Knowledge of alfalfa diseases can help explain stand loss and steer you toward the right disease-resistant varieties, points out Jim Isleib, Michigan State University Extension field crops educator.
The most serious in alfalfa are stem and root diseases, he says.
“Plant breeders have incorporated disease resistance into modern alfalfa varieties, mostly eliminating the problems of bacterial wilt and fusarium wilt,” says Isleib. “However, phytophthora root rot, anthracnose and sometimes verticillium wilt can be problems under the right conditions. Researchers in Minnesota and Wisconsin have identified aphanomyces root rot (race 2) and brown root rot as ‘emerging’ alfalfa root diseases in their states.”
Varieties resistant to aphanomyces root rot have been available for many years. But reports in Wisconsin and Minnesota of aphanomyces in alfalfa have become more frequent in recent years, possibly due to the emergence of the race 2 strain.
The pathogen can apparently persist in soil or in other plants for long periods. Seedlings may die (damping off) at an early stage of development. Older seedlings are yellow and stunted. When aphanomyces and phytophthora root rot occur together, the problem is much worse.
Brown root rot was reported in alfalfa for the first time in Wisconsin and Minnesota in 2003. The disease has probably been present for much longer, but not recognized, Isleib says. The fungus is slow-growing and prefers cool soil temperatures of less than 60°F. Symptoms may not become visible until after the third winter due to slow disease development.
The fungus rots plant roots mostly in spring and fall and can severely weaken plants and contribute to winterkill. Stunted and dead plants appear in two- to three-year-old stands with brown sunken lesions on taproots. Infected plants may start to grow in spring, then stop growing and die because the taproot has rotted off. The fungus also causes disease in other perennial forage legumes and winter wheat.
For more details on alfalfa root health, read Alfalfa Root Health and Disease Management: A Foundation for Maximizing Production Potential and Stand Life by University of Minnesota Extension.
For information on selecting disease-resistant alfalfa varieties for Michigan, see Michigan State University Extension Bulletin E-2787, Perennial Legume and Grass Forage Variety Selection for Michigan.
For Wisconsin information that includes aphanomyces resistance, see Forage Variety Update for Wisconsin – 2010 Trial Results by University of Wisconsin Extension.