No. 13 Article 6/June 18, 2004

Wet Soil Conditions Favorable for Phytophthora Rot of Soybean

The continuing wet weather in many parts of Illinois this spring is not only favorable for ducks and frogs, it also has been favorable for Phytophthora rot of soybean. Numerous problems similar to Phytophthora rot have been reported recently, and Phytophthora has been confirmed in some cases. An article in issue no. 9 (May 21, 2004) of the Bulletin discussed seedling diseases in general. Further, as noted in that article, we are still interested in getting samples of soybean seedlings (up to about V4) from Illinois for a seedling disease survey. This article summarizes key information about Phytophthora rot and the major races (and pathotypes) of this pathogen that are known to occur in Illinois.

Phytophthora rot kills soybean at all growth stages, from planting to maturity. The impact of this disease in Illinois varies from year to year and among locations, depending in part on soil drainage and the occurrence of wet soil conditions early in the growing season. A related pathogen, Pythium, causes similar symptoms under similar conditions, and it is difficult to distinguish from Phytophthora without laboratory examinations.

Symptoms. Phytophthora sojae can rot seeds prior to emergence and can cause pre- and postemergence damping off. At the V1 stage, infected stems appear bruised and are soft, secondary roots may be rotted, and plants frequently wilt and die. Older plants may die throughout the season. Dark brown lesions form on the roots, root rot develops, and a distinct dark brown discoloration of the stem may extend from below the soil line upward into the branches on older plants. Leaves also turn yellow and stay attached after plant death. In tolerant varieties or varieties with partial resistance, plants may be stunted, and lateral and tap roots may be brown and partially rotted, resulting in hidden damage that may reduce yields. Photographs of symptoms of Phytophthora rot can be found at the UIUC Field Crop Disease Web site.

Management. Phytophthora rot of soybean can often be managed with host resistance, soil drainage control, and fungicidal seed treatments such as metalaxyl (Allegiance, Gustafson) and mefenoxam (Apron XL, Syngenta Crop Protection.). Resistant varieties have been the cornerstone for management of this disease in Illinois. Phy-tophthora rot, however, remains significant in Illinois in part because host resistance is not effective in all areas. Failures of resistance in soybean cultivars with the major Rps resistance genes (Rps1a, 1c, and 1k) available commercially often have been reported by producers and crop advisers.

Races and pathotypes of P. sojae in Illinois. We recently completed a survey of races and pathotypes of Phytophthora sojae in 24 Illinois counties.

The map shows the Illinois counties (shaded) where isolates of Phytophthora sojae were obtained from soil samples for the race and pathotype survey.

Races (and pathotypes) were determined based on which of eight different soybean varieties were killed after inoculation with an isolate in the greenhouse. The differential varieties each contain a different major Rps resistance gene or allele. We identified 16 races of P. sojae in Illinois and 6 additional virulence pathotypes that did not match currently defined races. The most common races and pathotypes are shown in Table 1. This research, which was funded by the Illinois Soybean Checkoff, indicated that the major resistance genes Rps1c or Rps1k should still be effective in most fields. However, these common resistance genes are no longer effective in many other fields in Illinois.--Dean Malvick

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