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Be Aware of Aphanomyces Root Rot of Alfalfa Caused by Different Races of Aphanomyces in Illinois

April 19, 2002
Over the past year, I have delivered information on Aphanomyces root rot of alfalfa via newsletter articles and meetings in Illinois. This disease can be severe, and alfalfa growers should be aware of it. Many growers have suggested that this disease may be a problem in their fields after they have seen a description of this disease and symptoms. Aphanomyces root rot has been known to be an important alfalfa disease in the Midwest for only about 15 to 20 years. It has been known to be an important disease of processing pea and snap bean for much longer. We have no reason to suspect it is a new disease of alfalfa, but it is likely that it has been one of the diseases causing serious damage to seedlings, as well as reduced yields in established stands, for many years.

A problem with Aphanomyces root rot is that it is difficult to diagnose in the field. This disease is caused by the soilborne fungal-like pathogen Aphanomyces euteiches, which infects alfalfa roots in slowly drained fields and during extended periods of rain. This disease is typically most damaging to seedlings. It can dramatically reduce stands and can reduce vigor and yield of plants that survive infection.

How can you know if Aphanomyces root rot is causing damage in your fields? Aphanomyces root rot is not the only disease that causes poor growth of seedling alfalfa. Phytophthora root rot and Pythium seed and root rot are other diseases caused by similar pathogens that occur in wet or poorly drained soils and often appear first in low areas in fields. Look for symptoms of Aphanomyces root rot, conditions that favor this disease, and other clues related to alfalfa cultivar and seed treatments.


Seedlings infected with Aphanomyces on the left and healthy seedlings on the right.

Plants infected with Aphanomyces usually become stunted and chlorotic (yellow) before they wilt and die. Cotyledons usually turn yellow first, and seedlings often develop a purple tint before they die. The similar pathogens Phytophthora and Pythium tend to kill seedlings quickly before plants become severely chlorotic or discolored. Other diseases that affect seedlings (such as Rhizoctonia seedling rot) are not as strongly associated with wet conditions. You can also help narrow down the problem to Aphano-myces root rot by looking at your alfalfa variety and seed treatments. Most certified varieties have resistance to Phytophthora root rot, while many don't have resistance to Aphanomyces. Futhermore, this is made even more complex due to different races of the Aphanomyces pathogen, as described below. In addition, determine if you have planted seed treated with fungicides such as Apron-FL, Apron XL, or Allegiance-FL. These fungicides will reduce early seedling rot and damping-off caused by Pythium and Phytophthora, but they do not control Aphanomyces. Thus, if you have wet or slowly drained soil conditions, seedlings dying and with symptoms typical of Aphanomyces, and you planted treated seed of a variety with resistance to Phytophthora, then Aphanomyces root rot may be causing damage. Final confirmation can obtained by laboratory diagnosis if samples are sent to the University of Illinois Plant Clinic in Urbana, (217)333-0519, http://www.cropsci.uiuc.edu/research/clinic/clinic.html.

A year ago we did not know anything about the distribution or characteristics of Aphanomyces associated with alfalfa root rot in Illinois. This disease had been confirmed to be a serious problem in nearby states, including Wisconsin, Iowa, and Kentucky, but it had not been studied in Illinois. I initiated a research project on Aphano-myces root rot in Illinois last summer, and we know a bit more now than we did last year at this time--and we will know much more in 6 months. We suspected that this disease is a problem in Illinois fields, and we have preliminary evidence to suggest that this is true. We now know that Aphanomyces is widely distributed in Illinois, and race 2 of this pathogen appears to be common in some areas.


Alfalfa cultivar with resistance to Aphanomyces root rot in the center row that is bordered by rows of susceptible cultivars. The stand count and plant height are two to three times greater for the resistant cultivar.

How can you manage Aphanomyces root rot, and why does it matter if race 2 is common or not in Illinois? This disease can best be managed by improving drainage or avoiding poorly drained fields, and by using Aphanomyces-resistant alfalfa varieties. As noted previously, fungicides are not available for control of Aphanomyces root rot of alfalfa. Races of Aphanomyces are important because they determine which alfalfa varieties will be resistant to this disease. Alfalfa varieties rated highly resistant (HR) or resistant (R) to Aphanomyces root rot should be planted where slowly drained soils occur and where Aphanomyces may be a problem. However, most varieties with resistance to Aphanomyces are resistant only to race 1, and these can be killed by race 2 of Aphanomyces. Thus, if race 2 is common in the soils of your fields, most alfalfa varieties will do little to help you manage this disease. Several commercial alfalfa varieties are available that have resistance to both races of Aphanomyces. If resistance to race 2 is not specified for an Aphanomyces-resistant alfalfa cultivar, then you can assume it is resistant only to race 1. The potential value of Aphanomyces resistance and resistance to race 2 can be most easily seen in side-by-side comparison of resistant and susceptible varieties. The "story" of Aphanomyces root rot of alfalfa in Illinois is continuing to develop. Check back in the Bulletin for updates on this disease in the future, and contact me by e-mail or telephone if you want more information at any time.--Dean Malvick

Author: Dean Malvick


The Pest Management and Crop Development Bulletin
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