Despite this spring's cold temperatures, alfalfa leaf diseases are light. We have examined a number of alfalfa fields around Ames and in northern Iowa, which included fields in the first, second, and third years after planting. The levels of foliar diseases are very low. The northern half of Iowa has not had excessive rains necessary to spread these diseases.
In Iowa, three foliar diseases are commonly seen in spring: spring black stem, Leptosphaerulina leaf spot, and downy mildew. We did not observe downy mildew in any of the fields we examined. We found spring black stem in every field, but it was limited to the lower four leaves with very few lesions on stems. We occasionally found Leptosphaerulina leaf spot. In a normal year, the last two diseases are present in every alfalfa field.
The situation may vary in some areas, however. Extension field specialists in southern Iowa reported relatively severe Leptosphaerulina leaf spot. The following information may help you identify these diseases if you have leaf spot problems in your alfalfa.
Spring black stem produces numerous small, dark-brown to black spots on the lower leaves, petioles, and stems (see top photo). Irregularly shaped lesions on leaves increase in size and coalesce. Lesions on stems and petioles enlarge and may blacken large areas near the base of the plant (see middle photo). The fungus that causes this disease is dispersed by splashing rain. If infections are severe in fields, early cutting generally is recommended to avoid defoliation damage. Cutting as early as mid-bud stage may be necessary when the disease is severe.
Leptosphaerulina leaf spot attacks mainly leaves. Both young and old leaves are susceptible to infections. Lesions often start as small, black spots and remain as "pepper spots" or enlarge to "eyespots." The lesions have light brown to tan centers with darker brown borders and often are surrounded by a chlorotic area (see photo, page 73). The spread and control of this disease are similar to spring black stem.
Downy mildew is caused by the fungus, Peronospora trifoliorum. The fungus infects alfalfa in spring when the temperature is low and moisture is high. Symptoms of this disease are chlorotic blotches on the upper leaf surface, and a white to gray mold growth on the lower leaf surface (see photo, page 73). Sometimes the leaf may be pale in color. Only young tissues are susceptible to infection. The fungus survives in shoots over summer and spreads in the fall. A light occurrence of the disease this spring means a small risk in the fall. Resistance to this disease is available.
Finally, last week's cold temperatures caused damage in some plants, typically seen as wilting of the top portions of plants. Be aware that these symptoms can be misidentified as symptoms of a fungal disease.
This article originally appeared on pages 73-74 of the IC-478(10) -- May 26, 1997 issue.