Crop Scene Investigation - 6 Solved: Fusarium wilt was behind Blake's dying soybeans

Thursday, January 31, 2008

by DEAN SHANTZ

Fusarium wilt (also called soybean blight) was the culprit causing patches of dying soybean plants throughout Blake's field.

The disease is caused by a common soil-borne fungus and is predominant when soybeans are under drought stress and root development is inhibited. Both factors can interfere with the normal activity of the plant's root system and make it vulnerable to root diseases. Two key stressors were present in Blake's field - soil compaction and lack of moisture.

Fusarium wilt lives up to its name by causing the soybean plant to wilt. It shares this characteristic with Phytophthora root rot, which also exhibits a droopy appearance. But there are two clues that help to rule out Phytophthora.

Typically, a plant affected with Phytophthora will die, but the leaves will stay on. In severe cases of Fusarium wilt, the leaves will fall off the plant and only the petiole (stem) will remain. The stem provides another big hint in evaluating the difference between these two diseases. The stems of plants affected with Phytophthora exhibit lesions or decay above the soil line. Plants with Fusarium wilt, however, lack any sign of above ground stem damage.

Sudden Death Syndrome (SDS) is also often mistaken as Fusarium wilt. The diseases are similar as they both cause leaves to turn yellowish. The key difference is that SDS has a distinct spotting or speckling between the veins known as interveinal chlorosis. Fusarium wilt doesn't distinguish between the vein and the rest of the leaf.

Blake and I talked about how the similarities of these diseases highlight the importance of digging a little deeper before jumping to a diagnosis. We also cleared up the misconception that root rots are only seen in a really wet year. Drought hurts too.

Since we can't control how much (or how little) moisture Mother Nature provides, we concluded that overall good agronomics is the best management strategy when it comes to Fusarium wilt. This includes using certified seed that has been treated to protect against disease. Planting into good soil conditions at the proper depth is also necessary to control seedling stress. Finally, teaming up with an agronomist is a way to keep grower stress to a minimum, too.

Congratulations to Jonathan Zettler, Teeswater, ON, for his correct answer. BF

Dean Shantz, CCA, is a Field Agronomist for NK® Brand, Syngenta Seeds.