Epidemiological studies on the infection process and symptom expression of soybean sudden death syndrome

Sudden death syndrome (SDS) caused by Fusarium virguliforme (Aoki, O'Donnel, Homma & Lattanzi) is one of the most important soybean (Glycine max (L.) Merr.) diseases in the US. Management strategies currently available for this disease are not always effective, partly due to the high variability in symptom expression that occurs in field environments. To clarify the relationship between progress of root rot and foliar symptoms, soybean seedlings were inoculated at five inoculum densities and were destructively sampled over a 50 day period. Disease severity and area under disease progress curve (AUDPC) increased in response to increasing inoculum density (P < 0.01), particularly for foliar symptoms. Root rot severity evaluated 15 to 30 days after inoculation (DAI) was most highly correlated (r >0.8, P < 0.01) with foliar severity at 40 and 50 DAI, while weak correlations were found when roots and leaves were assessed simultaneously. Rate of disease progress generally increased as inoculum densities increased for both root and foliar symptoms. Root biomass was reduced by up to 80% at the three highest inoculum densities. To study the effect of plant age on symptom expression, plants were sown at intervals over a five week period to obtain plants at different stages of development at the time of inoculation. Plants were incubated in growth chambers to 17°C for 7 days and then to 24°C, and assessed for root rot and foliar severity at 18 and 38 DAI. Root rot developed on plants inoculated at all ages, but plants inoculated 0 days after planting (DAP) had the highest (P < 0.01) root rot severity compared to plants inoculated at older ages. Foliar symptoms were severe on plants inoculated 0 DAP, but never developed on plants inoculated at all other ages. Fungal colonization of the xylem was more frequent (56%) in plants inoculated 0 DAP than on plants inoculated at later stages (2-14%). These findings show that soybean roots are susceptible to infection at different vegetative growth stages but that plants become less susceptible to xylem colonization and development of foliar symptoms as they mature. The interaction between plant age at inoculation and soil temperature was studied in greenhouse conditions. Soybeans were grown at 17, 23 and 29°C and inoculated at 0, 3, 7 and 13 DAP. Root rot developed in all inoculated plants, but severity decreased with increasing temperature and root age at inoculation. Foliar symptoms also became less severe at warmer soil temperatures, but severity was greatly dependent on plant age at inoculation. At 17°C, plants inoculated at all ages except 13 DAP developed foliar symptoms, while at 29°C only plants inoculated 0 DAP showed foliar symptoms for the duration of the experiment. Plants inoculated 0 DAP showed severe root rot and foliar symptoms at all temperatures. Root growth rate and root length were negatively correlated with root rot AUDPC and root rot rate, and positively correlated with root dry weight. This suggests that accelerated root growth in warm soils restricts xylem colonization and reduces the window for infections conducive to foliar symptoms. The fact that temperature did not affect disease severity on plants inoculated 0 DAP, may explain why delayed planting may not prevent severe epidemics if infection occurs shortly after planting.