Year in and year out, soybean growers across Missouri and surrounding states have no shortage of pests. In 2019, in addition to normal challenges, weed control was more difficult due to compressed windows for timely herbicide application. On the other hand, insect and disease pressure was lower than expected. That’s not to say that many growers did not benefit from measures taken to control insects and diseases. However, 2019 may be one to remember due to an emerging threat that gained a foothold in our area: white mold.
For two consecutive years, this fungal disease has been detected in northwest and central Missouri. Perhaps the most serious late-season soybean disease in much of the upper Midwest, white mold proliferates in cool, moist conditions, dense canopies and high-yield environments. Yield losses are estimated at 2 to 5 bushels for every 10% of plants infected with the disease. White mold infections can commonly exceed 50% in areas where the disease is well established. It overwinters as sclerotia in the soil and persists for long periods, making crop rotation ineffective at reducing disease potential.
Scouting for white mold is more difficult than foliar diseases. The first sign of white mold development is not on the plant but on the soil. Overwintering sclerotia develop into mushroom-like structures called apothecia. These structures appear at or near canopy on the soil and produce spores that infect the soybean plant.
The second sign of development is lesions below the soybean nodes where white, fluffy mycelial growth occurs, giving white mold its name. The mold appears after spores from the apothecia infect the soybean plants through the flowers.
The last symptoms are the most visible—wilting of soybean leaves and plant death—but these issues show up far too late to indicate the need for treatment.
Being primarily a stem disease, white mold is difficult to control with fungicides. Disease hiding deep within the soybean canopy is difficult to reach. Further compounding the issue is the fact that many fungicides that control foliar diseases are ineffective against white mold, making product selection critical. Common fungicide practices to control white mold are a triazole application at R1 (early flowering) followed by another application at R3.
To ensure fungicide applications are effective, first understand that not all triazoles are created equal. Certain products are much more effective against white mold than others. Selecting a fungicide active ingredient is just as important as choosing the class of chemistry. Including the herbicide Cobra at the R1-R2 application of fungicide will improve white mold control. Cobra activates systemic acquired resistance in soybeans, which means it sensitizes the plants to white mold and allows them to trigger defense mechanisms quicker.
Other management practices for white mold often have to be incorporated with chemical control to be effective. Chief among those is canopy management. While lush, dense canopies provide several benefits for weed control, they provide an ideal home for white mold to develop and a physical barrier between the disease and applied fungicides. Fields with known histories of white mold are best suited for 30-inch rows, allowing fungicides to cover the stems more effectively and air flow to slow white mold progression.
Similarly, variety selection becomes important, not just for plant resistance but also plant architecture. While bush-type varieties are often considered more desirable than pole-type soybeans, the opposite is true if white mold management is a driving factor. A pole variety provides fewer branches, more upright growth and less canopy cover.
Prevention is one of the best management plans for white mold, especially without widespread infestations. Sclerotia can be transported from field to field, by tillage equipment and combines, in residue and in soil. Thorough equipment cleaning between fields may be worthwhile.
While we don’t know to what extent we will have to deal with white mold in the future, we do know it now resides in Missouri. Going forward, control starts with a good integrated pest management plan, considering all practices and the threat level of a particular field. It’s paramount to understand the biology of the pest to include cultural, biological and chemical strategies at the appropriate time. Our biggest challenge as new pests develop is to understand the limitation of our current control systems and determine when something new needs to be incorporated. Increasing understanding of white mold is the first necessary step
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