No. 22/August 29, 1997
Urgent!! Urgent!! Urgent!! (Anthracnose)
We have over the past few days received calls concerning yellowing and dying of the uppermost portions of soybean plants throughout entire fields. These calls have come from much of the state, particularly western and northern Illinois.
The basic symptoms observed are a discoloration of the leaf margins, followed by entire leaves turning yellow or in some cases actually drying up and dying. The disease is limited to the uppermost leaves and the terminal bud area and does not appear to be economically damaging because it affects only the top leaves.
Soybean anthracnose infections throughout Illinois are much more common along the stems as beans begin to mature. The spores are windblown from crop debris into the upper or lower canopy. The fruiting structures of the causal fungus produce clusters of small hairlike projections (setae) that protrude from the stem or other infection points and can be seen with a hand lens or a microscope. These setae are diagnostic for fungi in the genus Colletotrichum.
However, another phase of this disease is known as tipblight. This phase is characterized by the upper leaves and pods turning yellow prematurely or turning brown and dying. Other symptoms include death of major veins, leaf rolling and necrosis (death), petiole (leaf stem) cankering, and stunting of plants. Entire fields are affected, and the damage is usually seen after rainy periods. Premature defoliation caused by the fungus girdling and killing the petioles may cause economic losses. Affected plants and the entire field may senesce prematurely, and seed size and number can be reduced.
Plants are susceptible to anthracnose infection in all stages of development, particularly from bloom to podfill. Warm, moist weather favors stem and pod infections. Spores of this fungus cannot survive drying. Five hours of air-drying can reduce spore viability by 98 percent.
Control of anthracnose infections usually is needed only in seed-production fields. Spraying with commercial fungicides is helpful in reducing losses; but, at this time of year and with the limited extent of damage, I do not believe this will be economical in a grain-production field. Although the disease may appear to be quite extensive, a close examination of plants should show that only a small portion of the upper areas is affected and the rest of the plant should be fully productive.
H. Walker Kirby, Department of Crop Sciences, (217)333-8414