By Adriana Murillo-Williams and Paul D. Esker er.al
What are the conditions that favor stem diseases?
Weather drives plant disease development. While many regions in Pennsylvania continue experiencing a precipitation deficit that keeps soybean foliar diseases at bay, the situation differs for stem diseases, with reports of sudden death syndrome, anthracnose, and white mold in different parts of the Commonwealth. If conditions have been so dry, why are these diseases occurring? Microclimate variability can be very high, even within the same field, and conditions within the canopy may vary greatly in terms of temperature and humidity compared to open air. Therefore, a single shower or a thunderstorm may provide the right conditions for infection and disease development.
Sudden death syndrome (SDS)
During the Spring, many areas around Pennsylvania received above-average precipitation. The pathogen (Fusarium virguliforme) survives from season to season in the form of survival structures called chlamydospores. It infects seedlings through the roots early in the season when conditions are cool and wet. Infection is favored by soybean cyst nematode (SCN) infestations. Although infection occurs in the Spring, the symptoms develop during the reproductive stages and start as an interveinal chlorosis that becomes necrotic, leading to leaf death (Figure 1).
Anthracnose
The causal agent (Colletotricum truncatum) is seedborne, survives in crop debris, and may infect different plant parts anytime during the growing season when conditions are warm and humid (12 or more hours of free moisture) (1). Initial symptoms include brown spots on the stems and petioles.
White mold
Sclerotinia stem rot, or soybean white mold, is caused by the fungus Sclerotinia sclerotiorum. The fungus infects the stems through senescing flowers and other plant parts. Infection occurs when conditions are moist and cool and are favored by canopy closure. For disease development, canopy temperatures of <82°F and 42-72 h leaf wetness are required (2). Initial symptoms include a gray to green discoloration of the upper leaves, which turns necrotic, leading to death of the upper leaves. At this point, symptoms on the stem can be observed on the lower nodes.
Source : psu.edu