Home | Past Issues

Issue No. 19, Article 3/July 31, 2009

Get to Know the Common Foliar Diseases of Soybean

Some foliar diseases of soybean have been present for the past several weeks, and if frequent rainfall continues, disease spread likely will continue as well. Several of the most common foliar diseases of soybean are described here.

Bacterial blight. Bacterial blight, which is very common every year, is already present in Illinois soybean fields this season. Symptoms are small angular lesions on the leaves surrounded by yellow halos. Some soybean varieties have good levels of resistance to bacterial blight, while others are susceptible. A bacterial blight race survey conducted in Illinois in 2007 indicated that race 4 was the most prevalent in the state, and most soybean varieties are susceptible to it. In general, bacterial blight is not considered to cause much yield reduction to soybean in a typical year.

Bacterial blight on soybean. Note the yellow halos around the spots.

Bacterial pustule. Bacterial pustule generally is less common than bacterial blight in Illinois soybean fields. However, bacterial pustule observations have already occurred this year. Symptoms of bacterial pustule are very similar to those in bacterial blight--angular lesions on the leaves with yellow halos--except that pustules are also formed. Because it can be difficult to see the pustules with the naked eye, a magnification device (hand lens, microscope, etc.) is needed for diagnosis. The pustules formed by the bacterial pustule pathogen look very similar to those formed by the soybean rust pathogen, except that with bacterial pustule, pustules can be on both the upper side and underside of the leaves. With soybean rust, pustules are formed on the under side of the leaves only.

Bacterial pustule symptoms on soybean.

Magnification of bacterial pustules on a soybean leaflet.

Septoria brown spot. Septoria brown spot is observed every year in almost every soybean field in Illinois. Because of the frequent rainfall this year, the disease is already widespread. Symptoms are observed as brown spots on the leaves, which coalesce as time goes on. Leaf yellowing begins to occur, and eventually affected leaves turn completely chlorotic (yellow) and fall off prematurely. In most years, Septoria brown spot only causes defoliation of the lower leaves, and no resultant yield loss. However, in years with frequent rainfall in July and August, yield loss can occur.

Septoria brown spot on soybean.

Soybean rust. Soybean rust has been observed in Illinois every year since 2006; however, all of these observations occurred very late in the season, and yield was not affected. Symptoms of soybean rust are small, dark, circular lesions on the leaves, with pustules present on the underside of the leaves. These pustules are extremely difficult to recognize with the naked eye, and magnification is needed to diagnose soybean rust. Any suspicious leaves should be sent to the University of Illinois Plant Clinic for diagnosis. Information on the monitoring effort for soybean rust in Illinois and in North America was published in issue 13 of the Bulletin (June 19).

Magnification of soybean rust pustules on the underside of an infected soybean leaf.

Frogeye leaf spot. In some years, frogeye leaf spot can be observed across the entire state, but it is most common in southern Illinois. Frogeye leaf spot symptoms are circular tan to gray spots on the leaves, surrounded by very pronounced dark purple margins. Under severe disease pressure on susceptible varieties, these spots can also be found on the stems and pods. Varieties differ in their susceptibility to frogeye leaf spot. Research has indicated that the Rcs3 gene for resistance is effective against all races of the disease present in Illinois.

Frogeye leaf spot on soybean.

Cercospora blight. Cercospora blight can be observed throughout the entire state in wet years. Cercospora blight symptoms appear as leaves that are discolored purple to dark maroon. Affected leaves can defoliate prematurely, causing yield loss under severe disease pressure. The fungus that causes Cercospora blight also causes purple seed stain of soybean seed.

Soybean leaflet affected by Cercospora blight.

Downy mildew. Downy mildew is another common soybean disease found frequently in Illinois. It appears as light yellow "blotches" on the upper leaf surface, with gray "tufts" appearing on the lower leaf surface directly under the yellow blotches. In general, downy mildew is not considered to cause much yield loss of soybean in our state.

Upper soybean leaf surface affected by downy mildew.

Foliar fungicides will protect against Septoria brown spot, soybean rust, frogeye leaf spot, and Cercospora blight, but not against the other diseases just described. In University of Illinois and Southern Illinois University foliar fungicide trials on soybean (conducted by Dr. Jason Bond), the biggest yield benefits have been observed when moderate to high disease pressure is present (an average yield benefit of 3 bushels per acre in low disease pressure vs. 9 bushels per acre in moderate to high disease pressure).

Comparison of average yield responses from foliar fungicides in Southern Illinois University and University of Illinois research trials with low disease pressure versus moderate to high disease pressure on susceptible varieties (data courtesy J.P. Bond and C.A. Bradley).

Appreciation is extended to the Illinois Soybean Association for providing funding to conduct foliar fungicide research trials throughout the state.--Carl A. Bradley

Carl A. Bradley

Click here for a print-friendly version of this article

Return to table of contents