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New Mexico State University

New Mexico State University

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Hot, Sticky Weather Favors Chile Disease

LAS CRUCES -- High temperatures and humidity in southern New Mexico could bring on problems in chile with powdery mildew, a yield-reducing disease that damaged crops in 1996 and 1997.

Powdery mildew fungus causes plant leaves to curl, exposing chile pods to sunburn injury.

The amount of damage depends on when powdery mildew strikes said Natalie Goldberg, plant pathologist with New Mexico State University's Cooperative Extension Service.

"If it occurs in July, we can see significant losses by the end of the season," she said. "But if the disease is delayed until August or September, our losses are generally reduced. In fact, when the disease occurs very late in the season, it may actually provide a little bit of a benefit in defoliating the plants in time for red chile harvest."

Growers can take several steps to prevent crop damage, starting with thinning their chile.

"Anything that reduces humidity in the plant canopy is going to slow the disease, so good thinning of the fields and making sure plants are not overcrowded can be helpful," Goldberg said.

Powdery mildew attacks the lower leaves of chile plants first. The disease causes a small lesion on the upper sides of leaves and a grayish powdery growth on the undersides. It gradually works its way up the plant as the disease develops.

Sulfur is the only fungicide registered for powdery mildew in chile.

"Sulfur needs to be applied as a preventive measure because it's not very effective once the disease has any headway," she said. "Growers who can get the sulfur on early on the undersides of leaves and lower part of the plant canopy can see some reduction in disease severity."

The smaller plants are, the easier it is to get ground rigs into the fields to apply sulfur, she said. The material must be reapplied if rain washes it off.

Growers have another treatment option until the end of July, Goldberg said. An emergency exemption for use of the fungicide myclobutanil, sold under the trade name Nova, expires Aug. 1, 1998, for New Mexico.

Rare only five years ago, powdery mildew appears to be entrenched in southern New Mexico. "Unfortunately this disease can survive on a wide range of host plants," Goldberg said. "It's probably here to stay."

The disease affects lettuce, many weeds, solanaceous plants like tomatoes, and native ornamental plants such as Mexican Bird of Paradise and Mexican Evening Primrose, she said.