Seed and seedling disease of corn and soybean in Ohio: The role of Fusarium graminearum, Pythium species diversity, fungicide sensitivity, Pythium community composition, and soil properties in disease severity

Both Pythium and Fusarium spp. can be readily isolated from corn and soybeans plants where stand establishment is an issue. In the first study, isolations were made from symptomatic corn and soybean seeds and seedlings collected in Ohio from fields with stand establishment problems, and in a second study soil was collected from 88 locations in Ohio using a stratified sampling strategy and baiting procedure. The results of this study provide a comprehensive view of the Pythium species present in Ohio, their pathogenicity, response to fungicide seed treatment, distribution in Ohio, and response to abiotic properties of the soil, as well as the pathogenicity and fungicide sensitivity of F. graminearum to corn and soybean seedlings. The overall goal was to determine which species of Pythium and Fusarium are most commonly associated with seed and seedling disease of corn and soybean, and determine how widespread these pathogens are in the state. In addition, determine which soil properties pH, Ca, Mg, K, P, CEC, OM, percent sand silt and clay, and field capacity are associated with disease caused by Pythium spp., and the formation of Pythium communities. A total of 124 isolates representing 12 different species of Pythium, and 112 isolates of F. graminearum were recovered from diseased corn and soybean seedlings from the first study. The results demonstrated that there is a large diversity of pathogenic Pythium species in the soils of Ohio. These included several species that were reported for the first time as pathogens of corn and/or soybeans and also include the description of the new species Pythium delawarii. These investigations also provided the first report of P. attrantheridium as a pathogen of both corn and soybean; the first report of P. dissotocum as a pathogen of soybean; and the first report of P. inflatum as a seed and seedling pathogen of corn and soybean in North America. Of the five commercial seed-treatment fungicides tested, none provided adequate control for all twelve Pythium species tested, and fludioxonil was the only fungicide which provided sufficient inhibition of F. graminearum mycelial growth. However several fludioxonil resistant mutants were identified during the sensitivity experiments. F. graminearum was found to be highly pathogenic to both corn and soybean seedlings in the both the in vitro and soil bioassay pathogenicity test, indicating the importance of F. graminearum as a pathogen of both corn and soybean seedlings. P. irregulare, P. inflatum, P. torulosum, P. ultimum var ultimum, P. ultimum var sporangiiferum and P. dissotocum were the most frequently recovered of the 21 species identified in the second study. Greater disease incidence levels were associated with soils with lower levels of Ca, Mg, CEC, and organic matter. Five Pythium communities were identified in this study using the Jaccard similarity index and Ward's minimum variance to group locations which have the same Pythium species present. Canonical Discriminant Analysis (CDA) was used to determine if there was a strong association between abiotic components pH, Ca, Mg, organic matter, percent clay and field capacity of the soil and the structure of Pythium communities. The results of this study provide a comprehensive view of Pythium species present in Ohio agronomic soils, the distribution in Ohio, and response to abiotic properties of the soil through a large scale sampling strategy that included representative locations from the corn and soybean production region of the state.