Soybean sudden death syndrome is characterized by root rot followed by the development of foliar symptoms. However, it is not known how time of infection affects disease severity. Soybean plants were inoculated at 0, 4, 7, 14, 21, 28, and 35 days after planting (DAP) by drenching potting media with conidia of Fusarium virguliforme, then incubating in growth chambers at 17°C for 7 days followed by 24°C for 31 days. Root rot and severity of foliar symptoms were assessed 18 and 38 days after inoculation (DAI). Root rot developed on plants inoculated at all ages but plants inoculated at seed stage (0 DAP) had the highest (P < 0.01) root rot severity (>90%). At 38 DAI, foliar symptoms were severe (>80%) on plants inoculated at 0 DAP but did not develop on plants inoculated at all other ages. Xylem colonization by F. virguliforme was more frequent in plants inoculated at 0 DAP than on plants inoculated at later stages. The results of this study suggest that soybean roots become less susceptible to xylem colonization and the subsequent development of foliar symptoms as plants mature. Therefore, practices aimed at protecting seed and seedling roots from infection may improve soybean sudden death management.
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