The wet Illinois weather this spring has favored development of Aphanomyces root rot of alfalfa. This disease has caused significant damage in some areas to alfalfa seeded this spring.
The photos below show damage due to Aphanomyces root rot in one central Illinois alfalfa field.
Aphanomyces root rot is caused by the fungal-like pathogen Aphanomyces euteiches. The pathogen causes the most damage to newly seeded alfalfa stands in wet weather and slowly drained soils. Aphanomyces root rot is usually not a significant problem when soils are fairly dry for 4 to 6 weeks after planting, but the pathogen can survive in soil for long periods and cause serious damage when soils are wet after planting. Results can be thin stands, increased weed pressure, reduced nodulation and nitrogen fixation, and reduced vigor and yield of plants that survive infection. Aphanomyces euteiches seems to occur in most Illinois alfalfa fields, with both races 1 and 2 widespread.
Diagnosis of Aphanomyces root rot can be challenging in the field. Infected seedlings usually are discolored and stunted. Cotyledons become yellow, and the seedlings usually develop a purple tint. The stunted and discolored seedlings are often mixed in a stand with taller, healthy seedlings. Established stands with high disease pressure may be stunted and thinned. Pythium and Phytophthora damping-off and root rot are also favored by wet soil conditions, but these diseases tend to kill seedlings quickly, before plants become severely chlorotic or discolored.
Aphanomyces root rot is managed by planting resistant or highly resistant varieties and by avoiding slowly drained fields. Based on recent research in Illinois, varieties with resistance to both Aphanomyces races 1 and 2 should provide the best control for many, if not most, fields in the state. Seed and seedling diseases caused by Pythium and Phytophthora can be managed with the seed-treatment fungicides Apron XL (mefenoxam) and Allegiance (metalaxyl), but effective fungicides and seed treatments are not available for control of Aphanomyces root rot of alfalfa. --Dean Malvick