Sudden death syndrome (SDS) of soybean is favored by planting in cool soil but epidemics can be severe even when planting occurs later in the season into warmer soil. Our objective was to determine how soil temperature affects susceptibility of plants exposed to Fusarium virguliforme at different ages. Soybean plants were grown in rhizotrons in water baths at 17, 23, and 29°C. Subsets of plants were inoculated 0, 3, 7, and 13 days after planting (DAP) by drenching soil with a conidial suspension. Root rot developed in all inoculated plants but severity decreased with increasing temperature and plant age at inoculation. Severity of foliar symptoms also decreased with increasing plant age. Whereas plants inoculated 0 DAP developed severe foliar symptoms at all temperatures, plants inoculated 3 and 7 DAP developed symptoms only at 17 and 23°C, and those inoculated 13 DAP never developed foliar symptoms at any temperature. Root length at inoculation was negatively correlated with disease severity. Our findings suggest that roots are most susceptible to infection during the first days after seed germination and that accelerated root growth in warmer temperatures reduces susceptibility to root infection conducive to foliar symptoms. However, soil temperature may not affect infections that occur as soon as seeds germinate.
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