As the wheat harvest in Kansas is getting started, it's crucial to remember the diseases that can impact grain quality or seed viability. By now, most disease management decisions have been made, but it's not too late to implement strategies to mitigate losses on heavily infected fields. Moreover, it's the right time to consider variety selection and seed preparation for the next season, which can significantly influence the future of your crop.
Black point and sooty molds
With recent rains slowing the progress of wheat harvest, some areas of the state will begin to see sooty molds (Figure 1) and grain with a discoloration known as black point (Figure 2). Both of these problems are caused by molds that colonize mature wheat close to harvest.
These molds are normally not aggressive pathogens in wheat, but they can rapidly colonize mature plants. We see this issue most commonly when rain re-wets mature plants and causes harvest delays. Sooty molds are often a cosmetic problem because the mold growth is very superficial on the chaff and glumes (Figure 1). If the timing of the rain coincides with the late stages of kernel development, the molds can begin to colonize the outer layers of the wheat kernel, resulting in a gray-black discoloration called black point (Figure 2). Commonly, the embryo end of the kernel is most discolored, but entire kernels can become gray or black as a result of the black point. There is no way to manage this disease as all fungicides are long past their labeled application windows. Although not typically yield-limiting, the fungi that cause black point can cause problems with germination and may reduce seedling vigor. Seed lots with symptoms of black point should be tested for germination. If black point is causing germination problems, fungicide seed treatments can often improve the germination and ensure good stand establishment.
Figure 1. Wheat head with symptoms of sooty mold. Photo by Erick DeWolf, K-State Research and Extension.
Figure 2. Wheat kernels showing symptoms of black point. In severe cases, the entire kernel may appear darkened. Photo by Kelsey Andersen Onofre, K-State Research and Extension.
Fusarium head blight (scab)
Fusarium head blight (scab) has been reported in some Kansas counties where wet weather was present during and after flowering. This disease (Figure 3) can lead to lightweight, damaged, “scabby” kernels (Figure 4), which may contain the mycotoxin deoxynivalenol (DON). DON is sometimes referred to as “vomitoxin” because it can lead to vomiting and feed refusal in animals and is strictly regulated in finished products destined for human consumption.
Fields known to have high levels of Fusarium head blight should be harvested separately from unaffected fields when possible. It may be prudent to harvest infected fields as early as possible to avoid further accumulation of DON in grain. Producers should carefully consider whether they want to blend seed from infected and uninfected fields, as this may result in reduced quality of uninfected grain. Research has indicated that adjusting combine fan speeds may help ‘blow out’ lightweight Fusarium-damaged kernels. Fan speeds that are too high, however, may remove too much healthy grain. Additionally, blowing out scabby kernels may increase the abundance of volunteer wheat.
If you consider saving seed from affected fields, the seed should be professionally cleaned. The pathogen that causes Fusarium head blight can result in seedling disease and decreased emergence. If seed from fields with Fusarium head blight is saved, a seed treatment can improve emergence, as these scabby seeds can result in seedling blights. Planting infected seed will not result in head blight symptoms at heading in subsequent seasons.
Figure 3. Symptoms of Fusarium head blight (head scab) appear as bleached spikelets anywhere on the head and will spread through the head when conditions are favorable. When there is high humidity, orange-colored fungal structures can be seen in some infected heads. Photo by Kelsey Andersen Onofre, K-State Research and Extension.
Figure 4. Chalky wheat kernels resulting from a severe infection of Fusarium head blight. Photo by Erick DeWolf, K-State Research and Extension.
Common bunt (stinking smut)
Common bunt is typically only detected at harvest when grain appears to have a black, dusty appearance. Grain may also have a foul, “fishlike” odor. Infection actually occurs in the fall, when wheat seeds infested with spores germinate; however, symptoms are not present until the dough growth stages. Kernels infected by common bunt are dark and discolored and are filled with black spores called teliospores (Figure 5). Teliospores can be released when wheat is harvested and, in severe cases, may cause a black cloud behind the combine. Heavily infected fields can have reduced grain quality and yield. Very ‘smutty’ grain can be discounted. The best way to manage common bunt is to purchase certified, fungicide-treated seed or to have saved seed commercially cleaned and fungicide-treated.
Figure 5. Wheat infected with common bunt. Photo by Kelsey Andersen Onofre, K-State Research and Extension.
Loose smut
Loose smut can be easily identified after heading by masses of black spores that appear on heads in place of spikelets (Figure 6). By harvest, most of the black, powdery spores have been blown away by wind, leaving only the central stem of the head (rachis). Individual heads with loose smut will not produce any grain. Earlier in the growing season, spores from infected heads may have spread to neighboring plants and infected developing seeds. Cool, wet weather during the flowering period favors disease spread. Although this disease will not affect grain quality in the current season, the disease can persist within the infected kernels if the grain is saved for seed. The best option for controlling infected seeds is using a fungicide seed treatment. Coverage is key to ensuring the success of the treatment. More information about wheat seed treatment options can be found in this K-State wheat seed treatment publication: https://bookstore.ksre.ksu.edu/pubs/MF2955.pdf.
Figure 6. Loose smut symptoms caused by the fungus Ustilago tritici. Photo by Kelsey Andersen Onofre, K-State Research and Extension.
Kelsey Andersen Onofre, Extension Wheat Pathologist
andersenk@ksu.edu