Spring Black Stem of Alfalfa

One of the most prevalent foliage diseases of alfalfa in Utah is spring black stem. Caused by the fungus Phoma medicaginis (=Ascochyta imperfecta), it occurs annually during Utah springs and can persist into summer if cool, wet weather continues. It will recur again when fall rains begin. Because of the environmental conditions, the first cutting is usually the most damaged by the disease.

SYMPTOMS

This disease is most serious during cool, rainy springs. All above-ground parts of the plant are susceptible to infection. Small, dark brown to black, elongated lesions develop on young stems and petioles. Small shoots may be girdled by lesions and die, as can young buds. On leaves, the lesions are dark but irregular in shape. Lesions often merge to cover a large part of the leaf. Affected leaves become chlorotic (yellow) and wither before dropping. The lowest leaves are affected first, and the disease advances up the plant as long as humidity remains high in the canopy. Protein content of affected hay is reduced by black stem. Stems become brittle and entirely blackened with severe fungal infections. Sometimes the disease extends into the crown and upper root. Pods in seed crops may become infected if maturing in cool, wet weather.

Summer weather conditions are less conducive to this disease. It usually abates during warm, dry periods, but may recur in wet falls. Inoculum builds up during the fall and winter. The fungus overwinters in fallen infected leaves and on the stubble. Small, dark, round pycnidia form and can be seen partially imbedded in the stubble. Spores produced in the pycnidia ooze out in the spring and infect shoots as they emerge. Water, wind, and insects help spread spores to new infection sites. Pea aphid infestations promote disease by the production of honeydew which stimulates the fungus.

CONTROL

There are no currently registered fungicides, nor are there any varieties resistant to spring black stem. However, there are some cultural techniques which will reduce losses:

• Sprinkler irrigation contributes to high relative humidity in the canopy which is conducive for the disease. Avoid irrigation techniques which keep the canopy moist for long periods, especially during rainy weather.
• The primary damage caused by the disease is the loss of leaves and corresponding reduction in the protein. Therefore, early cutting is advised when the disease starts to advance up the plant, causing a loss of foliage.

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician

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