Purple seed stain, also known as Cercospora leaf blight, has been positively identified on soybeans in several Kansas fields this year. This disease is most common when moisture is abundant during the reproductive stages of growth and where soybeans have followed soybeans in the rotation. While it occurs at low levels almost every year, in some years, such as 2014, it is much more abundant. Producers may wonder what effect it has on yields and seed quality.
The first signs of a problem occur about the time of seed set. Upper leaves exposed to the sun develop a distinct purplish color (Figure 1). The discoloration can deepen and extend over the entire upper surface of affected leaves, giving them a leathery, dark, reddish purple appearance. Later, reddish purple, angular- to irregular-shaped lesions may appear on both the upper and lower leaf surfaces.
When pods are infected, the disease causes a noticeable purple discoloration of the seed, as if the seed were stained with grape juice (Figure 2). Most often, only part of the seed is discolored.
A fungicide seed treatment may reduce early seed-borne infection. Planting untreated, purple-stained seed can serve as a source of inoculum to the developing crop. Plants developing from infected seed can be killed or stunted. Planting untreated, infected seed is not recommended.
The severity of purple seed stain at harvest time however, is usually not related to its presence on the planted seed. Environmental conditions at the time of flowering have the most influence on the incidence and severity of the discoloration. Conditions that favor infection at this time include high temperatures (greater than 80 degrees), rainfall, and high humidity. The fungus survives on crop residue so the disease is most likely to be a problem in continuously planted, no-till fields.
If the disease infects leaves early, there may be some yield loss -- usually less than 10 percent. Purple seed stain does not usually affect seed quality for processing, but the seed discoloration may result in a reduction in price at the point of sale.
The most effective control measures include using disease-free or treated seed and crop rotation. Using treated seed will help reduce seedling loss, but will have little or no effect on later-season infections of foliage or seed. Fungicides applied at the R3 to R5 growth stages can be used to improve seed quality and occasionally limit yield losses, but this is rarely needed in Kansas. There are no thresholds for treatment and by the time a grower realizes that treatment is needed, it is usually too late.
Varieties show differences in susceptibility to purple seed stain, but recent data on which varieties are more resistant than others is not available.