No. 20 Article 7/August 10, 2007

Late-Season Soybean Diseases Showing Up

Reports of sudden death syndrome, brown stem rot, and white mold are coming in from different areas of Illinois. All three of these diseases typically begin expressing their symptoms in the latter part of the season.

Sudden death syndrome (SDS) is caused by the soilborne fungus Fusarium virguliforme (previously known as Fusarium solani f. sp. glycines). This fungus infects roots early in the season, especially when soils are cool and wet. The fungus produces a toxin, which causes foliar symptoms later in the season. These foliar symptoms first appear as yellow spots between the primary veins of the leaves. The yellow spots coalesce to cause interveinal chlorosis (between-vein yellowing) and then interveinal necrosis (between-vein death). The veins on the leaves of affected plants remain green. On some occasions, fungal masses, which are blue in color, may be observed on the roots of affected plants; the blue fungal mass is not always present or obvious on the roots of SDS-affected plants, however.

Range of foliar symptoms on soybean caused by sudden death syndrome.

Brown stem rot (BSR) is a disease caused by the soilborne fungus Phialophora gregata. Soybean stems must be split open to observe the symptom of internal browning caused by BSR. The BSR fungus can also (but does not always) cause foliar symptoms identical to the foliar symptoms caused by SDS.

Internal browning of a soybean stem caused by brown stem rot (Photo courtesy of A. Robertson, Iowa State University).

Unfortunately, in-season control of SDS and BSR is not available, though there are soybean varieties with partial resistance to both diseases. Risk of these diseases can be reduced by avoiding planting too early, when soils can be cool and moist. Both diseases have been shown to be more severe when soybean cyst nematode (SCN) is also present in the field. In addition to helping manage SCN, planting an SCN-resistant variety may also provide some benefit against SDS and BSR.

White mold, also known as Sclerotinia stem rot, is caused by the fungus Sclerotinia sclerotiorum. This disease is found in central and northern Illinois but generally not in southern Illinois. White mold has a very unique disease cycle, and environmental conditions play the biggest role in its incidence and severity. The white mold fungus survives in the soil as black "sclerotia," which are very dense and can endure temperature and moisture extremes. The viability of these sclerotia can last for multiple years. Under the right conditions, the sclerotia germinate and form tiny mushroom structures, known as "apothecia." These apothecia produce spores (ascospores) that are discharged and become airborne. Ascospores that land on a senescing soybean petal can cause infection. The disease requires adequate soil moisture for the sclerotia to germinate and foliage wetness for the ascospores to cause infections. The white mold fungus thrives in cool, wet conditions, but it shuts down when temperatures exceed 85°F or if foliage is not wet from rain or dew.

White fungal growth of the white mold fungus on the base of a soybean plant (Bradley photo).

Although complete resistance to white mold is not available, there are soybean varieties with partial resistance. Topsin M and Domark fungicides are registered on soybean for white mold control. Topsin M has been evaluated for white mold control in University of Illinois research trials (see Table 1). In these trials, Topsin M was effective at controlling white mold, but timing was critical. Also, the yield benefit of controlling white mold with fungicides is only fully realized when disease incidence is moderately high.--Carl A. Bradley

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