Epidemiology Of Plant Pathogens Research Articles

An Agent Based Model Shows How Mixed Infections Drive Multi-Year Pathotype Dynamics in a Plant-Virus System

Mathematical models are widely used to understand the evolution and epidemiology of plant pathogens under a variety of scenarios. We use here this approach to analyze the effects of different traits intrinsic and extrinsic to plant-virus interactions on the dynamics of virus pathotypes in genetically heterogeneous plant-virus systems. For this, we propose an agent-based epidemiological model that includes epidemiologically significant pathogen life-history traits related to virulence, transmission, and survival in the environment, and allows to integrate long and short scale transmission, primary and secondary infections, and within-host pathogen competition in mixed infections. The study focusses on the tobamovirus-pepper pathosystem. Model simulations allowed to integrate pleiotropic effects of resistance-breaking mutations on different virus life-history traits into net costs of resistance-breaking, allowing predictions on multi-year pathotype dynamics. We also explored the effects of two control measures, the use of host resistance and roguin of symptomatic plants, that modify epidemiological attributes of the pathogens, to understand how their populations will respond to evolutionary pressures. One major conclusion points to the importance of pathogen competition within mixed-infected hosts as a component of the overall fitness of each pathogen that, thus, drives their multiyear dynamics.

No Evidence for Seed Transmission of Tomato Yellow Leaf Curl Sardinia Virus in Tomato

Seed transmission is an important factor in the epidemiology of plant pathogens. Geminiviruses are serious pests spread in tropical and subtropical regions. They are transmitted by hemipteran insects, but a few cases of transmission through seeds were recently reported. Here, we investigated the tomato seed transmissibility of the begomovirus tomato yellow leaf curl Sardinia virus (TYLCSV), one of the agents inducing the tomato yellow leaf curl disease, heavily affecting tomato crops in the Mediterranean area. None of the 180 seedlings originating from TYLCSV-infected plants showed any phenotypic alteration typical of virus infection. Moreover, whole viral genomic molecules could not be detected in their cotyledons and true leaves, neither by membrane hybridization nor by rolling-circle amplification followed by PCR, indicating that TYLCSV is not a seed-transmissible pathogen for tomato. Examining the localization of TYLCSV DNA in progenitor plants, we detected the virus genome by PCR in all vegetative and reproductive tissues, but viral genomic and replicative forms were found only in leaves, flowers and fruit flesh, not in seeds and embryos. Closer investigations allowed us to discover for the first time that these embryos were superficially contaminated by TYLCSV DNA but whole genomic molecules were not detectable. Therefore, the inability of TYLCSV genomic molecules to colonize tomato embryos during infection justifies the lack of seed transmissibility observed in this host.

High Genetic Diversity in Predominantly Clonal Populations of the Powdery Mildew Fungus Podosphaera leucotricha from U.S. Apple Orchards.

Apple powdery mildew (APM), caused by Podosphaera leucotricha, is a constant threat to apple production worldwide. Very little is known about the biology and population structure of this pathogen in the United States and other growing regions, which affects APM management. A total of 253 P. leucotricha isolates, sampled from 10 apple orchards in Washington, New York, and Virginia, were genetically characterized with novel single sequence repeat and mating type markers. Eighty-three multilocus genotypes (MLGs) were identified, most of which were unique to a given orchard. Each isolate carried either a MAT1-1 or a MAT1-2 idiomorph at the mating type locus, indicating that P. leucotricha is heterothallic. Virulence tests on detached apple leaves showed that the 10 most frequent P. leucotricha MLGs were avirulent on a line containing a major resistance gene. Analysis of molecular variance showed significant differentiation (P < 0.001) among populations, a result supported by principal coordinate analysis revealing three genetic groups, each represented by nonoverlapping MLGs from Washington, New York, and Virginia. A Bayesian cluster analysis showed genetic heterogeneity between Washington populations, and a relative migration analysis indicated substantial gene flow among neighboring orchards. Random mating tests indicated that APM epidemics during the active cycle were dominated by clonal reproduction. However, the presence of sexual structures in orchards, the likelihood that five repeated MLGs resulted from sexual reproduction, and high genotypic diversity observed in some populations suggest that sexual spores play some role in APM epidemics. IMPORTANCE Understanding the population biology and epidemiology of plant pathogens is essential to develop effective strategies for controlling plant diseases. Herein, we gathered insights into the population biology of P. leucotricha populations from conventional and organic apple orchards in the United States. We showed genetic heterogeneity between P. leucotricha populations in Washington and structure between populations from different U.S. regions, suggesting that short-distance spore dispersal plays an important role in the disease's epidemiology. We presented evidence that P. leucotricha is heterothallic and that populations likely result from a mixed (i.e., sexual and asexual) reproductive system, revealing that the sexual stage contributes to apple powdery mildew epidemics. We showed that the major resistance gene Pl-1 is valuable for apple breeding because virulent isolates have most likely not emerged yet in U.S. commercial orchards. These results will be important to achieve sustainability of disease management strategies and maintenance of plant health in apple orchards.

Beyond the shape: molecular systematics and phytopathological diagnostic

Crop protection can be implemented by several strategies, among them prophylaxis guarantees profitable productions and a slight environmental impact. Diagnosis of pathogens exploited different strategies, according to the organisms to be detected. Historically, fungi have been identified by morphological characters, bacteria by physiological tests and viruses by symptoms on indexing plants. Immunological assays (devised to detect bacteria and viruses) at first, and nucleic acid based assays (available for all biotic pathogens) later, reduced strategy discrepancies. The fast evolution in regulation and techniques that we are living nowadays, deeply changed the terms. It is, now,possible to identify all the pathogens affecting a crop in a single sample (multiplexing) and to examine a high number of samples at a time.We can state that there is no pathogen that cannot be identified through assays that guarantee the sensitivity and the specificity required by certification schemes, eradication procedures and quarantine protocols. The same fast technical evolution renders the exploitation of the new sophisticate and powerful tools more and more cheap and simple. At the present stage, a deeper knowledge of the biology and the epidemiology of plant pathogens changes the problem from technical to conceptual. Conventional fungal taxonomy is no more apt to depict frameworks to house the biological complexity of fungal pathogens; molecular phylogeny opened new horizons and posed new questions. Molecular systematics can bring into harmony systematic schemes, biological complexity and phytopathological aspects. To explain concepts, examples including toxigenic Fusarium and Diaporthe helianthi, as a quarantine pathogen, will be discussed.

Molecular epidemiology of forest pathogens: from genes to landscape

In the last two decades, important developments in molecular biology have significantly improved our capacity to address questions in population biology, which could have an impact on our understanding of the epidemiology of plant pathogens. In turn, some of the molecular tools that are now available, in particular those based on the polymerase chain reaction, have made it possible to detect and monitor pathogens directly from the environment, by-passing the need for culturing. This has had a large impact in forest pathology, where hosts and pathogens often cover large ranges of latitude, longitude, and altitude. Genomics data as well are being generated in forest pathology, with fungal and tree genomes being completely sequenced. The integration of genomic and environmental data is a promising area of forest pathology, and this review will address some of the issues and potential future directions in this field.