You May Not Like the Rain, but Water-Molds Do

June 19, 2019
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By:  Stephanie Karhoff, Ohio State University

Late-seson field symptoms of Phytophthora root and stem rot on a susceptible soybean variety.

A rare appearance from the sun allowed farmers in Northwestern Ohio to progress with soybean planting, but with more rain in the forecast (see the most recent weather outlook in the C.O.R.N. newsletter here), there is concern that early-season seedling diseases may develop.

The two main diseases of concern are Phytophthora root and stem rot and Pythium seed and root rot. Phytophthora root and stem rot is caused by Phytophthora sojae, and Pythium seed and root rot is caused by multiple species of Pythium. Both of these soybean pathogens are water-molds, and as the name implies, thrive in wet conditions, which allow their spores (called zoospores) to move towards and infect soybean roots.

Phytophthora root and stem rot occurs most commonly in high-clay soils after early-season rainfall events when soil temperatures are above 60 degrees Fahrenheit. Early symptoms include rapid yellowing and wilting, followed by root rot and ultimately reduced stands. These symptoms may be confused with flooding injury. The easiest way to differentiate the two is to look at the color of the roots. White roots with a rat-tail appearance indicate flooding, and brown to black roots indicate Phytophthora. The main management strategy is variety selection. The two types of resistance available for Phytophthora root and stem rot are Rps genes and partial resistance (often referred to as tolerance by seed companies). In areas with a history of Phytophthora root and stem rot, it is best to select a variety with a combination of both types of resistance, and use a metalaxyl, mefanoxam, or oxathiapiprolin seed treatment.

Infection of seeds and seedlings by Pythium species lead to similar symptoms as Phytophthora sojae, and Pythium species are typically part of the seedling disease complex (i.e. multiple seedling pathogens infecting the soybean). Cool, wet planting conditions favor development of Pythium seed and root rot.

For more information about soybean diseases, check out the OSU Soybean Disease Website at https://u.osu.edu/osusoybeandisease/.