Effect of co-inoculations with Pythium and Fusarium species on seedling disease development of soybean

Abstract

Multiple pathogens are frequently recovered together from diseased soybean seedlings, suggesting a seedling disease complex. Pythium sylvaticum and P. irregulare are prevalent oomycetes and Fusarium oxysporum and F. graminearum are prevalent fungi associated with seedling disease of soybean. We hypothesized that infection with multiple seedling disease pathogens would cause disease symptoms to be more severe than either pathogen alone. A cup assay was used to evaluate the effect of co-inoculation with Pythium and Fusarium species on soybean seedling disease development. Cups were inoculated with either Pythium alone, Fusarium alone, or co-inoculated with both genera. Non-inoculated cups were used as a control. Cups were sampled at 2, 4, 6, 8 and 10 days after planting (DAP) and disease severity (DS), emergence (EME) and wet root weight (WRW) were determined. Isolations on selective media were done to determine pathogen prevalence. Seedling disease was more severe in cups inoculated with Pythium compared with those inoculated with Fusarium alone. No difference in disease development was observed for the Pythium species tested, but more severe disease occurred on seedlings inoculated with F. graminearum compared with those inoculated with F. oxysporum. Generally, there was little difference in disease development between treatments inoculated with Pythium alone and those co-inoculated with Pythium and Fusarium. Over time, an increase in DS and WRW was observed in Pythium+Fusarium treatments and there was no effect of time on EME. Under the conditions used in these experiments, co-inoculation with Pythium and Fusarium did not result in more severe seedling disease on soybean.