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Extension > Agriculture > Crops > Forage Production > Pest management > Aphanomyces root rot: Widespread distribution of Race 2

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Aphanomyces root rot: Widespread distribution of Race 2

Deborah Samac, Adjunct Professor, Department of Plant Pathology


Picture 1. Aphanomyces on alfalfa seedling.

Strong seedling establishment in alfalfa is important to achieve the plant density needed to out–compete weeds and produce high biomass yields. But, establishing alfalfa can be challenging because alfalfa seeds and seedlings are vulnerable to several pathogens present in soil. Wet soil conditions favor the development of diseases caused by "water molds," namely Phytophthora root rot (PRR), Aphanomyces root rot (ARR), and damping off (rotting). The pathogens causing these diseases produce mobile swimming spores called zoospores that require water for development and infection of alfalfa. When rain is excessive after sowing, stand establishment may fail due to seed and seedling rot caused by these pathogens. Both ARR and PRR can also attack adult plants under wet soil conditions.

Symptoms of Aphanomyces infection

Seedlings infected by ARR become stunted and chlorotic (yellow) before they wilt and die and infected seedlings usually remain upright. In adult plants the root mass is reduced and lateral roots have brown decay. A brown lesion on the taproot may mark the location where lateral roots were rotted off. Nodules are frequently absent or decaying. Foliage is stunted, becomes chlorotic and resembles symptoms of nitrogen deficiency. Infected plants are often slow to regrow or may fail to grow after harvest or winter dormancy. Chlorotic foliar symptoms occur in soils that are deficient in sulfur. So it is prudent to examine roots of plants for evidence of root rot as well as to test plant tissue or soil for sulfur content to determine the cause of the symptoms.

Symptoms of Phytopthora root rot

Seedlings infected by PRR collapse and decay rapidly. In established stands when soils remain wet, PRR attacks lateral roots and the taproot. The rotted tissue turns dark brown–black forming a pencil point–like symptom and foliage turns yellow or reddish.

Seedling protection

To help protect seedlings from seed rot and damping–off and PRR, the majority of alfalfa seed is treated with the systemic fungicide mefenoxam, (Apron XL®). However, Apron XL does not provide protection against ARR. Varieties with resistance to ARR are available, but the majority of varieties have resistance to only one race (race 1) of the pathogen. A recent survey of 45 alfalfa fields in Minnesota found that a second race (race 2) is widespread in the state and more common than race 1. The same situation has also been found in Wisconsin and New York. Evidence is mounting that additional races of ARR are present in alfalfa fields that can overcome race 1+2 resistance. Recently, the fungicide Stamina, which protects seedlings against ARR, was labeled for use as an alfalfa seed treatment and is being used along with Apron XL to help boost protection from soilborne pathogens.

A soil test is available to determine which races of ARR are present in soil. To have soil tested, contact Deborah Samac at

Be on the alert for Anthracnose on alfalfa

Picture 2. Anthracnose on alfalfa

Anthracnose is a serious stem and crown rot disease of alfalfa that kills individual plants and causes rapid stand decline. It has been controlled in the past by plant resistance, but new pathogen types have been found. The disease may be on the increase in the Midwest.

Symptoms of Anthracnose

Disease symptoms include the following:

Anthracnose is caused by Colletotrichum trifolii, a fungus that produces masses of tiny spores on infected stems and crowns. During periods of warm, rainy weather, spores are splashed from infected to healthy plants. Lesions develop on stems, causing stems to wilt and eventually die. The pathogen grows from stem tissue into the plant crown, and causes a crown rot, which ultimately kills the plant.

Please report disease observations to Deborah Samac ( or (612) 625-1243).

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