2005 Year in Review

From the Fields

Soybean disease-management programs need to start with selecting a variety with resistance to the common diseases for the area. Phytophthora root rot (PRR) is considered by many to be the second most yield-reducing disease of soybeans in the United States. High-yielding public and private varieties are available with resistance.

The impact of this disease in the state varies from year to year depending greatly upon the occurrence of wet weather early in the season. In addition to wet weather, other conditions that favor the disease include low, poorly drained areas, clay and compacted soils. This fungal disease attacks and kills soybeans from planting to harvest.

There are three key ways to manage PRR: improve field drainage, plant high-quality seed resistant to the disease, and use fungicidal seed treatments.

In managing PRR, there is the option of race-specific or field resistance (partial resistance) seed sources. Race-specific varieties contain one or more genes (Rps) with resistance to specific races of the disease-causing organism. This type of resistance, also called race specific resistance, is active from the time of planting until full maturity. It fails only where races occur that are not controlled by genes in the plant.

Field or partial resistance provides a broad form of resistance to all races of the PRR organism. It may not provide the level of protection needed where pathogen population levels are extremely high. Field resistance is not active in the early seedling stage, and plants are considered susceptible to PRR until one or two true leaves have developed.

Seed treatments such as metalaxyl (Allegiance) and mefenoxam (Apron XL) can be effective against PRR in the first few weeks after planting and would be recommended as good insurance.

Specific resistance to PRR is controlled by major resistance genes such as Rps1a, Rps1c or Rps1k. Over 70 races of PRR have been identified; all have not been detected in Illinois. According to Dean Malvick, University of Illinois Extension plant pathologist, the disease remains significant in Illinois in part because many different races are known that defeat different resistance (Rps) genes in soybean and multiple races can occur in the same field.

More information on PRR is in the University of Illinois Extension publication titled Pocket Guide to Soybean Diseases in the Midwestern U.S., number C1380. Extension offices can assist individuals in getting a copy.

by  Editor, theCity1.com
February 7, 2005