Fusarium Wilt or Blight in Soybeans

Fusarium wilt or blight of soybeans can affect soybeans at any stage of development and is very apparent in southwest. It is caused by the very common soil-borne fungus, Fusarium oxysporum which is different to Fusarium virguliforme (previously-Fusarium solani f. sp. glycines) which is responsible for Sudden Death Syndrome (SDS). Much of what has been called SDS has indeed been general Fusarium wilt so be careful. The dry conditions until recently have not been ideal for SDS but it could still occur in fields with a history of SDS.

Fusarium oxysporum causes a wilting of the soybean plant and is often misdiagnosed as Phytophthora root rot/ wilt as well. Affected plants have a wilting of the stem tips and the upper leaves are scorched. The middle and lower leaves can turn yellow or have pale (dull) yellow spots. In severe cases the leaves will dry up and drop prematurely leaving the petiole behind. The leaf symptoms are different from SDS or brown stem rot where distinct yellow speckling or spotting between the veins (interveinal chlorosis) occurs and as the disease progresses these areas become larger and in severe cases the entire area between the veins becomes brown (necrotic) and dry.

Unlike Phytopthora there also is no evidence of a stem lesion or external decay that goes above the soil line. Cutting the roots and stems lengthwise will reveal a browning of the vascular tissue and pith. This may be confused with early-season brown stem rot. Fusarium infested roots often have red, orange or white mycelium visible.

Why so much this year? Fusarium wilt and other root rot diseases are most problematic when soybeans are under water and root stress. In addition. Soybean cyst nematode can further stress an already compromised plant and allow easy access for many soil-borne root pathogens. Stress can interfere with the normal activity of the plant's root system and therefore, will affect growth and potentially increase root diseases. Stunting, overall poor growth and wilting are all symptoms of root stress. Root stress will also often resemble nutrient deficiencies.