Source Of Inoculum For Epidemics Research Articles

Spatial Genetic Structure and Population Dynamics of Gummy Stem Blight Fungi Within and Among Watermelon Fields in the Southeastern United States.

The epidemiology of gummy stem blight (GSB) of cucurbits, particularly the sources of inoculum for epidemics, and the regional population genetic structure of the causal fungi Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae), S. citrulli, and S. caricae are not well understood. Our goal was to better understand the population structure and fine-scale spatial genetic structure of Stagonosporopsis spp. in the southeastern United States. Overall, 528 isolates collected from nine fields in 2012, 2013, and 2014 were genotyped with 16 microsatellite markers. In 2013, S. caricae was first detected in the southeastern United States; however, S. citrulli remained the dominant species, representing 96.4% of the isolates. Principal coordinates analysis, discriminant analysis of principle components, and analysis of molecular variance indicated that most populations of S. citrulli were genotypically diverse, yet dominated by widely distributed clones that contributed to regional population structure. Spatial genetic structure resulting from aggregation of clonal genotypes at distances of less than 10 meters was detected within two of three fields in which isolate location was recorded. Studies on the epidemiological and fitness differences between S. citrulli and S. caricae and of prevalent and widespread clones will provide insight into the population structure and species dynamics observed in GSB epidemics.

Genetic Diversity and Population Structure of Cucurbit Gummy Stem Blight Fungi Based on Microsatellite Markers.

Combining population genetics with epidemiology provides insight into the population biology of pathogens, which could lead to improved management of plant diseases. Gummy stem blight, caused by three closely related species of Stagonosporopsis-Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae), S. citrulli, and S. caricae-is a devastating disease of cucurbits worldwide. Sources of inoculum for epidemics, mechanisms of dispersal, and the mating system of these species are not well understood. To improve our knowledge of gummy stem blight epidemiology, we developed 18 polymorphic microsatellite markers by combining microsatellite motif enrichment with next-generation sequencing. When tested on 46 isolates from diverse cucurbit hosts and regions, the markers were robust for the dominant and widely distributed S. citrulli. Within this species, we found no population structure based on broad-scale geographic region or host of origin. Using the microsatellites, a rapid polymerase chain reaction-based method was developed to distinguish the three morphologically similar species causing gummy stem blight. To better understand dispersal, reproduction, and fine-scale genetic diversity of S. citrulli within and among watermelon fields, 155 isolates from two field populations in Georgia, United States were genotyped with the 18 microsatellite loci. Although dominant and widespread clones were detected, we found relatively high genotypic diversity and recombinant genotypes consistent with outcrossing. Significant population genetic structure between the two field populations demonstrated that there is regional geographic structure and limited dispersal among fields. This study provides insight into the fine-scale genetic diversity and reproductive biology of the gummy stem blight pathogen S. citrulli in the field.

The Spread of a Released Clone of Gibberella zeae from Different Amounts of Infested Corn Residue.

Corn residue is a significant source of inoculum for epidemics of Fusarium head blight (FHB) in wheat and barley, but little is known about the influence of different amounts of corn residue on FHB. We monitored the spread of a released clone of Gibberella zeae (Fusarium graminearum), causal agent of FHB, from small 0.84-m-diameter research plots containing 45, 200, or 410 g of infested corn stalk pieces in winter wheat and barley fields in Virginia over 3 years (2008 to 2010). The fungus was recaptured through the collection of wheat and barley spikes at 0 and 3 m from the source and the released clone was identified in heterogeneous background populations using amplified fragment length polymorphisms. Results showed a slightly greater intensity of recovery of the clone at a greater distance when more infested residue was present. Plots containing larger amounts of inoculum (410 g) generally resulted in a smaller decline of recovery of the clone at 3 m from the source, indicating a greater spread from the larger inoculum source. The clone was also recovered at distances ≥18 m from inoculum sources. Larger amounts of corn residue generally had less influence on clone recovery in plots containing a moderately resistant wheat cultivar than those containing a susceptible wheat cultivar.

Genetic Diversity in Poplar Leaf Rust (Melampsora medusae f. sp. deltoidae) in the Zones of Host Sympatry and Allopatry

ABSTRACT Poplar leaf rust caused by Melampsora medusae f. sp. deltoidae is a widespread disease in North America, where epidemics occur within zones of sympatry and allopatry of telial hosts (Populus spp.) and aecial hosts (Larix spp.). To test the hypothesis that epidemics originate in the zone of sympatry where the rust can complete its life cycle, populations in sympatry and allopatry were analyzed with single-strand conformational polymorphism for codominant detection of alleles directly from uredinia. More alleles were detected in rust populations in the zone of host sympatry than in allopatry. Almost all alleles found in the zone of allopatry were a subset of the allelic diversity present in the zone of host sympatry. Distance analyses clustered populations according to geographic origin, but not sampling year or type of stand (plantation or natural stands). Large differences in allelic and genotypic frequency were observed between years in allopatry but not in sympatry, suggesting new colonizations in allopatric populations. Our results point to a dynamic and complex pattern of inoculum dissemination in polar leaf rust. The hypothesis most consistent with our results is that populations in sympatry represent a source of inoculum for epidemics, with some annual recolonization in allopatry, possibly via intermediate population jumps.

Potential of fungal antagonists for biocontrol of Fusarium spp. in wheat and maize through competition in crop debris

Pathogenic Fusarium spp. cause head blight in wheat or ear rot in maize leading to yield losses and also a reduction in quality due to mycotoxin contamination of the grain. Infected crop residues are the main inoculum source for epidemics. Saprophytic fungi, obtained from cereal tissues or necrotic tissues of other crops, were screened for their ability to colonise wheat straw and maize stalks and to suppress sporulation of pathogenic Fusarium spp. Results of bio-assays conducted under controlled conditions were variable among Fusarium spp. and host substrates for most antagonists tested, such as yeasts, Trichoderma spp. and non-pathogenic Fusarium spp. Isolates of Clonostachys rosea consistently suppressed sporulation of F. culmorum and F. graminearum on wheat straw, and of F. culmorum, F. graminearum, F. proliferatum and F. verticillioides on maize stalks. Isolates of C. rosea, C. cladosporioides and F. equiseti were applied to pieces of maize stalks or flowering ears in preliminary experiments conducted under field conditions. The colonisation of stalk pieces by pathogenic Fusarium spp. was assessed after 9 months. Colonisation of stalk pieces by pathogenic Fusarium spp. was significantly reduced at several sampling dates. However, results obtained with the antagonists were not consistent for all sampling dates and between experiments.

Detection of Pseudomonas syringae pv. aptata in Irrigation Water Retention Basins by Immunofluorescence Colony-staining

Bacterial blight of cantaloupe (Cucumis melo) caused by Pseudomonas syringae pv. aptata was first observed in south-western France and has since spread to all cantaloupe-growing areas of this country. Use of pesticides registered for this disease has proved ineffective and no commercial cultivars of cantaloupe are resistant to this blight. To develop control strategies for this disease, the principal sources of inoculum were investigated. Among the different sources of inoculum studied, we report the isolation of P. syringae pv. aptata from irrigation water retention basins in south-western France using the immunofluorescence colony-staining (IFC) method. In this study, the pathogen was detected at a low concentration (12 and 70cful−1) in two different retention basins. These results suggest that P. syringae pv. aptata can survive in water used to irrigate cantaloupe crops and could be a source of inoculum for epidemics of bacterial blight. To develop control strategies for this bacterial disease, the importance of water retention basins as sources of inoculum for bacterial blight of cantaloupe needs to be evaluated relative to other potential sources such as seeds, plants from nurseries and plant debris in the soil.