Fusarium is a very common soil fungus, and more than 10 different species are known to infect soybean roots and cause root rot. The species Fusarium oxysporum is responsible for causing Fusarium wilt. Although Fusarium root rot is a widespread disease in the United States, the economic impact on yield is not well documented.
Symptoms of Fusarium wilt are more noticeable under reduced moisture and hot conditions and are often misdiagnosed as those of Phytophthora root rot. Infected plants have brown vascular tissue in the roots and stems and show wilting of the stem tips. However, external decay or stem lesions are not seen above the soil line. Foliar symptoms include scorching of the upper leaves, while middle and lower canopy leaves can turn chlorotic and later wither and drop from the plant.
Young plants are at the greatest risk to root rots caused by Fusarium species. Infected plants may exhibit poor or slow emergence, and seedlings are often stunted and weak. Seedlings with root rot have reddish-brown to dark brown discolored roots. Infected plants may have poor root systems and poor nodulation, which may cause the plants to wilt and die.The fungus survives in the soil either as spores or as mycelium in plant residue. Certain weeds may serve as hosts to some pathogenic Fusarium species. The fungi can infect plants at any stage of soybean development but infection is particularly favored when plants are weakened. 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 compromised root systems.
The best time to scout for this disease is VE through R6; after frequent rains. Be sure to look closely at roots as they often are missing many lateral roots.
Variety selection: Varieties have varying levels of susceptibility, but no resistant varieties have been described.
Stress factors: 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.
Seed treatments: If Fusarium is a problem in a field, fungicide seed treatments may protect seedlings in subsequent years.
Planting date: Plant soybeans in problematic fields when soils are warmer.
Photos by Loren Giesler