Figure 1. (Above) Stemphylium leaf spot Figure 2. (Left) Close-up of Stemphylium leaf spot lesions on an alfalfa leaflet
Figure 3. Spring black stem and leaf spot (Photo credit: John Watkins)

May 7, 2010

The cool, wet weather that has prevailed this spring has favored the development of leaf spot diseases of alfalfa. One of these diseases, Stemphylium leaf spot (Figure 1), was recently identified in south central Nebraska. Symptoms include light tan, almost white, irregular lesions with a dark brown border (Figure 2). Sporulation occurs within the lesions and spores are spread by wind and splashing water from rain or irrigation. The disease is favored by cool weather and moisture. Forage quality is reduced when disease severity is high.

Another disease showing up in alfalfa fields is spring black stem and leaf spot (Figure 3). Symptoms of spring black stem and leaf spot include numerous small, dark brown to black spots that develop in early spring on the lower leaves, petioles, and stems and progress up the plant. Affected leaves turn yellow. The spots later enlarge and coalesce, and may blacken large areas on stems and petioles. The causal fungus overwinters on crop residue. Therefore, the disease is more severe in no-till fields. The fungus also causes a crown and root rot. Symptoms of spring black stem and leaf spot were observed throughout a sample from northeast Nebraska submitted this week to the Plant and Pest Diagnostic Clinic.

Both Stemphylium leaf spot and spring black stem and leaf spot can be managed by early harvesting to reduce leaf loss. A fungicide application may not be profitable unless the crop is being raised for seed.

Brandy VanDeWalle
Extension Educator, Fillmore County
Jennifer Rees
Extension Educator, Clay County
Stephen Wegulo
Extension Plant Pathologist, Lincoln