warming is likely. Mycorrhizal and N2-fixing relationships are generally enhanced by CO2 enrichment, but effects of warming are highly variable. Data suggest that energy flow through fungal pathways may be enhanced relative to bacterial pathways by both warming and atmospheric CO2 enrichment. Whether the shift toward fungal domination of soils will increase soilborne fungal disease occurrence in the future is still an open question. Plant heat and drought tolerance, along with resistance to pathogens in warmer and wetter soils, may be achieved, to some unknown extent, by exploitation and management of beneficial soil organisms. Further study is needed to develop a more holistic understanding of the effects of climate change on belowground processes.

Potential strategies and future requirements for plant disease management under a changing climate

Abstract: Climate change will probably influence the occurrence, prevalence and severity of plant diseases. This will also affect disease management with regard to timing, preference and efficacy of chemical, physical and biological measures of control and their utilization within integrated pest management (IPM) strategies. Prediction of future requirements in disease management is of great interest for agroindustries, extension services and practical farmers. A comprehensive analysis of potential climate-change effects on disease control is difficult because current knowledge is limited and fragmented. This review reveals that certain existing preventive plant protection measures, such as use of a diversity of crop species in cropping systems, adjustment of sowing or planting dates, use of crop cultivars with superior resistance and/or tolerance to diseases and abiotic stress, use of reliable tools to forecast disease epidemics, application of IPM strategies, and effective quarantine systems, may become particularly important in the future. Effective crop protection technologies are available and will provide appropriate tools to adapt to altered climatic conditions, although the complexity of future risks for plant disease management may be considerable, particularly if new crops are introduced in an area. Overall, the challenge of adapting disease control measures to climate change is not likely to be fundamentally different from the adjustments to technological innovations or changes in the economic framework already required in current crop protection. Potential beneficial effects of climate change, such as longer growing seasons, fewer frosts and shifted precipitation patterns, must not be neglected, as they could counteract the presumed enhancement of particular diseases.

Diseases in tropical and plantation crops as affected by climate changes: current knowledge and perspectives

Abstract: Tropical and plantation crops include important crops for food security and alternative energy resources. Even so, there are few studies on the impact of climate change on diseases of these crops. Findings from previous studies concerning some climate-change effects on diseases of coffee, sugarcane, eucalyptus, cassava, citrus, banana, pineapple, cashew, coconut and papaya have been summarized to provide a context. By reviewing available methods to evaluate the impact of climate change on diseases of tropical and plantation crops, we present trends for some diseases and their management strategies, identify critical gaps in knowledge, and suggest experimental and analytical approaches to advance knowledge. As the projected climate conditions will probably vary greatly in the future from continent to continent and from developed to developing countries, studies must be conducted under tropical regions considering their specific environmental conditions. Multifactor studies under realistic field situations, such as free air CO2 enrichment with increasing CO2 and O3 concentrations incorporating spectral reflectance measures in situ for realistic assessment of plant growth, are a way forward. Effects of a changing climate on chemical and biological controls are discussed in the context of changing global outlook on environmental demands for the future.

The dose rate debate: Does the risk of fungicide resistance increase or decrease with dose?

Abstract: This paper reviews the evidence relating to the question: does the risk of fungicide resistance increase or decrease with dose? The development of fungicide resistance progresses through three key phases. During the 'emergence phase' the resistant strain has to arise through mutation and invasion. During the subsequent 'selection phase', the resistant strain is present in the pathogen population and the fraction of the pathogen population carrying the resistance increases due to the selection pressure caused by the fungicide. During the final phase of 'adjustment', the dose or choice of fungicide may need to be changed to maintain effective control over a pathogen population where resistance has developed to intermediate levels. Emergence phase: no experimental publications and only one model study report on the emergence phase, and we conclude that work in this area is needed. Selection phase: all the published experimental work, and virtually all model studies, relate to the selection phase. Seven peer reviewed and four non-peer reviewed publications report experimental evidence. All show increased selection for fungicide resistance with increased fungicide dose, except for one peer reviewed publication that does not detect any selection irrespective of dose and one conference proceedings publication which claims evidence for increased selection at a lower dose. In the mathematical models published, no evidence has been found that a lower dose could lead to a higher risk of fungicide resistance selection. We discuss areas of the dose rate debate that need further study. These include further work on pathogen-fungicide combinations where the pathogen develops partial resistance to the fungicide and work on the emergence phase.

Geographic distribution of plant pathogens in response to climate change

Abstract: Geographic distributions of pathogens are the outcome of dynamic processes involving host availability, susceptibility and abundance, suitability of climate conditions, and historical contingency including evolutionary change. Distributions have changed fast and are changing fast in response to many factors, including climatic change. The response time of arable agriculture is intrinsically fast, but perennial crops and especially forests are unlikely to adapt easily. Predictions of many of the variables needed to predict changes in pathogen range are still rather uncertain, and their effects will be profoundly modified by changes elsewhere in the agricultural system, including both economic changes affecting growing systems and hosts and evolutionary changes in pathogens and hosts. Tools to predict changes based on environmental correlations depend on good primary data, which is often absent, and need to be checked against the historical record, which remains very poor for almost all pathogens. We argue that at present the uncertainty in predictions of change is so great that the important adaptive response is to monitor changes and to retain the capacity to innovate, both by access to economic capital with reasonably long-term rates of return and by retaining wide scientific expertise, including currently less fashionable specialisms.

Early stages of septoria tritici blotch epidemics of winter wheat: build-up, overseasoning, and release of primary inoculum

Abstract: Septoria tritici blotch (STB), caused by Mycosphaerella graminicola, is the most prevalent disease of wheat worldwide Primary inoculum and the early stages of STB epidemics are still not fully understood and deserve attention for improving management strategies The inoculum build-up and overseasoning involves various fungal structures (ascospores, pycnidiospores, mycelium) and plant material (wheat seeds, stubble and debris; wheat volunteers; other grasses) Their respective importance is assessed in this review Among the mechanisms involved in the early stages of epidemics and in the year-to-year disease transmission, infection by ascospores wind-dispersed from either distant or local infected wheat debris is the most significant Nevertheless, infection by pycnidiospores splash-dispersed either from neighbouring wheat debris or from senescent basal leaves has also been inferred from indirect evidence Mycosphaerella graminicola has rarely been isolated from seeds so that infected seed, although suspected as a source of primary inoculum for a long time, is considered as an epidemiologically anecdotal source Mycosphaerella graminicola can infect a few grasses other than wheat but the function of these grasses as alternative hosts in natural conditions remains unclear Additionally, wheat volunteers are suspected to be sources of STB inoculum for new crops This body of evidence is summarized in a spatio-temporal representation of a STB epidemic aimed at highlighting the nature, sources and release of inoculum in the early stages of the epidemic

Phosphite primed defence responses and enhanced expression of defence genes in Arabidopsis thaliana infected with Phytophthora cinnamomi

Abstract: This paper describes the effect of phosphite (Phi), a systemic chemical, on the induction of defence responses in Phytophthora cinnamomi-infected Arabidopsis thaliana accessions Ler and Col-0. Application of Phi to non-inoculated A. thaliana seedlings of accession Ler elevated transcription of defence genes in the salicylic acid (PR1 and PR5) and jasmonic acid/ethylene (THI2.1 and PDF1.2) pathways. Furthermore, a systemic increase in the expression of the PR1 gene was demonstrated in Phi-treated seedlings using the transgenic line PR1::GUS in the presence/absence of the pathogen by 72h after inoculation. The cells of Phi-treated A. thaliana Ler leaves responded to P. cinnamomi zoospore inoculation with a rapid increase in callose deposition and hydrogen peroxide (H2O2) production. Phi treatment resulted in the production of callose papillae 6h earlier than in non-Phi-treated inoculated seedlings and enhanced the production of H2O2 in the leaves of A. thaliana at the site of hyphal penetration and in cells away from the inoculation point. By 24h after infection, clear differences in the amount of H2O2 production were observed between the Phi-treated and non-Phi-treated plants. These rapid host responses did not occur in non-Phi-treated inoculated seedlings. There was also a significant (P<0·001) decrease in lesion size in Phi-treated plants. These results indicate that Phi primes the plant for a rapid and intense response to infection involving heightened activation of a range of defence responses.

Complexity in climate-change impacts: an analytical framework for effects mediated by plant disease

Abstract: The impacts of climate change on ecosystem services are complex in the sense that effective prediction requires consideration of a wide range of factors. Useful analysis of climate-change impacts on crops and native plant systems will often require consideration of the wide array of other biota that interact with plants, including plant diseases, animal herbivores, and weeds. We present a framework for analysis of complexity in climate-change effects mediated by plant disease. This framework can support evaluation of the level of model complexity likely to be required for analysing climate-change impacts mediated by disease. Our analysis incorporates consideration of the following set of questions for a particular host, pathogen, host–pathogen combination, or geographic region. 1. Are multiple biological interactions important? 2. Are there environmental thresholds for population responses? 3. Are there indirect effects of global change factors on disease development? 4. Are spatial components of epidemic processes affected by climate? 5. Are there feedback loops for management? 6. Are networks for intervention technologies slower than epidemic networks? 7. Are there effects of plant disease on multiple ecosystem services? 8. Are there feedback loops from plant disease to climate change? Evaluation of these questions will help in gauging system complexity, as illustrated for fusarium head blight and potato late blight. In practice, it may be necessary to expand models to include more components, identify those components that are the most important, and synthesize such models to include the optimal level of complexity for planning and research prioritization.

Susceptibility of grapevine pruning wounds to infection by Lasiodiplodia theobromae and Neofusicoccum parvum

Abstract: The susceptibility of 1- and 2-year-old grapevine wood to botryosphaeria canker caused by Lasiodiplodia theobromae and Neofusicoccum parvum was evaluated in California in two seasons. In the 2007 dormant season, pruning-wound susceptibility was highest when wounds were inoculated immediately after pruning in December (80% of pruning wounds were infected in Chardonnay for both fungal species and 75% and 98% in Cabernet Sauvignon for N. parvum and L. theobromae, respectively). In the 2008 dormant season, pruning-wound susceptibility was highest in November in Chardonnay (86% and 93% for N. parvum and L. theobromae, respectively) and in December in Cabernet Sauvignon (71% and 75% for N. parvum and L. theobromae, respectively). The lowest infection rates (13–35%) were observed when vines were pruned and inoculated in March in both dormant seasons and for both cultivars. Susceptibility of pruning wounds did not differ significantly (P = 0·7612) between 1- and 2-year-old wood and consequently, pruning-wound protection treatments should be applied to all wounds. In conclusion, grapevine pruning wounds were susceptible to infection by L. theobromae and N. parvum to varying extents throughout the dormant season in California (November–March), but, overall, susceptibility of pruning wounds was highest when inoculations were done immediately after pruning and decreased significantly as the interval between pruning and inoculation increased. Results of this study suggest that pruning grapevines in late winter (March) in California would significantly reduce the risk of infection by L. theobromae and N. parvum.

Rapid phylogenetic identification of members of the Pseudomonas syringae species complex using the rpoD locus

Abstract: Phylogenies based on four loci confirmed the relatedness of all nine validly published species type strains within the Pseudomonas syringae species complex. To further establish the phylogenetic structure within the complex, all 67 pathovar type strains (with defined host ranges) were sequenced using a 578-nucleotide rpoD locus. Since this locus encompassed that used in a previous seven-locus study, it was possible to relate these strains to the existing phylogroup, genomospecies and binomial classifications. All species type strains were distinguished by relatively long branch lengths with all four loci, except for P. savastanoi, P. ficuserectae, P. meliae, P. amygdali and P. tremae, which were attributed to phylogroup 3. The grouping of P. tremae with these genomospecies-2 species was surprising since this species was previously designated as the sole representative of genomospecies 5. The oat pathogen P. syringae pv. coronafaciens was also distinguished by relatively long branch lengths with all four loci. The rpoD phylogeny grouped all the pathovar type strains into major clades that corresponded to previously defined phylogroups, except for two genomospecies-7 strains and P. caricapapayae, which were identified as a new phylogroup (6). There was good correlation between phylogroup and genomospecies classifications, except that two genomospecies-8 strains (P. avellanae and P. syringae pv. theae) were found as a distinct clade within phylogroup 1 along with P. syringae pvs morsprunorum and actinidiae. The rpoD locus will provide a common reference framework to improve monitoring and surveillance of these important pathogens.

Potential biological control of clubroot on canola and crucifer vegetable crops

Abstract: Clubroot caused by Plasmodiophora brassicae is an emerging threat to canola (Brassica napus) production in western Canada, and a serious disease on crucifer vegetable crops in eastern Canada. In this study, seven biological control agents and two fungicides were evaluated as soil drenches or seed treatments for control of clubroot. Under growth cabinet conditions, a soil-drench application of formulated biocontrol agents Bacillus subtilis and Gliocladium catenulatum reduced clubroot severity by more than 80% relative to pathogen-inoculated controls on a highly susceptible canola cultivar. This efficacy was similar to that of the fungicides fluazinam and cyazofamid. Under high disease pressure in greenhouse conditions, the biocontrol agents were less effective than the fungicides. Additionally, all of the treatments delivered as a seed coating were less effective than the soil drench. In field trials conducted in 2009, different treatments consisting of a commercial formulation of B. subtilis, G. catenulatum, fluazinam or cyazofamid were applied as an in-furrow drench at 500 L ha−1 water volume to one susceptible and one resistant cultivar at two sites seeded to canola in Alberta and one site of Chinese cabbage in Ontario. There was no substantial impact on the susceptible canola cultivar, but all of the treatments reduced clubroot on the susceptible cultivar of Chinese cabbage, lowering disease severity by 54–84%. There was a period of 4 weeks without rain after the canola was seeded, which likely contributed to the low treatment efficacy on canola. Under growth cabinet conditions, fluazinam and B. subtilis products became substantially less effective after 2 weeks in a dry soil, but cyazofamid retained its efficacy for at least 4 weeks.

Infection processes and soft wheat response to root rot and crown rot caused by Fusarium culmorum

Abstract: An isolate of the fungus Fusarium culmorum constitutively expressing green fluorescent protein was used to investigate the infection process and host response of primary seedling roots and stem base leaf sheaths of soft wheat cv. Genio. Disease progress was assessed macroscopically by visual symptoms, microscopically by confocal laser scanning microscopy (CLSM) and via gene expression analysis of fungal and wheat genes by real-time quantitative RT-PCR. In the roots, CLSM investigations revealed an initial intercellular and subsequent intracellular colonization by fungal hyphae. The fungus invaded the rhizodermal layer and cortex but was not seen to colonize the stele. The fungus consistently expressed TRI5 (24, 48 and 96 h post-inoculation), indicating that trichothecenes were being synthesized throughout this phase of infection and colonization. The expression of the six host defence-associated genes (Wheatwin 1-2, PR1, Chitinase, PAL, WIR1 and LOX )i ncreased early in infection and decreased during later stages. In the stem base, CLSM observations revealed the fungus sequentially penetrating though the first, second and third basal leaf sheaths. Expression of TRI5 was initiated early in the infection of each leaf sheath. The expression of the host defence-associated genes varied over time and across leaf sheaths, and all were also expressed in leaf sheaths which had not yet been in contact with the fungus. Expression of LOX and WIR1 were particularly enhanced in the third leaf sheath.

Pathogen dynamics in a crop canopy and their evolution under changing climate

Abstract: Canopy-level interactions have been largely ignored in epidemiological models and their applications in defining disease risks under climate change, although these interactions are important for disease management. This paper uses anthracnose of Stylosanthes scabra as a case study and reviews research on dynamics of the pathogen (Colletotrichum gloeosporioides) at the canopy level and pathogen evolution under changing climate. It argues that linking of pathogen dynamics, crop growth and climate models is essential in predicting disease risks under climate change. A plant functional-structural model was used to couple S. scabra growth and architecture with disease under ambient and elevated CO(2). A level of induced resistance in plants with enlarged canopy determined anthracnose severity at elevated CO(2). Moisture-related microclimatic variables determined infection at ambient but not at elevated CO(2). At high CO(2) increased disease level from raised pathogen fecundity in enlarged canopy accelerated pathogen evolution after 25 sequential infection cycles. Modelling of pathogen dynamics under climate change currently suffers from a paucity of quantitative data, mismatch of scales in coupling climate and disease models, and model uncertainties. Further experimental research on interactions of biotic and abiotic factors on plant diseases under climate change and validation of models are essential prior to their use in climate-change prediction. Understanding and anticipating trends in host-pathogen evolution under climate change will improve the durability of resistance and lay the foundation for increased crop adaptation through pre-emptive plant breeding.

Ditylenchus gigas n. sp. parasitizing broad bean: a new stem nematode singled out from the Ditylenchus dipsaci species complex using a polyphasic approach with molecular phylogeny

Abstract: Morphologial, biochemical, molecular and karyological analyses of different populations and races of the stem and bulb nematode Ditylenchus dipsaci have suggested that it represents a species complex, of which only D. dipsaci sensu stricto and its morphologically larger variant, known as the giant race of the stem and bulb nematode, are plant parasites of economic importance. The present study singles out the giant race from this complex, herein described as a new species named Ditylenchus gigas n. sp., on the basis of morphological and molecular data obtained from several populations collected from broad beans in southern Italy, southern Spain and Lebanon. The new species epithet, which refers to the large body size of the nematode with respect to the normal races, must be considered to be conspecific with the D. dipsaci ‘giant race’ from Fabaceae in recent literature. Morphologically, the new species is characterized by a body size 1AE5‐2 times longer than the ‘normal race’, stylet delicate (11AE5‐13AE0 lm long) with knobs distinctly sloping backwards, and long post-vulval uterine sac (81‐150 lm long). Results of molecular analysis of rDNA sequences including the ITS1-5.8S-ITS2 region, the D2‐D3 fragment of the 28S gene, the small 18S subunit, the partial mitochondrial gene for cytochrome c oxidase I (mtCOI), and hsp90 gene sequences, support the new taxonomic species status for the former D. dipsaci giant race from Vicia faba, and clearly distinguish D. gigas n. sp. from D. dipsaci sensu stricto.

Wax matters: absence of very-long-chain aldehydes from the leaf cuticular wax of the glossy11 mutant of maize compromises the prepenetration processes of Blumeria graminis

Abstract: Conidial germination and differentiation, the so-called prepenetration processes, of the barley powdery mildew fungus (Blumeria graminis f.sp. hordei) are triggered in vitro by very-long-chain aldehydes, minor constituents of barley leaf wax. However, until now it has not been demonstrated that these cuticle-derived molecules also play a significant role in the initiation and promotion of the fungal prepenetration processes in vivo, on the surface of a living plant leaf. In the maize (Zea mays) wax mutant glossy11, which is completely devoid of cuticular very-long-chain aldehydes, germination and appressorial differentiation of B. graminis were strongly impeded. Spraying the mutant leaf surface with aldehyde-containing wild-type wax or pure n-hexacosanal (C26-aldehyde) fully restored fungal prepenetration, whereas maize wild-type leaf surfaces coated with n-docosanoic acid exhibited reduced conidial germination rates of 23%, and only 5% of the conidia differentiated infection structures. In vitro studies were performed to further corroborate the extensive prevention of fungal germination and differentiation in response to artificial surfaces coated with aldehyde-deficient maize wax. Because of its phenotype affecting the B. graminis prepenetration processes, the glossy11 mutation of maize may become a valuable molecular target and genetic tool that could provide a means of developing basal powdery mildew resistance in the globally important crops wheat and barley.

Colletotrichum acutatum interactions with unripe and ripe strawberry fruits and differential responses at histological and transcriptional levels

Abstract: Microscopic investigations were conducted into the interaction of Colletotrichum acutatum on white and red strawberry (Fragaria ×ananassa) fruit surfaces. The results showed that, whilst the early interaction events were similar in both white and red fruits, after 24 h fungal colonization dramatically varied: in white fruits C. acutatum became quiescent as melanized appressoria, but on red fruits it displayed subcuticular necrotrophic invasion. A microarray analysis of white and red strawberries after 24 h of interaction with C. acutatum was performed, in order to reveal differences in gene expression possibly related to the different susceptibility of unripe and ripe fruits. Epi/catechin-related genes and fatty acid metabolism genes, involved in the production of quiescence-related molecules such as flavan-3-ols, proanthocyanidins and antifungal dienes, were found to be regulated during strawberry ripening, supporting a role for these molecules as preformed defence mechanisms. Besides several genes commonly regulated upon pathogen interaction, different genes were specifically transcribed only in white or red challenged fruits; a number of these, such as those coding for lectin and polyphenol oxidase, possibly account for specific pathogen-induced responses. The putative biological role of these genes in the different susceptibility of fruits to C. acutatum is discussed.

Use of a real‐time LAMP isothermal assay for detecting 16SrII and XII phytoplasmas in fruit and weeds of the Ethiopian Rift Valley

Abstract: A study to investigate the association of phytoplasmas with papaya dieback and citrus decline syndromes in Ethiopia was carried out between July 2009 and February 2010, with sampling performed in major papaya- and citrus-growing areas of the Rift Valley. Samples of plants with symptoms were collected from papaya, citrus and suspected phytoplasma weed hosts and crops in and around the papaya and citrus fields studied. Nested polymerase chain reaction (nested-PCR) was used for initial characterization, using primers that amplify regions of the 16S rRNA and secA genes, and results were then confirmed with rapid real-time group-specific LAMP (loop-mediated isothermal amplification) assays. The results identified the occurrence of a phytoplasma belonging to the stolbur (16SrXII-A) group in papaya plants showing dieback symptoms, whilst no phytoplasmas were found associated with citrus decline. These results contradict previous reports that a 16SrII phytoplasma was associated with both papaya dieback and citrus decline in Ethiopia, but correspond with the association of a 16SrXII phytoplasmas with Nivum-Haamir-Dieback of papaya in Israel and papaya dieback in Australia. No 16SrXII phytoplasmas were found in any of the weeds and potential alternative hosts studied, although 16SrII phytoplasmas were consistently found in Parthenium hysterophorus weed plants. These results indicate that a 16SrXII phytoplasma is associated with papaya dieback in Ethiopia, whilst the causal agent of citrus decline is not a phytoplasma and remains unidentified.

Effects of plant height on type I and type II resistance to fusarium head blight in wheat

Abstract: By carefully separating type I and type II resistances, the possible effects of plant height on fusarium head blight (FHB) resistance in wheat were assessed using near-isogenic lines (NILs) for several different reduced-height (Rht) genes. Tall isolines all gave better type I resistance than their respective dwarf counterparts when assessed at their natural heights. These differences largely disappeared when the dwarf isolines were physically raised so that their spikes were positioned at the same height as those of their respective tall counterparts. The effects of plant height on type II resistance was less clear. For those NIL pairs which showed significant differences, it was the dwarf isolines which gave better resistance. As the Rht genes involved in these NILs locate at different genomic regions, the differences in FHB between the dwarf and tall isolines are unlikely to be the result of linkages between each of the different Rht loci with a beneficial or a deleterious gene affecting type I or type II resistance. Rather, the different FHB resistances are probably caused by direct or indirect effects of height difference per se, and microclimate may have contributed to the better type I resistance of the tall plants. Thus, caution should be exercised when attempting to exploit any of the FHB resistant loci co-located with Rht genes.

Characterization of Botryosphaeriaceae from plantation-grown Eucalyptus species in South China

Abstract: Bilateral agreement between South Africa and China funded through projects 30771732, 2007DFA31190 and 2006BAD08A11; members of the Tree Protection Co-operative Programme (TPCP)

Influence of cultivar resistance and inoculum density on root hair infection of canola (Brassica napus) by Plasmodiophora brassicae

Abstract: The impact of cultivar resistance and inoculum density on the incidence of primary infection of canola root hairs by Plasmodiophora brassicae, the causal agent of clubroot, was assessed by microscopy. The incidence of root hair infection in both a resistant and a susceptible cultivar increased with increasing inoculum density, but was two- to threefold higher in the susceptible cultivar; the relationship between root hair infection and inoculum density was also substantially stronger and more consistent in the susceptible cultivar. In the susceptible cultivar, the root hair infection rate peaked between 6 and 8 days after sowing and then declined. In the resistant cultivar, it increased over the 14-day duration of each study. It appears that examination of root hair infection by microscopy in a bait crop of susceptible canola could serve as a useful tool for estimating P. brassicae inoculum levels in soil. In a separate trial, the relationship between inoculum density and clubroot severity, plant growth parameters, and seed yield was assessed under greenhouse conditions. Inoculum density in the susceptible genotype was strongly and positively correlated with clubroot severity and negatively correlated with plant height and seed yield. In addition, a single cropping cycle of the susceptible cultivar contributed significantly higher levels of resting spores to the soil in a greenhouse test than did a cycle of the resistant cultivar, as assessed by quantitative PCR and microscope analysis.

Epidemiology, histopathology and aetiology of olive anthracnose caused by Colletotrichum acutatum and C. gloeosporioides in Portugal

Abstract: Anthracnose is an important disease affecting mature olive fruits, causing significant yield losses, and poor fruit and oil quality. In Portugal, high anthracnose incidence was recorded during 2003-2007 with 41% of 908 orchards surveyed displaying disease symptoms. In another 14% of the orchards, the pathogen was recorded in symptomless plants. Disease severity was on average 36%, frequently reaching 100%. In Portugal, anthracnose is endemic to neglected orchards of susceptible cultivars, but under favourable conditions it can also severely affect less susceptible cultivars. Pathogens were genetically heterogeneous, with Colletotrichum acutatum genetic group A2 as the most frequent (80%), followed by group A4 (12%) and group A5 along with C. gloeosporioides (3-4%), while groups A3 and A6 of C. acutatum were sporadic. Important geographic variations were observed in the frequencies of these populations, accompanied by year-to-year populational shifts. Epidemiology and histopathology studies showed the presence of the pathogens on vegetative organs year-round, particularly on olive leaves and branches, and on weeds. These represent inoculum reservoirs where secondary conidiation occurs, and conidia are then dispersed by spring rains reaching flowers and young fruits or by autumn rains reaching pre-mature fruits. Unripe fruits were colonized without showing symptoms up to penetration of the cuticle, but further colonization and symptom production was completed only as fruits matured. These findings challenge current control practices, particularly the timing of fungicide treatment, and contribute to improved disease management.

Direct evidence of surface infestation of seeds and tubers by Plasmodiophora brassicae and quantification of spore loads

Abstract: Using quantitative PCR, DNA of Plasmodiophora brassicae, the causal agent of clubroot, was detected and quantified on canola, pea and wheat seeds, as well as on potato tubers, all harvested from clubroot-infested fields in Alberta, Canada. Quantifiable levels of infestation were found on seven of the 46 samples analysed, and ranged from <1·0 × 103 to 3·4 × 104 resting spores per 10 g seeds; the vast majority (80–100%) of resting spores on these samples were viable, as determined by Evan’s blue vital staining. However, the levels of infestation found were generally lower than that required to cause consistent clubroot symptoms in greenhouse plant bioassays. While the occurrence of P. brassicae resting spores on seeds and tubers harvested from clubroot-infested fields suggests that seedborne dissemination of this pathogen is possible, practices such as commercial seed cleaning may be sufficient to effectively mitigate this risk.

Trichothecene genotypes of Gibberella zeae from winter wheat fields in the eastern USA

Abstract: Fusarium head blight (FHB), caused principally by Gibberella zeae (Fusarium graminearum), is a devastating disease of small grains such as wheat and barley worldwide. Grain infected with G. zeae may be contaminated with trichothecene mycotoxins such as deoxynivalenol (DON) and nivalenol (NIV). Strains of G. zeae that produce DON may also produce acetylated derivatives of DON: 3-acetyl-DON (3-ADON) and 15-acetyl-DON (15-ADON). Gradients (clines) of 3-ADON genotypes in Canada have raised questions about the distribution of G. zeae trichothecene genotypes in wheat fields in the eastern USA. Tri3 and Tri12 genotypes were evaluated in 998 isolates of G. zeae collected from 39 winter wheat fields in New York (NY), Pennsylvania (PA), Maryland (MD), Virginia (VA), Kentucky (KY) and North Carolina (NC). Ninety-two percent (919/998) of the isolates were 15-ADON, 7% (69/998) were 3-ADON, and 1% (10/998) was NIV. A phylogenetic analysis based on portions of three genes (PHO, RED and URA) from 23 isolates revealed two species of Fusarium (F. graminearum sensu stricto and one isolate of F. cerealis (synonym F. crookwellense)). An increasing trend of 3-ADON genotypes was observed from NC (south) to NY (north). Punctuated episodes of atmospheric transport may favour a higher frequency of 3-ADON genotypes in the northeastern USA, near Canada, compared with the mid-Atlantic states. Discoveries of the NIV genotype in NY and NC indicate the need for more intensive sampling in the surrounding regions.