Crop Scene Investigation - 26 Solved: What's ailing Art's alfalfa?

Sunday, February 6, 2011

Similar to phytophthora root rot, aphanomyces is considered a major cause of disease in alfalfa seedlings, particularly in wet soil conditions. But, while there are seed treatments available to protect seed against phytophthora and other soil-borne diseases, no seed treatment product offers protection against aphanomyces.

Joel Bagg, a forage specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, notes that aphanomyces also attacks adult alfalfa plants and can dramatically reduce the yield and vigour of established stands. This was the case with Art's third-year alfalfa field.

Bagg says typical symptoms in established stands are stunted, yellow plants that have no fine, fibrous roots. Lateral roots are often rotted and even absent. Established stands that survive the initial infection are typically thin, yellow and weedy, and show reduced rhizobia nodulation. Regrowth is slow with poor vigour, and therefore yields are low.

Bagg adds that the disease's impact is most often seen in dry years because affected alfalfa plants, suffering from stunted roots, don't have the moisture- and nutrient-gathering capacity of healthy plants. The reduced nodulation also limits the plant's ability to fix nitrogen, so the plant appears to be nitrogen deficient.

Phytophthora tends to kill seedlings more quickly and extensively than aphanomyces, but aphanomyces is considered more chronic. Aphanomyces is less likely to cause seedling death, but more likely to result in stunted, low-yielding alfalfa crops. Fungicides containing metalaxyl such as Apron are active against phytophthora, but not aphanomyces.

Bagg says the best way to manage the disease is through the use of resistant varieties. Race 1 and race 2 isolates of aphanomyces have been identified. Race 2 is more virulent and may be more common than race 1. Many recommended alfalfa varieties in Ontario are resistant to race 1, but only a handful are resistant to race 2. Unfortunately for Art, he was not using a race 2 resistant variety. Congratulations to Ray Brubacher, Massey, for his persistence and for his correct solution. BF