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New Mexico State University

New Mexico State University

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Powdery Mildew Found on Chile Plants

LAS CRUCES - A familiar fungal disease -- powdery mildew -- has been detected on chile pepper plants in the Las Cruces area.

"High humidity such as we've seen the past couple of weeks is a key condition for this increasingly common disease," said Natalie Goldberg, plant pathologist with New Mexico State University's Cooperative Extension Service. Only six years ago, powdery mildew on chile was considered uncommon in New Mexico, she said. But favorable conditions for the disease and an overwintering population of the fungus have made it an annual problem for chile growers.

Powdery mildew is a fungal leaf disease that attacks chile plants of all ages and varieties. A severe infection can prematurely defoliate chile plants, reducing yield and quality.

"A telltale sign of this disease is the presence of a white, powdery growth on the underside of leaves," Goldberg said. "As the infection progresses, the leaves curl up and expose the powdery fungus."

Eventually, leaves fall from plants, exposing developing fruit to direct sunlight. Sunburned fruit can be a secondary problem. The disease problem also is likely to be more devastating earlier in the growing season when favorable conditions occur.

"Powdery mildew has occurred at economically damaging levels for the past few years," Goldberg said. "We know that a four- to five-day period of maximum humidity near 100 percent and minimum humidity above 35 percent is excellent powdery mildew weather. A few good thunderstorms, and the disease begins to appear."

Powdery mildew is very difficult to manage once infection is noticeable. Protective fungicide sprays with sulfur can be effective, Goldberg said. "Unfortunately, conditions in New Mexico make practical use of this material difficult. Sulfur can cause damage to the plants if temperatures rise above 90 degrees in the three days following application."

In addition to proper timing of sulfur applications, thorough coverage is vital for successful treatment. In chile, the fungus first attacks the underside of the older leaves, then spreads throughout the plant. So sulfur must be applied to the underside of leaves throughout the canopy.

Although additional chemicals are being evaluated for use on chile to control powdery mildew, none are currently registered. However, on July 19, New Mexico received a Section 18 emergency exemption for Nova (myclobutanil) to help manage this disease. For more information on this exemption, contact the Pesticides Bureau at the New Mexico Department of Agriculture.

"From now on, it's safe to say that powdery mildew will be a disease of concern for New Mexico chile growers," Goldberg said. "The severity of the disease each year will depend on environmental conditions; the more rain and high humidity during the hot summer months, the more severe our problem will be."