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The Conditions Are Right for Foliar Diseases of Alfalfa in Illinois

May 3, 2002
The recent wet and cool weather in Illinois, along with frost in parts of the state, following a period of rapid growth of alfalfa, is creating favorable conditions for foliar diseases. Alfalfa foliar diseases are often common at this time of the year, especially in the lower part of the plant. Spring black stem and leaf spot was recently reported by Matt Montgomery of the Sangamon/Menard Extension unit. This disease is common in Illinois. Omar Koester, from the Monroe/Randolph Extension offices, reports major problems with alfalfa weevil but minimal problems with alfalfa foliar diseases. However, foliar diseases can develop quickly during favorable conditions.

Many of the foliar diseases appear similar at first look (i.e., as small brown spots, but they often can be distinguished with a closer look). A few scattered spots on the leaves and stems are not something to be concerned about, but these diseases can develop to the point where they cause severe defoliation, leading to yield loss, reduced quality, and plant stress. The fungi that cause most foliar diseases overwinter on old leaves and stems, then produce spores in the spring that spread to the new growth. Here is a short summary of some common fungal leaf spot diseases of alfalfa.

Spring leaf spot and black stem (caused by the fungus Phoma medicaginis) is a very common disease at this time of the year (and in the fall). This disease often starts on the lower leaves, petioles, and stems and moves upward. Dark brown to black spots develop and may form into large spots. Lesions on the stems can become severe and may girdle the stems. This disease can cause defoliation when weather is cool and wet, and is usually most severe in the lower part of the canopy. Phoma is also one of several different pathogens that can cause crown and root rot.

Lepto leaf spot (sometimes called "pepper spot") can also occur at this time of the year and can be a problem throughout cool, wet summers. Symptoms of Lepto leaf spot (caused by the fungus Leptosphaerulina briosiana) are light brown spots, about 1/32 inch or slightly larger on leaflets, and often are surrounded by chlorotic halos. When this disease is severe, it can cause severe defoliation.

Common leaf spot may also be widespread in parts of Illinois throughout the summer. Like most foliar diseases, it doesn't kill plants but can cause defoliation. This disease (caused by the fungus Pseudopeziza medicaginis) causes small brown spots, about 1/32 inch in diameter, that usually remain as separate lesions without a chlorotic halo. It usually starts on the lower foliage and moves up the plant, and is more severe after the first cutting of the season.

Although the name is similar, the disease "summer" leaf spot and black stem is distinctly different from "spring" leaf spot and black stem. Summer leaf spot and black stem usually is not seen on the first cutting and begins later on the second and following cuttings. It is caused by the fungus Cercospora medicaginis. Lesions on leaves and stems are typically chocolate brown at first and often are fairly large (1/8 to 1/4 inch in diameter). The lesions develop and turn to a silvery tint and often have a yellow halo around them.

Management of foliar diseases of alfalfa is based primarily on cutting to reduce yield and quality losses due to defoliation. Stands should be harvested as soon as possible if foliar diseases are severe, even if it is before the optimal harvest time based on the growth stage of the alfalfa. Good management and fertilization (especially potassium) practices may also help to reduce losses from foliar diseases. Choose well-adapted, high-yielding alfalfa varieties. Unfortunately there are no alfalfa cultivars available with high levels of resistance to foliar diseases; however, many of the new cultivars suffer less damage from these diseases than older varieties.--Dean Malvick

Author: Dean Malvick

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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