Fusarium root rot and wilt of soybean
Fusarium is a very common soil fungus, and more than 10 different species are known to cause root rot. However, the economic impact on yield is not well documented. Infected seedlings exhibit poor or slow emergence and are often stunted. Root rot appears as reddish-brown to dark brown discolored roots and poor nodulation. Foliar symptoms of Fusarium wilt include scorching of the upper leaves, while middle and lower canopy leaves can turn chlorotic and later wither and drop from the plant. Infected plants have brown vascular tissue in the roots and stems and show wilting of the stem tips. External decay or stem lesions are not seen above the soil line. Cool, wet soil favors early season infection, while hot, dry conditions favor later season symptoms.
The causal fungi survive in the soil or in plant residue. Certain weeds may serve as hosts to some pathogenic Fusarium species. Stresses such as herbicide injury, high soil pH, iron chlorosis, nematode feeding, and nutritional disorders can all predispose plants to infection. After infection, damage to plants can be worsened if soil moisture is limited because of the damaged root systems.
Reducing or eliminating stress factors, such as use of herbicides that cause injury to soybeans, wet soils, and soybean cyst nematode, can help reduce root rot problems. Growing varieties tolerant to iron deficiency chlorosis should be considered if the root rot seems associated with iron deficiency chlorosis. No resistant varieties are available, but fungicide seed treatments may protect seedlings.
Gallery Images: L. Giesler, A. Tenuta, M. Anderson, A. Sisson, and D. Mueller