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Check summer-seeded alfalfa to determine potential root rot damage to new stands

Richard Leep and Doo-Hong Min Published on 16 September 2009

In areas of excessive rainfall, many newly seeded alfalfa fields may show poor growth in wet or poorly drained fields which may be due to several diseases.

Seedling diseases should be suspected when emergence is poor or there are stunted, discolored or dead seedlings.

Aphanomyces root rot can cause death and stunting of seedlings as well as more subtle disease of established plants that can result in significant yield reduction. This disease is caused by a soilborne fungal-like pathogen. Other diseases that occur in wet or poorly drained soils include Phytophthora root rot and Pythium seed and root rot. Plants infected with Aphanomyces usually become stunted and chlorotic (yellow) before they wilt and die, whereas Phytophthora and Pythium tend to kill seedlings quickly before plants become severely chlorotic. Another clue to a problem with Aphanomyces is root rot of an alfalfa cultivar that is highly resistant to Phytophthora.

Perhaps because alfalfa disease that occurred in wet soil was attributed to Phytophthora, Aphanomyces root rot of alfalfa was not recognized as a serious problem until the early 1980s. Aphanomyces root rot is best managed by avoiding poorly drained soils and using Aphanomyces-resistant alfalfa varieties. However, even well-drained fields can become water-saturated due to abnormally high rainfall.

Fungicides are not available for control of Aphanomyces root rot of alfalfa. Phytophthora and Pythium root rots of seedlings can be controlled with fungicidal seed treatments, such as Allegiance-FL, Apron-XL or Apron-FL, but these seed treatments are not effective against Aphanomyces.

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. A list of varieties and their disease resistance can be found at the Michigan State University Forage Information Systems’ website at www.msue.msu.edu/fis/ and clicking on the Extension bar and then the Perennial Forage Legume and Grass Varieties for Michigan.

Control of Aphanomyces root rot became more challenging when different races of this pathogen were discovered. Many commercial alfalfa cultivars are now available that have resistance to race 1, the first race discovered. Another race (race 2) of Aphanomyces was identified in the early 1990s that overcomes race 1 resistance. Alfalfa cultivars developed for resistance to race 1 are killed by the aggressive race 2 isolates. Race 2 isolates have been identified in a number of states.

Alfalfa varieties with resistance only to race 1 may be genetically vulnerable to Aphanomyces root rot in many regions due to the presence of race 2. Several commercial alfalfa varieties are now 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 overall distribution and impact of races 1 and 2 of Aphanomyces are uncertain, but Aphanomyces root rot should be considered as a potential problem. If you have an alfalfa seeding that has failed this fall due to the excessive rainfall, it should be safe to replant with alfalfa again next spring since compounds which cause autotoxicity do not accumulate in seedlings. Phytophthora- and Aphanomyces-resistant varieties (treated with Apron XL) are recommended for replanting failed seedings. Spring alfalfa re-seeding should be done as early as possible in the spring.  FG

-Excerpts from Michigan State University Field Crop Advisory Team Alert newsletter, Vol. 23, No. 18

Richard Leep and Doo-Hong Min
Crop and Soil Sciences
Michigan State University

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