– Hosts and Symptoms:
– Peronospora trifoliorum infects numerous strains and varieties of alfalfa.
– Primary symptoms include chlorotic leaf blotches, rolled or downturned leaves, and stunted stems.
– Identification is based on moldy, downy growth on the underside of leaves.
– Only seedlings and young tissue are susceptible to infection.
– Potential for secondary infection occurs every five days during ideal conditions.
– Environment:
– Prefers high humidity and moderate to warm temperatures.
– Peak spore production and infection occurs around 65°F.
– Active in temperatures between 40 and 85°F.
– Primarily seen during cool, wet periods in summer or warm, dry periods in spring and fall.
– Usually found in the midwestern and southern United States.
– Management:
– Growing resistant varieties of alfalfa is a common form of control.
– Cutting the alfalfa crop early removes infectious conidia.
– Use of chemical control like metalaxyl and mefenoxam is effective for seedlings.
– Cultural controls are believed to be the most effective form of control.
– Alternative methods like salicylic acid, potassium phosphite, and neem oil have shown protective effects.
– References:
– UC IPM and NMSU provide guidelines on downy mildew on alfalfa.
– UW-Extension offers resources on alfalfa diseases.
– Pacific Northwest Pest Management Handbooks cover alfalfa downy mildew.
– Compendium of Alfalfa Diseases and Pests provides in-depth information.
– Study by Mohamed Morsy et al. explores biotic and abiotic compounds for disease control.
– Miscellaneous:
– Peronospora trifoliorum is an oomycete requiring free moisture to spread.
– Disease may overwinter in dead leaf debris, crown buds, or seeds.
– Infection occurs every five days during ideal conditions.
– Resistant varieties limit the disease’s ability to infect and survive.
– Chemical control with metalaxyl and mefenoxam suppresses the infectious stage.