Crops: Crop Scene Investigation - 15 Solved: What caused wilted patches in Paul's soybeans?

Tuesday, February 3, 2009

by DEB CAMPBELL

White mould, also known as sclerotinia, caused the wilted patches in Paul's soybeans.

It's fairly easy to make a visual diagnosis of this disease because of the white, cotton-like growth that develops on the affected plant's stem. Stems and pods infected with white mould can also appear pale brown and water-soaked. Frequently, small black bodies (sclerotia) can be seen on the stems of diseased plants. Plants are generally killed in patches late in the growing season.

Paul's field had a number of conditions that put his field at risk of white mould, including planting in narrow row widths, a high seeding rate and high soil fertility. These factors typically contribute to dense soybean canopies, but can also lead to the development of white mould when growing conditions are right.

The disease is most prevalent when cool, wet conditions occur during flowering or near harvest. Dense canopies hold moisture and therefore stay wetter for longer periods of time. Along the tree lines and in other sheltered areas of Paul's field, the disease was more prominent because there was less air movement. 

Somewhere along the line, Paul had an excellent crop of soybeans – very thick, very heavy, well podded, and a high population. The truth is you don't get white mould in a poor crop; you get white mould in a good crop. The thicker the crop, the less air movement and the greater the chance for diseases like white mould.

The fact that Paul used conventional tillage also increased his risk. With reduced tillage, the sclerotia of the white mould fungus are positioned in the upper soil profile and will die faster than sclerotia buried at greater depths. Deep burial tends to slow the mortality of sclerotia. Tillage activity is also likely to spread sclerotia within the field.

The impact on Paul's yield was significant because the disease developed early in the crop – the R4 stage – and because of the substantial size of the crop as well as the affected areas. 

To avoid white mould in the future, it is key to select soybean varieties with good white mould tolerance. Varieties with greater standability also reduce risk of mould developing. In addition, plant under a no-till system and in a wider row width – a 15 inch row width certainly helps to maintain the air movement in the canopy to keep it drier. Paul should also avoid planting host crops such as canola, edible beans, buckwheat and sunflowers for at least three years. Congratulations to Larry Davis, Burford, for his correct answer. BF

Deb Campbell, CCA, is a Field Agronomist for NK® Brand, Syngenta Seeds.