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Symptoms of foliar disease are becoming apparent in soybean fields. Soybean product susceptibility, field history, and weather conditions all factor into the incidence of foliar diseases of soybean, so scouting is important. Listed below are some brief descriptions of common soybean foliar diseases.
Septoria Brown Spot (Figure 1). Symptoms are generally mild during early growth stages. Small, irregular, dark-brown spots occur primarily on leaves. Symptoms occur first on lower leaves during warm, wet conditions and then progress to the upper leaves. Tissue surrounding lesions may be yellow. Late in the growing season leaves become rusty brown or yellow and drop prematurely.
Figure 1. Septoria brown spot in soybean.
Frogeye Leaf Spot(Figure 2). Circular to angular spots appear on leaves. Lesions begin as water-soaked spots and turn into brown spots with dark, reddish brown margins. The centers of older lesions usually appear gray. Lesions may occur on stems and pods. Infected seeds often have obvious gray or brown discoloration.
Figure 2. Circular spots caused by frogeye leaf spot in soybean.
Cercospora Leaf Blight (CLB) (Figure 3). CLB is typically first noticed when the upper leaves in the canopy turn yellow, usually with a bronze tint. “Bronzing” is the result of a multitude of lesions that have coalesced. When symptoms are severe, the upper surfaces of affected leaves have a puckered, leathery appearance. Severely affected leaves are blighted and eventually drop off the plant. Diseased petioles may remain attached to the stem.
Figure 3. Leaf infection of Cercospora leaf blight.
Pods are also commonly affected and both pods and seeds may have a purplish discoloration. This is known as “purple seed stain”. Extensive purple seed stain can reduce grain marketability. Delayed germination may also occur when seed discoloration exceeds 50% of the seedcoat1.
When weather conditions are favorable for disease development and incidence of disease is high, a foliar fungicide application may be beneficial. Fungicide applications in soybeans are generally not needed in the early vegetative growth stages (VE through V6). Fungicide applications for late-season diseases are generally made between R3 and R5 growth stages (pod development stages) and are the most critical period for seed yield. Leaf loss can significantly reduce yield if diseases attack during early seed filling. Spraying fungicides after R6 is generally no longer necessary and not recommended. The length of time foliar fungicides are active ranges from 14 to 21 days.
Sources: Septoria Brown Spot. University of Minnesota Extension. http://www.extension.umn.edu. Westphal, A., Abney, T.S., and Shaner, G. 2006. Diseases of soybean: frogeye leaf spot. BP-131-W. Purdue University Extension. http://extension.purdue.edu Cercospora leaf blight and purple seed stain. University of Minnesota. http://extension.umn.edu. Websites verified 06/20/2018. 180701153944