Aphanomyces Root Rot In Alfalfa
Aphanomyces root rot is caused by the fungus-like pathogen Aphanomyces euteiches. Similar to phytophthora root rot (Phytophthora medicaginis), it is considered a major cause of disease in alfalfa seedlings, particularly in wet soil conditions. Aphanomyces also attacks adult alfalfa plants and can dramatically reduce yield and vigour of established stands. Aphanomyces is confirmed in a widespread area in the mid-west and northeast United States, and is likely underestimated as an alfalfa pathogen in Ontario.
Aphanomyces is a water mould which requires saturated soil conditions for infection. The organism can survive in the soil for long periods. Infected seedlings are typically stunted but remain upright. Cotyledons are yellow or purplish, followed by chlorosis of other leaflets. Root systems will initially be tan coloured and then turn dark brown. Affected seedlings are frequently mixed with taller, healthy seedlings. Aphanomyces symptoms in new seedings are usually not apparent when soils are dry after seeding.
Classic symptoms in established stands are stunted, yellow plants. Look for the absence of the fine, fibrous roots. Lateral roots are often rotted and even absent. Established stands that survive the initial infection are typically thin, yellow and weedy, and show reduced rhizobia nodulation. Symptoms appear similar to nitrogen deficiency. Regrowth is slow with poor vigour, and therefore yields are low. Because of the stunted root system, infected alfalfa stands do very poorly during seasons with extended dry weather.
More Chronic Than Phytophthora
Phytophthora tends to kill seedlings more quickly and extensively than aphanomyces, by attacking the tap root. However, aphanomyces is considered more chronic. Aphanomyces is less likely to cause seedling death, but more likely to result in stunted, low yielding alfalfa crops. Fungicides containing metalaxyl (ie Apron) are active against phytophthora, but not aphanomyces. If a grower has used Apron treated seed of an aphanomyces susceptible variety, but has a seedling problem, aphanomyces infection could be the cause.
How Extensive Is It?
There are many alfalfa fields in Ontario that show visual symptoms of aphanomyces, although this has not been confirmed by laboratory analysis. A limited 1992 survey in southwestern Ontario indicated infection in 6 (in 5 different counties.) of 83 alfalfa fields surveyed (7%). Soil tests for aphanomyces are available in the US, but not yet in Ontario. Based on the rapid spread of aphanomyces in neighbouring States in the past decade or so, it seems very possible that it is also a significant alfalfa disease in Ontario.
Aphanomyces is managed by using resistant varieties, similar to what has been done with phytophthora root rot resistant alfalfa varieties. Race 1 and race 2 isolates of aphanomyces have been identified. Race 2 is more virulent. Many alfalfa varieties are resistant to race 1, but far fewer are resistant to race 2. Seed companies are working to make both race 1 and 2 resistant varieties commercially available in Ontario.
Figure 1 - Aphanomyces root rot of alfalfa seedling. (Photo courtesy of APS Press).
Figure 2 - Aphomyces symptoms of root rot (right) and healthy plants (left). (Photo courtesy of APS Press).
For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047