Source Of Primary Inoculum Research Articles

Ralstonia solanacearum causing potato bacterial wilt: host range and cultivars’ susceptibility in Rwanda

AbstractMembers of the Ralstonia solanacearum species complex (RSSC), causing potato bacterial wilt or brown rot, are highly contagious and there are no known cultivars with durable resistance to the pathogen. This study hypothesized (a) that crops intercropped or rotated with potato, plants in the same family, and plants grown in the neighbouring fields can host the pathogen and they can be potential sources of primary inoculum, and (b) that potato cultivars currently multiplied by the public tissue culture laboratory in Rwanda are less susceptible to the pathogen. Fourteen plant species and potato, and nine potato cultivars were tested for susceptibility to an RSSC phylotype II strain under greenhouse conditions. The bacteria induced symptoms on potato, tomato, tree tomato, sweet pepper, and eggplant only. Among the plant species with symptoms, potato, tomato, and tree tomato wilted completely. There was a significant difference in days to symptom expression and to complete wilting (p < .0001). While all tested potato cultivars were found to be susceptible, the number of days to first symptom expression, days to complete wilting, area under the disease progress curve (AUDPC), and the number and weight of harvested tubers varied considerably. Cultivars Cruza, Kinigi, and CIP‐58 were less susceptible whereas the cultivars Gikungu, Kirundo, and Victoria were highly susceptible. There is a strong need to search for other sources of resistance. The results indicate that some plant species that might serve as a reservoir of the bacterium should be avoided in the vicinity of potato crops.

Persimmon anthracnose: a comparative study of aggressiveness on shoot and fruit among Colletotrichum horii isolates in southern Brazil

ABSTRACT: The persimmon tree is known for its rusticity and productivity and was first introduced to Brazil in the late 19th century. However, anthracnose disease is causing immature fruit drop and severe disease symptoms in persimmon fruit, shoots, flowers, and twigs. The causal agent was first described as the fungal species, Colletotrichum horii, which was first confirmed using only the ITS region. In this study, we compared the aggressiveness of 13 isolates of Colletotrichum spp. obtained from fruit and shoots of persimmon grown in the Metropolitan Region of Curitiba, Paraná State, Brazil. A multilocus molecular analysis was carried out based on ITS, GPDH, and EF genes, and we confirmed that the isolates were confirmed as C. horii. All isolates were pathogenic for unwounded and wounded persimmon fruit but differed in aggressiveness. Only one isolate was non-pathogenic when inoculated into unwounded persimmon shoots. Most isolates caused cankers and shoot death whether shoots were wounded or unwounded. In this study, we emphasized the importance of shoots as a source of primary inoculum. In future studies, it will be critical to further elucidate the epidemiological basis of anthracnose disease by conducting field studies to establish a more effective strategy for disease control.

Detection of rice blast pathogen- Expedition from Petri plate to PCR tube - a mini review

Rice blast caused by Magnaporthe oryzae (anamorph: Pyricularia oryzae) is a serious disease of rice threatening its production worldwide. the pathogen has high genomic plasticity, wide host range and rapid rate of evolution resulting in fungicidal resistance and breakdown of resistant varieties. though integrated disease management modules have been adopted for the management of the pathogen, the disease continues to be a challenge to plant pathologists. The pathogen affects all the above-ground parts of the plant including seeds. Seeds are implicated as important overwintering source of inoculum of the pathogen and may serve as an important source of primary inoculum during the rice season. Further, the spread of pathogen to previously un- invaded areas has been found associated with the movement of the infected seeds. therefore, use of pathogen-free healthy seeds should be an integral part of the rice blast management module, for quarantine purposes. Further, with the appearance of wheat blast as a global threat to wheat production and invasion of rice blast pathogen to previously uninvaded areas, accurate detection of M. oryzae strains is of paramount importance. Conventionally used seed testing assays are time-consuming, labor-intensive and have low sensitivity and specificity. Advances in the detection strategies of rice blast pathogen from blotter test to high throughput real time PCR is remarkable. But still, commercial application of these technologies is lacking. the review focuses on the advances in detection strategies of M. oryaze and their utility as commercial protocols.

Pathogen Dispersal and Glomerella Leaf Spot Progress Within Apple Canopy in Brazil.

Glomerella leaf spot (GLS) of apple is caused by three different Colletotrichum species complexes. This study evaluated the dispersal of Colletotrichum spores related to GLS temporal progress and defoliation. Spores were monitored by air and water runoff in different plant heights, and the temporal progress of GLS and defoliation were assessed. Spores of the pathogen were first cached in the lower part of the tree closer to the ground, confirming the importance of dead leaves on the ground as main source of primary inoculum. In plots with high primary inoculum, the disease increases exponentially during favorable weather conditions. The highest initial inoculum was found in the lower part of the tree, but the highest rate of the disease progress in the upper.

Genome-wide association study of the seed transmission rate of soybean mosaic virus and associated traits using two diverse population panels

Genome-wide association analyses identified candidates for genes involved in restricting virus movement into embryonic tissues, suppressing virus-induced seed coat mottling and preserving yield in soybean plants infected with soybean mosaic virus. Soybean mosaic virus (SMV) causes significant reductions in soybean yield and seed quality. Because seedborne infections can serve as primary sources of inoculum for SMV infections, resistance to SMV seed transmission provides a means to limit the impacts of SMV. In this study, two diverse population panels, Pop1 and Pop2, composed of 409 and 199 soybean plant introductions, respectively, were evaluated for SMV seed transmission rate, seed coat mottling, and seed yield from SMV-infected plants. The phenotypic data and genotypic data from the SoySNP50K dataset were analyzed using GAPIT and rrBLUP. For SMV seed transmission rate, a single locus was identified on chromosome 9 in Pop1. For SMV-induced seed coat mottling, loci were identified on chromosome 9 in Pop1 and on chromosome 3 in Pop2. For seed yield from SMV-infected plants, a single locus was identified on chromosome 3 in Pop2 that was within the map interval of a previously described quantitative trait locus for seed number. The high linkage disequilibrium regions surrounding the markers on chromosomes 3 and 9 contained a predicted nonsense-mediated RNA decay gene, multiple pectin methylesterase inhibitor genes (involved in restricting virus movement), two chalcone synthase genes, and a homolog of the yeast Rtf1 gene (involved in RNA-mediated transcriptional gene silencing). The results of this study provided additional insight into the genetic architecture of these three important traits, suggested candidate genes for downstream functional validation, and suggested that genomic prediction would outperform marker-assisted selection for two of the four trait-marker associations.

Prevalence of Acidovorax citrulli in Commercial Cucurbit Seedlots During 2010-2018 in China.

Acidovorax citrulli is the causal agent of bacterial fruit blotch (BFB), a serious threat to cucurbit fruit and seed production worldwide. In recent years, the BFB has spread to many areas of China, mainly via the inadvertent distribution of contaminated commercial seeds. To assess the prevalence of seedborne A. citrulli in commercial watermelon and other cucurbitaceous seedlots in China, a 9-year survey was conducted between 2010 and 2018. A total of 4,839 seedlots of watermelon and other cucurbitaceous species were collected from 13 major seed production areas of China and tested by a semiselective media-based colony PCR technique for A. citrulli. Overall, A. citrulli was detected in 18.00% (871/4,839) of all cucurbitaceous seedlots. The bacterium was detected in 21.59% (38/176), 19.19% (33/172), 23.44% (214/913), 40.76% (247/606), 13.28% (85/640), 15.40% (95/617), 13.25% (73/551), 8.03% (48/598), and 6.71% (38/566) of all commercial seedlots tested from the 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, and 2018 growing seasons, respectively. Additionally, the prevalence of A. citrulli in cucurbit seedlots was determined for different seed production areas. The prevalence of A. citrulli in cucurbitaceous seedlots produced in Xinjiang, Gansu, Ningxia, Inner Mongolia, and 9 other provinces was 18.76% (582/3103), 26.34% (103/391), 21.47% (82/382), 11.11% (14/126), and 10.75% (90/837), respectively. This is the first survey for A. citrulli in commercial cucurbit seeds in China, and the relatively high prevalence suggests that commercial seeds represent a substantial source of primary inoculum that can threaten cucurbit seed and fruit production in China.

Weather-based predictive models for Diaporthe helianthi ascospore release in Uruguay

Stem canker (SC), caused by Diaporthe helianthi, is the most serious sunflower disease in Uruguay. Yield losses have been estimated up to 75%. Chemical control is one of the strategies used to manage this disease, but fungicide application should be done before symptoms are visible. Ascospores are the primary source of inoculum, they are produced in perithecia which develop in infected stubble and are dispersed by wind to infect plants. As in other monocyclic diseases, quantifying primary inoculum is essential to predict an epidemic. In this study, ascospores were trapped on microscope slides with solid petroleum jelly which were placed on top of flat open cages filled with natural infected stubble. Cages were placed outdoors, slides where replaced twice a week and stained ascospores were counted under the microscope. Our objective was to develop weather-based models to predict ascospore release levels of D. helianthi from infected stubble. Explanatory weather variables were calculated during the seven-day periods prior to each field weekly ascospore count using daily weather station data from La Estanzuela, Uruguay. Then, logistic models were fit to estimate probabilities of having severe or moderate to light levels of ascospore counts. The best models included variables associated to the precipitation and dew-induced wetness frequency, combinated with the simultaneous occurrence of high relative humidity or low thermal amplitude records. Estimating the evolution of ascospore release through the weather-based models might help to guide preventive fungicide applications to control stem canker in Uruguay.

Infection, Survival, and Growth of Clavibacter nebraskensis on Crop, Weed, and Prairie Plant Species.

Clavibacter nebraskensis is the causal agent of Goss's leaf blight and wilt, an important disease of maize in the United States and Canada. The epidemiology and ecology of this bacterial pathogen are poorly understood. Infested maize residue is often considered to be the primary source of inoculum for maize; however, the potential for many other plant species to be infected and serve as inoculum sources is unknown. The goal of this study was to determine if C. nebraskensis could infect, survive, and grow on common weed, crop, and grass species. Seedling leaves of 18 plant species that grow in maize production areas in the United States were inoculated with this pathogen in a controlled environment and in the field. Lesion development, bacterial streaming, and pathogen population size on leaves were then determined and used as criteria to evaluate host-pathogen interactions. Woolly cupgrass (Eriochloa villosa) and the native prairie grasses big bluestem (Andropogon gerardii) and little bluestem (Schizachyrium scoparium) developed lesions and bacterial streaming, identifying them as hosts and susceptible to infection. To our knowledge, this is the first report of these grass species being hosts of C. nebraskensis. Ten other grass species, including wheat and oats, were identified as potential sustaining hosts that maintained epiphytic or endophytic pathogen populations >106 colony forming units per leaf sample but displayed no evidence of infection. Five broadleaf species tested were nonhosts based on the three criteria. This study suggests that multiple plant species support infection and growth of C. nebraskensis and further elucidates the ecology of this pathogen and the epidemiology of Goss's wilt.

Relationship between Verticillium dahliae inoculum and sunflower wilt in Argentina

Leaf mottle and wilt (LM) of sunflower caused by Verticillium dahliae is the most important disease in Argentina. LM is a monocyclic disease where microsclerotia in the soil are the primary inoculum source and have an important impact on symptom expression (incidence and severity) and yield. The inoculum (microsclerotia/g of soil) and disease relationship is affected by several factors, such as temperature and soil moisture. The objective of this work was to determine the regression model that best fits the relationship between V. dahliae microsclerotia in soil and sunflower LM, over a wide environmental range. Sunflower plants were exposed to different microsclerotia densities in pasteurized and non-pasteurized soils in pots under greenhouse and field conditions. The microsclerotia in soil and LM relationship regression models were also tested in the field with natural soil borne microsclerotia infestation, in different sunflower cultivars and locations. After the flowering stage (± 8 days), disease severity was affected by inoculum density at every evaluation time (P-value≤0.0009). A 45% increase in the area under the disease progress curve was shown by soil pasteurization. Power regression model showed the highest concordance and Pearson correlation coefficients, parameter significance and no difference with the lowest Akaike index criterion value among all tested models in all experiments. This study provides a model (LM = 11.6*inoculum density^0.57) which describes the inoculum density and LM relationship. The use of this model might help farmers to predict an LM risk before sowing and to choose the appropriate field and the resistance level of the sunflower hybrid to be used.

Verification of integrated management of chickpea Fusarium wilt (Fusarium oxysporum f.sp. ciceris) in major chickpea growing areas of East Shewa Zone, Ethiopia

Fusarium wilt is caused by Fusarium oxysporum f.sp. ciceris, a major limiting chickpea productivity in Ethiopia. The present study was to validate the integrated management of chickpea Fusarium wilt. The verifications were conducted in Adea, Lume and Gimbichu districts. The experiment was laid out as a factorial combination a farmer’s field was used as replications. The management package consists of two varieties (Arerti and Habru), seed treatment with protective fungicides a rate of 2 g (a.i) kg/seed) and without and two seed bed type (raised and flat). There were three verifications per site. The pod borer management consists of timely insecticide application lambda cyhalothrin. The 2016 result showed significant variations among the treatment on Fusarium wilt and hundreds of seed weight. The lowest Fusarium wilt incidence (1.5%) was found on variety Arerti by raised bed and fungicide treated seed. The optimum yield (1747.5 kg/ha) obtained on flat plot with variety Habru use moisture in field, perhaps raised bed drain moisture. The correlation revealed that there was negative relationship between Fusarium wilt and yield. While, in 2017 significant variation is on Fusarium wilt and number of pods per plant. The highest Fusarium wilt incidence (24%) was on variety Arerti by flat plot without fungicide. Whereas, the highest pod per plant (54.4%) was on variety Arerti by raised bed with fungicide treated. There was clear difference among the seasons on Fusarium wilt incidences and yield. Thus, combining resistant cultivars by raised bed and fungicides treatment will reduce the Fusarium wilt incidence and optimize planting time to obtain attainable yields and ploughing interval in severely infected field reduces primary sources of inoculum. Key words: Chickpea, fungicides, Fusarium wilt, incidence, yield.

Pathogen and disease characteristics of marigold black spot in Beijing and surrounding areas

Annual epidemics of black spot, a serious disease of marigold (Tagetes erecta), developed in Beijing and surrounding areas from 2011 to 2015. Alternaria tagetica was isolated from marigold showing symptoms from Beijing and Hebei province, and pathogen characteristics and meteorological factors influencing disease development were investigated. The optimum growth temperature of the fungus was 20–25 °C. Field experiments conducted from 2011 to 2014 to evaluate diseased residual branches indicated that marigold residues from the previous year are a likely source of primary inoculum. Regression analysis showed that temperature and precipitation were explanatory factors for disease development from 2013 to 2015. Structural equation modelling indicated that the contributions of temperature degree‐day and rainfall to disease intensity were 27% and 30%, respectively. Augmented Dickey–Fuller test results indicated stability in the residual sequence and co‐integration between the variables daily average temperature, daily average precipitation, daily average humidity and daily disease intensity. Error‐correction modelling showed that temperature, lagged order of temperature, precipitation, lagged order of precipitation, lagged order of disease intensity and the error term were significant. Research indicated that A. tagetica was the primary pathogen of marigold black spot, and that crop rotation could play a useful role in management of the disease, whilst increased rainfall and reduced temperature provide superior conditions for disease spread and infection. There is potential for the results of this study to be used by municipal agriculture authorities in the prediction of marigold black spot in Beijing.

Determining the Sources of Primary and Secondary Inoculum and Seasonal Inoculum Dynamics of Fungal Pathogens Causing Fruit Rot of Deciduous Holly.

Fruit rot of deciduous holly, caused by species of the genera Alternaria, Colletotrichum, Diaporthe, and Epicoccum, is affecting plant production in Midwestern and Eastern U.S. nurseries. To determine the sources of inoculum, dormant twigs and mummified fruit were collected, and leaf spot development was monitored throughout the season from three Ohio nurseries over two consecutive years. Mummified fruit was the main source of primary inoculum for species of Alternaria and Epicoccum, whereas mummified fruit and bark were equally important for species of Colletotrichum and Diaporthe. Brown, irregular leaf spots developed in the summer, and disease incidence and severity increased along with leaf and fruit development. Coalesced leaf spots eventually resulted in early plant defoliation. When tested for their pathogenicity on fruit, leaf spot isolates were able to infect wounded mature fruit and induce rot symptoms, which indicated that leaf spots could serve as a source of secondary inoculum for fruit infections. In addition, spore traps were used to monitor seasonal inoculum abundance in the nurseries. Fruit rot pathogens were captured by the spore traps throughout the season, with peak dissemination occurring during flowering. In this study, we also attempted to understand the role of environmental factors on leaf spot development. Although leaf spot incidence and severity were negatively correlated to mean maximum, minimum and average temperature, a decrease in temperature also coincided with leaf senescence. The role of temperature on leaf spot development should be further studied to fully interpret these results.

Combined Metabarcoding and Co-occurrence Network Analysis to Profile the Bacterial, Fungal and Fusarium Communities and Their Interactions in Maize Stalks.

Fusarium Head Blight (FHB) is one of the most devastating diseases of cereals worldwide, threatening both crop production by affecting cereal grain development, and human and animal health by contaminating grains with mycotoxins. Despite that maize residues constitute the primary source of inoculum for Fusarium pathogenic species, the structure and diversity of Fusarium spp. and microbial communities in maize residues have received much less attention than in grains. In this study, a metabarcoding approach was used to study the bacterial, fungal and Fusarium communities encountered in maize stalks collected from 8 fields in Brittany, France, after maize harvest during fall 2015. Some predominant genera found in maize residues were cereal or maize pathogens, such as the fungal Fusarium, Acremonium, and Phoma genera, and the bacterial Pseudomonas and Erwinia genera. Furthermore, a high predominance of genera with previously reported biocontrol activity was found, including the bacterial Sphingomonas, Pedobacter, Flavobacterium, Pseudomonas, and Janthinobacterium genera; and the fungal Epicoccum, Articulospora, Exophiala, and Sarocladium genera. Among Fusarium spp., F. graminearum and F. avenaceum were dominant. We also found that the maize cultivar and previous crop could influence the structure of microbial communities. Using SparCC co-occurrence network analysis, significant negative correlations were obtained between Fusarium spp. responsible for FHB (including F. graminearum and F. avenaceum) and bacterial OTUs classified as Sphingomonas and fungal OTUs classified as Sarocladium and Epicoccum. Considering that isolates belonging to these taxa have already been associated with antagonist effect against different Fusarium spp. and/or other pathogenic microorganisms and due to their predominance and negative associations with Fusarium spp., they may be good candidates as biocontrol agents. Combining the use of Fusarium-specific primers with universal primers for bacteria and fungi allowed us to study the microbial communities, but also to track correlations between Fusarium spp. and other bacterial and fungal genera, using co-occurrence network analysis. Such approach could be a useful tool as part of a screening strategy for novel antagonist candidates against toxigenic Fusarium spp., allowing the selection of taxa of interest.

Differential Spread of Grapevine Red Blotch Virus in California and New York Vineyards

Limited information is available on the spread dynamics of grapevine red blotch virus (GRBV, genus Grablovirus, family Geminiviridae) in vineyards. In this study, we investigated red blotch disease progress in three vineyards with a disparate initial inoculum prevalence. Secondary spread was documented in Cabernet Sauvignon and Cabernet franc vineyards in California, but not in a Merlot vineyard in New York. Increase in annual disease incidence (4.8, 0.13, and 0%) was unrelated to the estimated initial source of inoculum at planting (1, 40, and 40%) in the Cabernet franc, Cabernet Sauvignon, and Merlot vineyards, respectively. Limited genetic diversity of GRBV populations in newly infected vines supported localized spread in California vineyards, and suggested the planting material as the primary source of inoculum. Among the community of hemipteran insects visiting two of the three study vineyards, populations of Spissistilus festinus, the vector of GRBV, were absent in the Merlot vineyard and low in the Cabernet Sauvignon vineyard. Furthermore, all cover crop samples collected from GRBV-infected California vineyards each spring of 2016 to 2018, particularly legume species which are preferred hosts of S. festinus, tested negative for GRBV, suggesting a minimal role, if any, in GRBV spread as inoculum reservoirs. Together our findings illustrate differential disease progress in distinct vineyard ecosystems, and support the elimination of virus inoculum sources as an actionable disease management strategy across vineyards.

Characterization of Sclerotinia sclerotiorum airborne inoculum, the widespread agent of white mould disease

A means to rationalize the use of fungicides for crop protection and to make agriculture friendlier to environment and human health is to develop disease-risk forecasting systems based on the assessment of airborne inoculum abundance. Sclerotinia sclerotiorum, the pandemic agent of white mould disease, is disseminated via the atmosphere in the form of ascospores. These airborne spores are the primary sources of inoculum initiating the majority of epidemics. However, for witloof chicory (Cichorium intybus var. foliosum L.), there is no data about airborne inoculum of S. sclerotiorum, which makes it difficult to develop a forecasting model. In the present study, we characterized the temporal evolution of the abundance and of the genetic characteristics of S. sclerotiorum inoculum on a witloof chicory field located in Northern France over a 3-year period. To our knowledge, this study provides the first quantification of viable airborne populations of this fungus in witloof chicory crops. Moreover, it provides the first genetic characterization of S. sclerotiorum airborne inoculum. The results show that viable ascospores were present through 80% of the sampling dates. A significant correlation between abundance of airborne ascospores and local relative humidity suggests a local origin of inoculum. However, the existence of a slight genetic differentiation between isolates carried by air masses coming from the West and from the North-West is compatible with the hypothesis of a distant origin of S. sclerotiorum inoculum. We discuss the additional studies that are envisioned to clarify the origin of S. sclerotiorum airborne inoculum in witloof chicory fields.

THE MAIN WAYS OF INFECTION OF GRAIN SORGHUM BY BURKHOLDERIA ANDROPOGONIS (BURKHOLDERIACEAE, BURKHOLDERIALES) IN THE FOREST-STEPPE OF THE MIDDLE VOLGA REGION

Bacterial leaf stripe, caused by Burkholderia andropogonis, is one of the main sorghum diseases in the forest-steppe of the Middle Volga region. The purpose of the research was to examine the main ways of infection of sorghum by Burkholderia andropogonis. The studies were carried out on experimental fields of the Volga region Scientific Research Institute of Selection and Seed-growing and in the laboratories of Plant Protection and Microbiology of the Samara State Agricultural Academy in 2010-2015. The severity of sorghum leaf stripe in the stage of physiological maturity was 9-60%, with a maximum in high-drought (2010), minimum in wet and cool (2011) years. Losses of grain sorghum yield from B. andropogonis were on average up to 30 %. The main overwintering and primary inoculum sources reported are infested seeds. Up to 97-100 % of the sorghum seeds were infected with B. andropogonis. After sowing infected seeds, bacteria spread through the vascular system in seedlings and developing plants, penetrating into emerging and ripening seeds. The main indirect vector of bacterial leaf stripe is the flea beetle Phyllotreta vittula feeding on sorghum leaves. In the first half of July, adults of the new generation of P. vittula facilitate the penetration of the pathogen from B. andropogonis exudate into the tissues of sorghum through wounds that feeding beetles form on the upper surface of young leaves. The role of flea beetles P. vittula in the spread of the bacteria B andropogonis in crops of sorghum recorded for the first time.

Apple nursery trees and irrigation water as potential external inoculum sources of apple replant disease in South Africa

Apple replant disease (ARD) is a biological phenomenon caused by soilborne agents that include selected species of fungi (Rhizoctonia and ‘Cylindrocarpon’-like), oomycetes (Pythium, Phytopythium and Phytophthora) and nematodes (Pratylenchus). Old orchard soils previously planted to apple or related species are a primary inoculum source of ARD pathogens. In the current study, nursery trees and irrigation water were investigated as potential external ARD inoculum sources in South Africa. Investigations conducted at five nurseries over two years revealed that roots of nursery trees were infested by several ARD agents. In the 2013 sampling year, Pythium irregulare and Pythium ultimum were obtained from trees in five and two of the nurseries, respectively, using isolation studies. Based on isolation studies conducted on individual trees in 2013, 47% and 4% of all surveyed trees contained P. irregulare and P. ultimum, respectively. In the 2014 season, real-time quantitative PCR (qPCR) analyses of root tissue from individual trees demonstrated that all of the nurseries and 95% of trees contained P. irregulare, whereas three nurseries and 41% of trees contained P. utlimum. The only other oomycete pathogens that were detected in nursery tree roots were Pythium spp. complex B2A and Pythium sylvaticum and each only occurred in one nursery in one of the sampling years. For all nurseries in both years, trees were consistently infected with ‘Cylindrocarpon’-like spp. Pratylenchus spp., which were only analysed in 2013, was present in all five nurseries and in 29% of the trees. Infestation levels were high, with 22% of trees having Pratylenchus root densities that exceeded 100 per 5 g of roots. Pythium irregulare was the dominant oomycete pathogen detected in irrigation water samples (31% to 76% of the samples) obtained from 13 orchards over two years on a monthly basis. The ARD pathogens P. ultimum, Phytopythium litorale and Pythium spp. complex B2A were rarely identified in irrigation water, along with five other non-pathogenic Pythium and Phytopythium species. The association of ARD causative agents with nursery trees and irrigation water suggests that these could function as potential ARD inoculum sources that might limit post-plant tree growth.

Validating Sclerotinia sclerotiorum Apothecial Models to Predict Sclerotinia Stem Rot in Soybean (Glycine max) Fields.

In soybean, Sclerotinia sclerotiorum apothecia are the sources of primary inoculum (ascospores) critical for Sclerotinia stem rot (SSR) development. We recently developed logistic regression models to predict the presence of apothecia in irrigated and nonirrigated soybean fields. In 2017, small-plot trials were established to validate two weather-based models (one for irrigated fields and one for nonirrigated fields) to predict SSR development. Additionally, apothecial scouting and disease monitoring were conducted in 60 commercial fields in three states between 2016 and 2017 to evaluate model accuracy across the growing region. Site-specific air temperature, relative humidity, and wind speed data were obtained through the Integrated Pest Information Platform for Extension and Education (iPiPE) and Dark Sky weather networks. Across all locations, iPiPE-driven model predictions during the soybean flowering period (R1 to R4 growth stages) explained end-of-season disease observations with an accuracy of 81.8% using a probability action threshold of 35%. Dark Sky data, incorporating bias corrections for weather variables, explained end-of-season disease observations with 87.9% accuracy (in 2017 commercial locations in Wisconsin) using a 40% probability threshold. Overall, these validations indicate that these two weather-based apothecial models, using either weather data source, provide disease risk predictions that both reduce unnecessary chemical application and accurately advise applications at critical times.

Saprotrophic survival of Magnaporthe oryzae in infested wheat residues

Wheat blast caused by Magnaporthe oryzae is a relatively new disease that has caused considerable losses in wheat fields of several South American countries, including Brazil. The 2016 report of wheat blast occurrence in Bangladesh raised concern in South Asia where wheat represents a significant crop. The sources of primary inoculum and survival from season to season of the fungus remain largely unknown. The effect of wheat residues on the onset of blast epidemics and the potential for survival of M. oryzae in the residues were studied under subtropical climatic conditions, in the South of Brazil. The objective of this study was to monitor the saprotrophic development of M. oryzae on wheat debris and explore the relative importance of crop residues as a source of inoculum. The wheat cultivars BRS 229 and Anahuac 75, moderately and highly susceptible to the disease, respectively, were inoculated with a spore suspension of 10−5 conidia mL−1 using an aggressive (Py 12.1.209) and a less aggressive (Py 12.1.132) isolate. At maturity, a portion of leaves, stems and spikes were detached from plants, and a group of ten lesions were randomly selected and marked on each type of plant organ. The air-dried plant organs were placed separately inside bags and exposed outside. The experiment was conducted over three different time intervals. Each 14 days, samples were taken from the field and tested for sporulation. The survival of the blast fungus decreased rapidly on the rachis when compared to stems and leaves. Sporulation of the fungus was observed on the wheat residues for up to five months. Based on the results of this study, the possibility that the causal agent of wheat blast survives under Brazilian conditions from one crop to another in wheat residues is very low. The management of crop residues is not a key point to control the development of wheat blast. A strong emphasis should be placed on the presence of other hosts.

Detection and chemical control of Cercospora sojina infecting soybean seed in Argentina

Frogeye leaf spot (FLS) of soybean caused by Cercospora sojina Hara is an important disease in the Argentine Pampas region. The primary inoculum sources of the disease are infected seeds and plant debris. Thus, use of pathogen-free or fungicide-treated seeds is crucial to prevent the introduction and further spread of the disease. The aims of this research were i) to quantify C. sojina incidence in harvested soybean seeds, ii) to compare media-based incubation methods for detection of the pathogen in seeds, and iii) to evaluate the effect of fungicide seed treatments in reducing FLS incidence. Among eight different detection methods evaluated, no difference in pathogen incidence was found, but the water restriction blotter method should be recommended as it is more time and cost-effective. Out of 13 different fungicide seed treatment combinations in three independent experiments, benzimidazole seed treatments resulted in zero FLS incidence. Any other of the tested fungicidal active ingredients either did not control the disease or did so poorly only when initial infection levels were low. Treating seeds with benzimidazole fungicides may help to reduce FLS incidence during initial soybean field stands when using infected seed lots.