Aphanomyces in Alfalfa Survey! Request for Soil Samples

Aphanomyces root rot is considered a major disease of alfalfa and the incidence of Aphanomyces in the mid-west and northeastern United States has increased in recent years. These observations are concerning since variety genetic resistance or tolerance has been the cornerstone of disease management in alfalfa production systems in Ontario.

Race 1 resistant varieties and the more recent development of race 2 resistant varieties are being used in the U.S. to manage this disease. An OMAFRA survey in 1992 detected the pathogen in 7% of the alfalfa fields tested but Aphanomyces distribution and its impact in Ontario is not well understood, and most likely is underestimated. For this reason, OMAFRA is conducting a new survey to update the distribution and race of Aphanomyces in the province.

We Need Your Help!

If you have or know of any fields with potential problems we want to know. Here is what to look for:

New seedings - Seedlings appear stunted, yellow and have a under developed root system which with very few lateral roots. The roots often are grey and watersoaked and as the disease develops they turn brown. The cotyledons and first true leaves can be yellow in colour.

Established stands- General symptoms include poor root development with the absence of the fine, fibrous roots and root hairs, along with reduced nodulation. Affected plants appear short, stunted and yellow in characteristic oval-shaped patches. There appears to be a definite separation between affected patches and normal plants. Because of the stunted root system, infected alfalfa stands do very poorly during extended dry weather, when these patches are particularly noticeable. Also mature plants are slow to break winter dormancy, slower to develop in the spring, lack vigour and regrowth following harvest is delayed.

How to Sample

Soil samples are to be collected from the upper 6 inches of soil and from 25 locations take a shovel or cup of soil from these locations throughout the field. Since a bio-assay is needed to test for Aphanomyces a large volume of soil is necessary to grow the plants therefore, 4 to 8 liters of soil (1-2 gallons). The collected soils must be kept in the cold or frozen if possible until delivered.

Fig. 1. Aphanomyces - new seedling

Fig. 2. Field with typical Aphanomyces injury
(Photo courtesy of Dean Malvick, U of Minnesota).

Who to Contact:

Joel Bagg: email joel.bagg@ontario.ca or telephone 705-324-5856
Albert Tenuta/Cheryl VanHerk: email albert.tenuta@ontario.ca or telephone 519-674-1694