How to Manage Pests

UC Pest Management Guidelines


Spring Black Stem

Pathogen: Phoma medicaginis

(Reviewed 3/17, updated 3/17)

In this Guideline:

Symptoms and signs

Spring black stem is a cool-season foliar disease. Symptoms include small, black-to-dark brown spots on lower leaves, petioles, and stems. The lesions are irregularly to triangularly shaped. As they increase in size, lesions coalesce and become light brown. Affected leaves turn yellow and often wither before falling. Lesions on stems and petioles enlarge, causing large areas near the base of the plant to turn black. Young shoots are often girdled and killed. Most damage occurs before the first cutting.

Comments on the Disease

The causal fungus produces brown-to-black fruiting bodies (pycnidia) on overwintered stem and leaf lesions. In early spring, spores released from pycnidia on dead stems during wet weather or overhead irrigation are splashed onto foliage and stems. In addition new shoots are infected as they grow through the crop residue or stubble. The fungus also may be seedborne.


Control measures include early cutting to reduce leaf loss, planting resistant cultivars, and planting pathogen-free seed. Crop rotation can reduce inoculum in the field. For more information, see CROP ROTATION.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430


R. M. Davis, Plant Pathology, UC Davis (Emeritus)
C. A. Frate, UC Cooperative Extension, Tulare County (Emeritus)
D. H. Putnam, Plant Sciences, UC Davis

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