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United States Department of Agriculture

Agricultural Research Service

Research Project: IMPROVING ALFALFA AND OTHER FORAGE CROPS FOR BIOENERGY, LIVESTOCK PRODUCTION, AND ENVIRONMENTAL PROTECTION

Location: Plant Science Research

Title: Aphanomyces root rot of alfalfa: widespread distribution of race 2

Authors
item SAMAC, DEBORAH
item Seitz, Victoria -
item Ristow, Patty -
item Rouse, Douglas -

Submitted to: Forage Focus
Publication Type: Trade Journal
Publication Acceptance Date: July 19, 2012
Publication Date: August 1, 2012
Citation: Samac, D.A., Seitz, V., Ristow, P., Rouse, D. 2012. Aphanomyces root rot: widespread distribution of race 2. Forage Focus. August 2012. p. 8-9.

Technical Abstract: The early spring of 2012 with prolonged wet soil conditions in many parts of the country resulted in reports of poor performance of alfalfa due to Aphanomyces root rot (ARR). Varieties with resistance to ARR are available, although fewer varieties have resistance to both race 1 and race 2 of the pathogen. Recently, the fungicide Stamina, which protects seedlings against ARR, was labeled for use as an alfalfa seed treatment and will be available on a limited basis for spring seeding in 2013. This article describes how to identify ARR, the test used to distinguish races, and what is known about distribution of race 2. 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. Roots of plants should be examined for evidence of root rot and plant tissue or soil tested for sulfur content to determine the cause of the symptoms. Surveys conducted within several States found that race 2 strains are as prevalent or more prevalent than race 1 strains and often both strains occur in the same field. This spring, race 2 was identified in New York alfalfa fields for the first time. Evidence is mounting that additional races of ARR are present in alfalfa fields that can overcome race 2 resistance. ARR is a widespread risk to alfalfa cultivars having resistance to only race 1. Identifying non-specific resistance is needed for improving alfalfa establishment and production in areas with ARR.

Last Modified: 9/10/2014
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