Diseases can cause economic losses in Mid-South soybeans. Until the early part of this century, many diseases could only be managed with resistant varieties or with cultural practices that were marginally effective.
Fortunately, there are now preventive and/or curative controls for most major diseases of soybeans. The accompanying table lists diseases and how they can be managed, prevented, or controlled.
Several important diseases (sudden death syndrome, stem canker, phytophthora root rot, charcoal rot, seed and seedling diseases) of soybeans have no curative control.
Sudden death syndrome and stem canker can be managed or avoided by using less-susceptible or resistant varieties.
Phytophthora root rot can be managed by using resistant varieties.
There are no known resistant varieties (only moderately resistant germplasm) or fungicide controls for charcoal rot.
Seed and seedling diseases (phomopsis, pythium, phytophthora, etc.) can be managed by using seed treatments.
Fungicides can be applied to prevent several prominent soybean diseases. Preventive fungicides are most effective when applied prior to or at the earliest appearance of a disease. The general recommendation is that the first application should be made at R3 or beginning of podset.
Fungicide application during early reproductive development to prevent foliar diseases in soybeans is economical in the Mid-South.
Asian soybean rust can be managed with preventive and curative foliar fungicides timed according to occurrence of rust in sentinel plots. Based on 2005 and 2006 history, rust may be avoided in the Mid-South by planting early-maturing varieties early so that R6 or full seed stage is reached before about Aug. 1.
Scouting should be used to detect the first occurrence of disease(s) or to accurately determine the occurrence of the reproductive stage recommended for the most effective preventive fungicide application prior to disease presence.
Cost and effectiveness of fungicide products should be evaluated when choosing options for disease management. Resistant varieties should be chosen based on level of pest tolerance and yield. Resistance and/or tolerance of most soybean varieties to major diseases can be found in Mid-South states' annual variety trial results and in seed company brochures.
Up-to-date guides with appropriate management/control measures for major Mid-South soybean diseases can be found in the crops and livestock section at http://www.lsuagcenter.com and at http://aes.missouri.edu/delta/muguide/g4452.stm.