Plant Pathology
Factsheet

Crown rots are chronic disease problems of alfalfa throughout the world. As the name implies, crown rots appear as rotted or corky areas in the crown and upper taproot portions of the plant. Color of affected tissues varies depending on which fungi are attacking the plant. Sometimes, shoots of affected plants show yellowing and wilting, but plants must be dug and taproots cut open to diagnose the problem.

In contrast to most diseases of alfalfa, crown rots are not a single disease but a complex of diseases with somewhat similar symptoms. Crown rots are caused by a great variety of fungi in the soil. Some fungi such as the anthracnose fungus, are the same as those that cause other diseases. Other fungi such as several Fusarium fungi, do not attack alfalfa except as crown invaders.

Crown rots cause loss of stand and forage yield in several ways. If crowns are rotted severely enough, plants can die simply by being choked off. Carbohydrates for winter survival are stored in the crown and upper taproot. By rotting this area, crown rots also make alfalfa plants more sensitive to winter kill. Some crown rot fungi produce toxins, thus weakening or even killing the plant.

MANAGEMENT

Use adapted varieties. Alfalfa breeders have not made an effort to breed for resistance to crown rots, primarily because literally dozens of fungi can cause these diseases. However, selecting a variety well-suited for local conditions will help in minimizing the impact of these diseases. Growers are encouraged to remain up-to-date on results of alfalfa variety trials conducted by the UK Department of Agronomy.

Minimize crown injury. Injury to crowns creates wounds that allow infection by crown rot fungi living in the soil. Minimize traffic and grazing in alfalfa stands when the soil is wet, to minimize crown injury,

Rotate alfalfa land to crops other than forage legumes. Rotation to other crops can help starve out some of the soilborne fungi that cause crown rot, because it deprives them of a food source.

Maintain good soil fertility, paying particular attention to potassium. Maintaining good soil fertility can help plants resist infections as they occur.

Allow adequate regrowth between cuttings. Allowing stands to grow until first flower can help plants maintain adequate carbohydrate reserves to resist infections. Note that postponing cutting much past first flower causes a decline in hay quality without improving disease control.