The cool temperatures this spring have slowed alfalfa growth and slow alfalfa growth can delay the buildup of foliar diseases. We checked alfalfa fields in Ames this weekend and observed foliar diseases on the bottom leaves of plants in some fields. The levels of these diseases were not high, but they may increase before the first cutting. Cool temperatures and frequent spring rains are favorable for the development of foliar diseases in Iowa. Keep an eye on the development of the following foliar diseases because high levels of these diseases can cause early defoliation.

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 large areas near the base of the plant. The fungus that causes this disease is dispersed by splashing rain. This disease is very common in Iowa and severe infection can result in defoliation.

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Leaf symptoms of spring black stem.

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Stem symptoms of spring black stem.

Common leaf spot looks similar to spring black stem, but appears only on the leaves and has larger, dark brown spots. This disease may not appear until a little later in the season.

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Common leaf spot.

Downy mildew is caused by the fungus Peronospora trifoliorum. This fungus infects alfalfa in spring when temperature is low and moisture is high. The weather conditions this spring are ideal for downy mildew. 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. The fungus survives in shoots over the summer and spreads in the fall. If the disease is a problem in your field, consider planting a resistant variety in your next planting.

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Leaf symptoms of downy mildew.

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.

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Leaf symptoms of Leptosphaerulina leaf spot.

When scouting, look into an alfalfa canopy for diseased leaves and stems because these diseases progress from lower portion of plants to the top. Pay special attention to fields that had disease problems last fall because they have greater risk than other fields for inoculum carryover from the winter. Also, fields in third year alfalfa have greater disease risk than first and second year alfalfa fields because the pathogens build up over time. Keep in mind that there are differences in disease tolerance between varieties. If disease level is high, 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.