There are more than a dozen diseases of alfalfa. They can be grouped into two categories: those that affect the stems, crowns, or roots and those affecting the foliage. The stem and root-rot diseases are the most serious. Major losses of stand have been caused by Phytophthora root rot, anthracnose, and Sclerotinia crown rot. Verticillium wilt is a constant threat because some farmers are still growing old varieties that lack resistance to this disease. Nearly all alfalfa varieties currently grown in Ohio have resistance to bacterial wilt and Fusarium wilt. The widespread use of resistant varieties has greatly diminished the significance of wilt diseases.
Phytophthora root rot has been responsible for loss of stand in the seedling year when rainfall is above average and/or surface and subsurface drainage is poor. It is caused by a soilborne fungus that becomes active under wet soil conditions and may attack both seedlings and older plants. Dark brown, decayed areas on the tap root two to three inches below the soil surface are early indicators of Phytophthora root rot. Soil moisture is the key factor affecting disease development. Any improvement in surface and subsurface drainage will reduce losses from this disease as well as from other less common root rots. Varieties with good levels of resistance are now available. Phytophthora specific fungicide seed treatments are available to help prevent loss of stands due to seedling damping-off. Fungicides are labeled for use at planting either as a broadcast application or incorporated on fertilizer granules and applied in a band beneath the seed.
Aphanomyces root rot may contribute to poor alfalfa establishment and reduced growth in wet soils. Seedlings may die (damping off) if infection occurs at an early stage of development. Older seedlings are yellowed and stunted. When Aphanomyces and Phytophthora occur together, they form a destructive disease complex. Alfalfa varieties with moderate to good levels of resistance are available for control of Aphanomyces root rot.
Anthracnose is a major cause of thinning of older alfalfa stands. Seedlings may be killed or crown and crown buds of older plants may be affected. Wilted and dead bleached stems are characteristic of the disease. Diamond-shaped lesions (cankers) with light-brown centers and dark margins develop on the lower stems. Anthracnose is a warm, wet-weather disease. Resistant varieties are available. Anthracnose has long been recognized as a destructive disease of red clover in the southern areas. The use of resistant varieties of red clover controls this problem.
Sclerotinia crown rot occurs almost exclusively on late-summer seedings, especially when minimum-tillage methods are used. Affected plants wilt and the cottony-white mold changes to hard black bodies on the crowns or lower stems. This disease is usually seen during the cooler periods of the year. Crop rotation, with two to three years away from alfalfa, aids in control.
Verticillium wilt may be spread from field to field with infested seed and in manure from animals fed infested hay. This soilborne disease usually does not become a problem until the third production year. It can be recognized on scattered plants that become yellow and stunted, then gradually die, leaving a thin, unproductive stand. Control is mainly through use of disease-free seed, resistant varieties, and crop rotation.
There are several foliage diseases of alfalfa. Any of these may cause considerable loss of leaves during periods of prolonged wet or humid weather. Little can be done about these diseases in the year that they occur. Proper crop rotation will help control these diseases in following years.