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Skaggs: Dry weather means fewer Japanese beetles, plant diseases

POSTED: September 5, 2008 5:01 a.m.

The drought of the past two years has made it increasingly difficult for gardening enthusiasts and landscapers alike. However, dry weather does bring about one nice benefit - fewer Japanese beetles and lawn diseases.

According to University of Georgia researcher Kris Braman, controlling Japanese beetle grubs and the adult beetle costs more than $460 million a year in the United States alone.

Adult Japanese beetles feed on the leaves of about 300 different landscape plants from roses and crape myrtles to fruits and vegetables. As grubs, they burrow underground and feed on plant roots. White grubs, as they are commonly known, are most damaging to turfgrass roots.

Braman theorizes that the Japanese beetle population is down because their life cycle relies on moisture. If it's too dry, the grubs cannot complete their development.

"Young Japanese beetle larvae need moisture to tunnel and search for food," Braman said. "If they survive in the drought to adulthood, they need wet areas to lay their eggs in, and there weren't many wet areas to be found this summer."

The drought has reduced the typical diseases found on turfgrass and landscape plants during high humidity conditions, but it also has opened the door for other diseases, said Alfredo Martinez, a plant pathologist with UGA Cooperative Extension.

Lawn diseases such as pythium blight and brown patch are not as numerous this year because of the lack of rain and regular watering. However, other turf diseases are opportunistic - taking advantage of dry, stressful conditions.

During drought, turf is stressed, "and we have a good amount of stressed turf areas in Georgia," he said. "We are seeing more cases of anthracnose and dollar spot, which are caused by organisms that take advantage of the lawns' stressed state."

"When there's not sufficient water, people tend to reduce or avoid fertilization all together," Martinez said. "They think it will burn the turf if they do. But low fertility promotes disease like dollar spot."

Jean Williams-Woodward, a UGA Extension plant pathologist who specializes in ornamentals, agrees. She says Leyland cypress trees are being hit harder by the drought-related diseases than most other trees and landscape plants.

"Seiridium canker disease is the main cause of tree decline," Williams-Woodward said. "Drought stress causes the cankers to enlarge about three times faster than they would on nonstressed trees."

Cankers form on the branches when the fungus enters through wounds or natural openings on the tree. Collectively, the cankers interfere with water flow, and, as a result, the branches die.

The drought has driven disease pressure in the landscape down, but there are always diseases that can persevere. For example, powdery mildew, a common disease of crape myrtles, can thrive in relatively dry conditions.

While powdery mildew is unsightly, it is primarily an aesthetic problem causing little to no harm to established trees and shrubs. At this late date, treatment is not needed.

Billy Skaggs is an agricultural agent and Hall County extension coordinator. Phone: 770-531-6988. Fax: 770-531-3994.



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