Phytophthora root rot has become one of the most important diseases of alfalfa in California. It is particularly a problem on heavy, poorly-drained soils, or on any soil where a layer of free moisture persists for some time after irrigation. Leaves of the diseased plants turn yellow and eventually the plants die. Reddish brown lesions are found on the roots of the infected plants with the woody tissue of the root showing a yellowing above and below the lesion. Improving the drainage is the best control for the disease, but in some soils this may not completely solve the problem.