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Aphanomyces: the Silent Yield Robber in Alfalfa

By Elia Romano

Are yields in your alfalfa fields lower than your expectations or maybe less than they have been in the past? Do you have areas in your field that are shorter and “pale green” or “yellow” looking? Have you ever dug roots in those areas and found that there are very few lateral roots and few or no nodules? Do you have springs in your hillsides or poorly drained soils? Have you noticed stunting and/or yellowing of your stand at establishment time? If you answered yes to any of these questions, there is a good chance that Aphanomyces root rot (ARR) is quietly stealing alfalfa tonnage right under your nose!

Unlike Phytophthora root rot, which severs the taproot and kills the plant, ARR affects the lateral root, causing stunted chlorotic plants that persist but have far less yield potential. The lateral roots are pruned and nodules are missing or damaged, causing poor nutrient uptake. It is primarily a seedling disease, although it can cause reduced yields in older stands. Seed treatments have little impact on ARR.

Aphanomyces is most severe in wet or flooded fields, and can be a problem on side-hills if there is a spring that keeps the soil saturated. Plants can recover when the soil dries out, but will not yield as well as unaffected, resistant plants.

Moving target

Like many fungal diseases, ARR has changed over time in response to management efforts. Within 10 years of its appearance in the mid-1980s, most new alfalfa varieties were resistant to Aphanomyces Race 1. However, in the early 1990s, Aphanomyces Race 2 was identified, and is now present throughout the Midwest.

Fortunately, breeding efforts have led to the development of alfalfa varieties with StandLife Genetics®. These high-yielding alfalfas have excellent resistance to Aphanomyces Race 2. Albert Lea seed offers three alfalfas with StandLife Genetics—Viking 390AP, Viking 357 and Foregrazer.

These alfalfas are the best weapons we have in the ongoing battle against ARR. New variants of Aphanomyces continue to be discovered, some of which are injurious to Race 2-resistant varieties. These are categorized as Race 3, and continued breeding for multi-race resistance will be required to stay ahead of this disease.

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