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Issue No. 15, Article 7/July 16, 2010

Soybean Foliar Disease Update

Several foliar diseases can currently be seen in Illinois soybean fields. Some diseases are more likely to cause yield loss than others, so proper identification is important.

Bacterial blight. Bacterial blight, caused by the bacterium Pseudomonas savastanoi, is present in fields across the state, but it appears to be most severe this year in areas of northern Illinois. Symptoms--brown angular spots with a pronounced yellow halo--may be confused with symptoms of Septoria brown spot (see the description below), but with bacterial blight there is a very pronounced yellow halo around each spot, whereas Septoria brown spot exhibits just general yellowing. In addition, bacterial blight generally is observed on the upper leaves, whereas Septoria brown spot is generally observed on the lower leaves first.

Symptoms of bacterial blight on a soybean leaflet.

Common foliar fungicides are not effective at controlling bacterial blight, so proper identification is important. Some products containing copper are labeled for control of bacterial blight; they must be applied very early in the disease cycle-typically before symptoms are observed-to be effective. Bacterial blight is not generally considered an economically important disease, and yield loss should be minimal.

Downy mildew. Downy mildew, caused by the Oomycete pathogen Peronospora manshurica, can be found in some fields this year but does not appear to be widespread in Illinois. Symptoms are pale yellow spots on the upper leaf surface; "tufts" may appear on the lower leaf surface under the yellow spots as the disease progresses. Foliar fungicide labels do not claim any control of downy mildew. The best management practices are planting resistant varieties, rotating crops, and avoiding planting seeds infected with the pathogen. Similar to bacterial blight, downy mildew generally is not considered economically important, and yield loss caused by it should be minimal.

Symptoms of downy mildew on the upper surface of a soybean leaflet.

Symptoms of downy mildew on the lower surface of a soybean leaflet.

Septoria brown spot. Septoria brown spot, caused by the fungus Septoria glycines, can be found in almost every field, generally every year. Symptoms are brown spots on the leaves that eventually coalesce to form large lesions; entire leaflets turn chlorotic and defoliate. Septoria brown spot is observed first on lower leaves, and the pathogen is spread to the upper leaves by splashing rain. In general, Septoria brown spot only becomes severe enough to cause yield losses when it causes defoliation in the upper canopy, and this generally occurs only when rainfall is frequent in July and August.

Symptoms of Septoria brown spot on soybean.

Frogeye leaf spot. Frogeye leaf spot, caused by the fungus Cercospora sojina, can be found in specific fields across the state. Frogeye leaf spot has been reported as observed in several southern Illinois counties and as far north as DeKalb County so far this year. In general, conditions have been favorable for this disease, with warm temperatures and adequate dew and rainfall in some areas. Symptoms are circular to oval tan spots with dark purple borders. Soybean varieties that have the Rcs3 gene for resistance to frogeye leaf spot appear to be resistant to all races of the disease that occur in Illinois. Varieties that are susceptible to frogeye leaf spot should be scouted at the R1 to R2 stages. If frogeye leaf spot is present, consider applying a foliar fungicide at the R3 stage. A summary of soybean foliar fungicide trials conducted over several years by University of Illinois and Southern Illinois University researchers indicate that when frogeye leaf spot pressure is moderate to high, the average yield response to foliar fungicides is 9 bushels per acre (compared to 3 in low-pressure environments). Foliar fungicide products containing a strobilurin fungicide active ingredient (azoxystrobin, fluoxastrobin, pyraclostrobin, or trifloxystrobin) appear to be the most effective in controlling frogeye leaf spot.

Symptoms of frogeye leaf spot on soybean.

Soybean rust. Soybean rust, caused by the fungus Phakospora pachyrhizi, seemed to have a late start this year in the southern U.S., which greatly reduces the risk of its causing yield losses in Illinois this season. Soybean rust is monitored throughout the U.S. through a network of sentinel plots. The most current information can be found at the Soybean Rust PIPE website (sbr.ipmpipe.org).

Screen capture from the Soybean Rust PIPE website (sbr.ipmpipe.org) showing the status of soybean rust in North America on July 14.

--Carl A. Bradley

Carl A. Bradley

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