Alfalfa seedling diseases in 1998

Wet conditions have delayed new alfalfa seedings in many parts of the state, and these conditions also may lead to problems with soilborne fungi. Several genera of fungi can attack alfalfa seedlings, including Phytophthora, Pythium, Aphanomyces, Fusarium, and Rhizoctonia. Traditionally, Phytophthora and Pythium have been considered the primary pathogens, but recently Aphanomyces euteiches has been more widely recognized. According to a survey we did in 1994, Aphanomyces is more common than Phytophthora in Iowa soils, and these two fungi should be considered equal threats to seedlings. Seedling diseases should be suspected when emergence is poor and/or there are obviously stunted, discolored, or dead seedlings. Like in other crops, alfalfa seedling diseases are more severe in wet conditions. Unlike other crops, alfalfa seedlings are very cold tolerant. Therefore, it is not necessary to delay planting to avoid disease, unless conditions are too wet.

Effect of Aphanomyces soil infestation on susceptible variety Vernal. Noninfested soil is on the right.
Aphanomyces symptoms on a susceptible variety (right). A resistant variety is on the left.

The best way to avoid seedling diseases is to plant varieties with an R or HR resistance rating to both Phytophthora and Aphanomyces. We have found that there are some strains of Aphanomyces in Iowa that kill even the resistant varieties, but it is still wise to use Aphanomyces resistance (see photo on page 61). To protect against Pythium, a fungicidal seed treatment is needed. Apron seed treatment is effective against Pythium and Phytophthora, but there are no registered seed treatments that have been shown to be effective against Aphanomyces. Ridomil is a soil fungicide registered for use in establishing alfalfa. It contains metalaxyl, the same active ingredient as Apron. Some studies have shown that Ridomil applied at seeding can be beneficial, but Apron seed treatment appears to be a more cost-effective way to control Pythium and Phytophthora.

If an alfalfa seeding fails, it is usually safe to replant alfalfa because the compounds that cause autotoxicity do not accumulate in seedlings. A Phytophthora- and Aphanomyces-resistant variety (treated with Apron) is recommended for replanting failed seedings. However, the timing is a factor because alfalfa seedings that are done too late in the spring may fail because of inadequate moisture.

This article originally appeared on pages 60-61 of the IC-480 (7) -- April 27, 1998 issue.

Updated 04/26/1998 - 1:00pm