Plants can survive effects of wet, warm weather

Last Updated: 
July 30, 2010

CORVALLIS, Ore. – Apple scab fungus that is making apple tree leaves curl and blacken is a result of an unusually wet spring in western Oregon. The same goes for black spot on rose bush leaves and other fungal diseases usually not as prolific in back yards.

"All kinds of things are going on," said Jay Pscheidt, plant pathologist at Oregon State University. "The cold, wet weather has kept pollinators away, and fungal and bacterial diseases are evident in unusual volumes.

"We're even seeing effects of winter injury from freezing temperatures in December of last year."

In most cases, there is little that can be done to treat already damaged fruit trees and shrubs, said Pscheidt, who advises not worrying if trees show symptoms of disease.

"There are a wide variety of cultural and biological techniques to manage or prevent disease," Pscheidt said. “Good sanitation is important.”

Pscheidt recommends removing and burning diseased leaves and old fruit from the tree and the ground. These should not be used for mulch or cold compost, which will not kill pathogens. "The best we can do is to plan and prepare for next year," Pscheidt said.

It's also important to select resistant cultivars. For example, Liberty, Prima, Akane, and Chehalis apples are resistant to apple scab, while Granny Smith and Gala are not. More information is available in an OSU Extension publication: "EC 631, Managing Diseases and Insects in Home Orchards," online.

Accurate diagnosis of plant disease also can prevent problems. Melodie Putnam, plant pathologist and chief diagnostician of the OSU Plant Clinic, recommends seeking advice from OSU Extension county offices first. At the plant clinic, most of the 2,300 yearly clients are commercial growers, but gardeners also are welcome to use its services, which include advice on disease management. Cost is $45 or $40 per sample if submitted via an OSU Extension county office.

Commercial fruit and berry growers are experiencing wet-weather problems to a lesser degree than are home owners, Pscheidt pointed out, primarily because they know and have implemented preventative measures.

"Also, many commercial orchards are located in areas of the state that are drier than the Willamette Valley and thus do not have as much disease pressure," he said.

Many of the disease management techniques or tactics that growers use come from the work of OSU researchers Pscheidt and his colleagues who study plant pathology and conduct disease management trials on a 50-acre plot of land east of Corvallis called the Botany and Plant Pathology Field Laboratory.

There, the scientists do the opposite of what they tell growers to do, and the orchards respond in kind with an abundance of fungal and bacterial diseases to be studied such as shot hole, peach leaf curl, shock virus, brown rot and powdery mildew, to name a few.

Although home owners might watch both the weather and their trees and plants with dismay, they are advised not to panic. "Their trees are not likely to die," Putnam said. "If there are no major problems, trees can sustain leaf loss."

Author: Judy Scott