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New Mexico State University

New Mexico State University

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Downy Mildew Hits Area Alfalfa

LAS CRUCES - Alfalfa growers can best rid their fields of downy mildew by harvesting early, said a New Mexico State University plant pathologist.


"For the past few weeks, environmental conditions in the Mesilla Valley have been just right for an outbreak of downy mildew on alfalfa," said Natalie Goldberg, plant pathologist with NMSU's Cooperative Extension Service.

Goldberg recommended early harvest and the selection of mildew tolerant cultivars which usually provide adequate control of downy mildew and will help to minimize the yield loss due to this disease.

"Downy mildew is caused by a fungus and usually occurs in spring before the first cutting of alfalfa, however, it may also occur in the fall and is particularly troublesome on newly germinating seedlings," said Goldberg. When seedlings are infected with the fungus, they are weakened and become susceptible to winterkill.

Downy mildew occurs during cool, moist periods in the spring or fall. The fungus grows at night when humidity is near 100 percent, Goldberg said. The fungus usually depends on wind and/or splashing rain to spread, and water on the leaf surface is required for germination and infection by the fungus. Infection can occur at temperatures between 40 and 85 degrees, with the peak occurring at 65 degrees.

"Because the fungus requires cool, moist conditions, it is usually only a problem on the first cutting of alfalfa," said Goldberg. After the first cutting, temperatures increase and limit the fungus' activity greatly reducing the damage.

Downy mildew begins as pale yellow spots on the leaflets. As the disease progresses, the spots may enlarge and eventually cover the entire leaflet. Infected leaflets become twisted and the margins curl down while the infected stems are usually larger than non--infected stems and produce bunchy, rosette-like growths at their tip, according to Goldberg.

"The tell-tale sign of downy mildew is the production of a white to grayish downy growth on the underside of infected leaflets," said Goldberg. "This fungal growth serves as inoculum for secondary spread of the fungus."

Only the young succulent tissue is susceptible to the fungus, according to Goldberg. Infection occurs either by direct penetration of the leaflet surface, or by entry of the fungus through the leaf pores called stomata. Under favorable conditions, secondary infection cycles occur every five days.

Spring seeding is recommended for areas where fall infections are common. "Metalaxyl, a systemic fungicide, is registered as a seed treatment for alfalfa," said Goldberg. In addition to seedling protection from downy mildew, this fungicide will help protect seedlings from other damaging fungi.