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Early-Season Foliar Diseases--Soybean

June 15, 2001
Many areas of the state have received a lot of rain over the past week. Blowing wind and rain help move pathogens from plant to plant. Wet, cool soil in flooded fields increases sporulation and movement of many soilborne pathogens. Recent weather conditions will probably increase the likelihood of infection by foliar and soilborne pathogens in the coming weeks.

Septoria leaf spot: Robert Bellm reported that the lower leaves of 6-inch-tall beans in Shelby County are getting brown spots, turning yellow, and falling off. These symptoms should not be a reason to replant. They are probably caused by the fungus Septoria glycines, the causal organism of brown spot on soybeans. Symptoms of Septoria leaf spot, also known as brown spot, appear as dark brown spots that are irregular in shape on the upper and lower surfaces of the unifoliate leaves. However, the pathogen can move upward into the plant canopy during periods of warm, moist weather, like last summer, which favored sporulation of the fungus. Small, brown fruiting bodies of the fungus, called pycnidia, are visible in the dead tissue of old lesions. Typically, this disease does not cause economic damage, but under conditions of continuous cropping of soybeans, infection by this pathogen can be severe and cause premature defoliation. However, crop rotation practices reduce the severity of this organism, but brown spot may still be observed on plants in the field.

Septoria leaf spot or brown spot.

Most of the previous disease articles in the Bulletin this season have discussed root rots caused by soilborne pathogens such as Pythium, Phytophthora, Rhizoctonia, and Fusarium. Recent weather conditions have increased the potential for infection by these organisms, and reports are slowly coming in. Mike Roegge reported poor soybean stands in a field that had been dry until last week, when it received 3 to 5 inches of rain in an hour. Some herbicide injury is present, but the roots appear to have a discolored, dry, sunken lesion on the taproot that appears to look like that caused by Rhizoctonia or Fusarium. You will recall that root rots caused by Pythium or Phytophthora organisms are wetter in nature. The photo below shows the discolored, dry, sunken lesion on the soybean taproots.

Soybean taproot with dry, sunken lesion.

Cool, wet soils and early-planted (late April and early May) beans make ideal conditions for the development of another soilborne pathogen, Fusarium solani f.sp. glycines, the causal organism of sudden death syndrome (SDS). Walking my research plots last week, I observed early stages of SDS as chlorosis of interveinal tissue on seedling leaves. The soybeans in my plots were in areas intentionally inoculated with the SDS pathogen. Soybeans were in the VC to V2 stages of growth. This week I noticed more definite interveinal chlorosis and necrosis of leaf tissue consistent with SDS. Random plants in the control plots that are not inoculated have some symptoms of SDS caused by natural populations of the pathogen. This pathogen has been known to cause foliar symptoms on young seedling plants; however, they often disappear as plants develop and soils dry out and then reappear in mid- to late August as plants mature. Please let me know if you notice SDS symptoms on your seedling beans. It is too late to implement any control measures, but hopefully weather conditions will allow the roots to grow well and regenerate sufficient root mass to replace any that may be lost due to infection by this organism. Just how early-season infection will affect yield is yet to be seen.

Symptoms of SDS on soybean seedlings.

When checking for symptoms of SDS, keep in mind that SDS occurs more frequently in fields under high production, in wet and compacted areas, and in fields with high populations of soybean cyst nematodes, Heterodera glycines. Research has shown that the soybean cyst nematode (SCN) is not required for infection by the SDS pathogen. The presence of SCN appears to increase plant stress when both organisms are present, which in turn can increase the severity of foliar symptoms compared to those caused by the SDS pathogen alone.--Loretta Ortiz-Ribbing

Author: Loretta Ortiz-Ribbing

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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