Alfalfa is turning green early in this warmer-than-usual spring and the frequent rains have promoted foliar diseases in parts of Iowa. Last week, ISU extension field specialists-crops Brian Lang (northeast) and Virgil Schmitt (east) reported spring black stem, and Mark Carlton (south central) reported Leptosphaerulina leaf spot.
A survey of alfalfa fields around Ames revealed both diseases. In most fields, spring black stem was found in many of the plants, but it was limited to the lower leaves. A relatively high level of spring black stem also was found in third-year alfalfa fields but not in fields of first- or second-year alfalfa. Although no severe defoliation has occurred in these fields, more rainfall before the first cutting may result in defoliation where lesions of spring black stem are visible in the middle of stems.
In two fields that were examined, scattered downy mildew was more visible than Leptosphaerulina leaf spot. However, these observations may differ from what you see in your fields because the occurrence of foliar diseases is determined by last year's disease and local precipitation. Scouting can help you determine if you should be concerned about foliar diseases. Use the following information to help you identify the diseases if you find that leaf spots are a problem in your alfalfa.
|||Leaf symptoms of spring black stem.|
|||Stem symptoms of spring black stem.|
Spring black stem produces numerous small, dark brown to black spots that first occur on the lower leaves and petioles, and later on stems. Irregularly shaped lesions on leaves increase in size and coalesce. Lesions on stems and petioles enlarge and may blacken sizable areas near the base of the plant. The fungus that causes this disease (Phoma medicaginis) is dispersed by splashing rain. Severe infection can result in defoliation; thus, early cutting generally is recommended so that defoliation can be avoided. Cutting as early as mid-bud stage may be necessary when the disease is severe.
|||Common leaf spot.|
Common leaf spot looks similar to spring black stem, but it appears only on the leaves and has larger, dark brown spots. This disease may not appear until later in the season than spring black stem.
|||Leaf symptoms of downy mildew.|
Downy mildew is caused by the fungus Peronospora trifoliorum. This fungus infects alfalfa in the spring when temperatures are low and moisture is high. Severe disease was observed in 1993 when the spring was wet and cold. Symptoms of this disease are chlorotic blotches on the upper leaf surface and a white to gray mold on the lower leaf surface. Sometimes, the color may be pale. Only young tissues are susceptible to infection. The fungus survives in shoots over the summer and spreads in the fall. If disease is a problem in your field, consider planting a resistant variety in the next planting.
|||Leaf symptoms of Leptosphaerulina leaf spot.|
Leptosphaerulina leaf spot attacks mainly leaves. Both young and old leaves are susceptible to infection. 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 are often surrounded by a chlorotic area. The mechanism of spread of this disease is similar to that of spring black stem.
Keep in mind that cold temperatures in early spring can cause damage to alfalfa and produce symptoms such as wilting in top portions of plants that can be mistaken for a disease problem.
This article originally appeared on pages 63-64 of the IC-480 (8) -- May 4, 1998 issue.