While June is the time to scout for weeds and perhaps a few pests, July is the time to scout for diseases and insects. In addition you also may see symptoms of nutrient deficiencies and general malaise of the soybean crop.
During July, you should scout for insects including the soybean aphid, Japanese beetle, bean leaf beetle and green cloverworm. Of these, the soybean aphid and Japanese beetle put soybeans at the most risk.
Aphids land on soybeans and suck out sap—this stresses the plant and reduces yield. The threshold for soybean aphids is 200 aphids per plant with moderate weather on the horizon. However a predicted hot and dry spell will knock down the population.
Japanese beetles feed on leaves and defoliate soybeans, which reduces yield. Spray with an insecticide when defoliation reaches 30% before bloom, or 20% between bloom and pod fill. Timely application of insecticides will keep these insects at bay.
Continue to monitor for signs of SCN (soybean cyst nematode) damage. Look for spots in the field that are yellow and stunted. You can scout your fields for SCN by digging plants no sooner than six weeks after planting. Dig up affected plants, going down 6 to 8 inches to get the roots. You can see adult female cysts on the root. However, use great care when handling roots because cysts will easily sluff off compared to nodules that stay attached to roots. Pull soil samples from infected areas and have them analyzed for egg count.
A number of diseases can impact soybeans in Illinois. Before you rush out to spray, determine if disease is even present and if a fungicide application is warranted. In soybeans, R1 or flowering begins around summer solstice, so by July it is time to start scouting for diseases prior to R3 (beginning pod), especially if the weather has been rainy and/or humid, which is favorable to disease development. The three diseases mainly to scout for in Illinois are white mold, frogeye leaf spot and Septoria brown spot.
White mold only appears in the northern half of Illinois and only during summers that are cool, moist and humid. White mold is most common under a closed crop canopy. Symptoms include the appearance of white cotton candy-like mold that appears at the stem nodes lower in the canopy. Leaves on infected plants will eventually turn brown, similar to sudden death syndrome. However, the presence of white mycelium is a dead giveaway for white mold.
Frogeye leaf spot appears as light gray lesions surrounded by dark borders on soybean leaves. These lesions come together to form larger irregular spots. Lesions also can appear on pods and stems. This disease can significantly reduce yields, especially when left untreated. Some biotypes of frogeye leaf spot are resistant to certain fungicides.
Septoria brown spot symptoms appear as small brownish or tannish lesions on leaf surfaces and are more common on lower leaves. Brown spot is common during wet seasons. It begins at the bottom of the plant and moves to the top. This disease can cause economic losses if left untreated, but rarely does. Observing a few brown spots on soybeans is very common and no cause for worry or action.
To help manage frogeye leaf spot and Septoria brown spot, application of a foliar fungicide is recommended. However, to control white mold, your fungicide selection needs to be more targeted as not all fungicides work on white mold.
Soybeans can exhibit nutrient deficiencies like any other crop. Iron, manganese and potassium deficiencies are three of which to be aware. Soil tests are a reliable indicator of possible potassium deficiency. However, for micronutrients rely on tissue tests. There are visual symptoms, but by the time you see them some yield will be lost. Foliar applications of manganese or iron are good management practices and multiple products are available.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at firstname.lastname@example.org