Several genera of soilborne 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 other crops, alfalfa seedling diseases are more severe in wet conditions. Unlike other crops, alfalfa seedlings are relatively cold-tolerant, therefore, it is not necessary to delay planting to avoid disease unless conditions are too wet.
Even though alfalfa seedlings are cold-tolerant, recent temperatures below 25° F may have resulted in cold or frost injury to seedlings. Frost-injured plants tend to be water-soaked and dark green to black, whereas diseased seedlings tend to be pale green to yellow or purple. Some chlorosis also may result from cold stress; these plants should recover after it warms up, but diseased seedlings will remain discolored and stunted. Seedlings that have completely collapsed may be victims of either seedling disease or frost injury.
||Seedlings of Aphanomyces-resistant and susceptible varieties grown in soil infested with the fungus
The best way to avoid seedling diseases is to plant varieties with a resistant (R) or highly resistant (HR) 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 a variety with Aphanomyces resistance. 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, however, most results I have seen do not show such a benefit. Apron seed treatment appears to be a much 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. Timing can be a factor in replanting because if it is done too late in the spring, there may be inadequate soil moisture.
This article originally appeared on pages 35-36 of the IC-478 (5) -- April 21, 1997 issue.