Development and Symptoms
Figure 1. (left) Browning discoloration of vascular system inside the stem from Fusarium wilt (right) White mycelium forming on outer surface of roots infected with Fusarium. Infected roots can also have red or orange mycelium. 
Figure 2. Area of field with severe wilting following drought and the same variety in unaffected areas in background.Fusarium is one of the most common soil borne diseases, mostly due to its ability to survive as mycelium in plant residue and spores in soil. Fusarium wilt can be caused by a complex of multiple soil-borne fungi, although is most often associated with Fusarium oxysporum. Fusarium is common across soybean production areas with over ten different species known to cause root rot. Root infection normally occurs in soybean early vegetative and reproductive stages and is often associated with cooler, wet soil conditions, but may occur at any growth stage. Vascular tissue inside the stem will begin to turn brown in color and continue to deteriorate as infections progress. The outer surface of roots can have a red, orange or white mycelium form on them (Figure 1). As the disease progresses, the upper leaves often show a scorched appearance and stem tips begin wilting (Figure 2). Leaves in the middle and lower canopy often show yellow spots. Wilting is a signal that vascular tissue is severely compromised and no longer able to supply sufficient water and nutrients to the leaves and stems, which is often exacerbated by drought conditions, further limiting available water.
Management
By the time symptoms are visible, there is little that can be done in that season, although several management options can reduce the risk of future problems. Seed applied fungicides create a protective layer that can reduce early season infection around young seedling roots.
Fusarium resistant soybeans are not available, although choosing varieties with defensive traits against other pests can indirectly reduce Fusarium infections. Root damage from soybean cyst nematode (SCN) feeding often serves as a point of entry for pathogens such as Fusarium. Addressing SCN problems with resistant varieties and nematicide seed treatments can help suppress nematodes and indirectly Fusarium. Fusarium is often the result of multiple compounding stressors that weaken the plant, making it more susceptible to infection. Poorly drained soils, compaction, iron deficiency chlorosis and herbicide injury are examples of stress that may predispose soybeans to Fusarium infection. Managing these stressors can often indirectly reduce future risk of Fusarium wilt.
Figure 3. Irregular patterns of wilting across field. Improved drainage over tile lines and on elevated areas of field were less affected.Easily Confused Diseases
Fusarium wilt may be easily mistaken for other diseases with similar symptoms. Correctly identifying the disease can be an important first step since management options can be different depending on the pathogen. The following chart can help rule out some diseases. For example, Fusarium wilt can look similar to Phytophthora at first glance but when examining exterior stems closely, you will see dark brown lesions extending up several nodes on the stem with Phytophthora infection, whereas stems with Fusarium wilt will look healthy.
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