Take Steps to Control Ornamental Plant Diseases

May 2, 2001

WOOSTER, Ohio - Homeowners and garden professionals should be taking steps now to control several ornamental plant diseases that can be problematic during wet spring weather, recommends an Ohio State University Extension agent.

Jim Chatfield, a Northeast District horticulture specialist, said apple scab of crabapple, black spot of rose, and Volutella leaf blight of pachysandra (a ground cover plant) are major fungal diseases that typically don't kill the plants, but are damaging enough to be an eyesore.

Apple scab causes yellowing and discoloration of crabapple foliage, wilt, leaf drop, and scabby lesions on the fruit. Chatfield said apple scab can be controlled through fungicide applications, but the timing is critical. "The ideal time to apply fungicides would be when the petals of flowering crabapples first begin to fall." In Wooster, he said, crab apples are beginning to flower now and will probably peak this week. "If you wait until you see the fungus on the leaves, then it's too late, and you're not going to be able to get rid of it."

Though fungicides are available to treat crabapples against the disease, Chatfield said the best method of control is to plant disease-resistant varieties. "The disease is easily controlled, easily dealt with, with good genetic resistance," he said. "The key is to pick a crabapple with the desired characteristics and has good resistance to apple scab."

Research conducted last year at Ohio Agricultural Research and Development Center's Secrest Arboretum in Wooster found that 33 of the 63 crabapple cultivars studied showed little or no susceptibility to apple scab. Chatfield said last year was a heavy year for apple scab due to the extremely wet spring. Cultivars found resistant to the disease include 'Adirondack', 'Camelot', 'Dolgo', 'Excalibur', Foxfire', 'Golden Raindrops', 'Hamlet', 'King Arthur', 'Purple Prince', 'Silver Moon', and 'Strawberry Parfait'.

Black spot of rose produces round-to-irregular black splotches and yellowing on the leaves. Leaf defoliation is common and repeated defoliation weakens the plant, leading to poor blooming and greater sensitivity to other stresses.

Chatfield said that some of the favorite varieties for rose lovers are prone to black spot, hence, fungicide applications may be a consideration. "The timing again is critical, though," he said. "You need to start spraying as soon as the leaves emerge and continue to spray throughout the summer. As long as it stays wet, the disease will be hard to control."

Good sanitation practices are also the key to keeping rose plants free of the disease, or at least, under control. Chatfield recommends removing diseased leaves and canes to help limit the black spot fungus. He also advises against using sprinkle irrigation to water the plants, as the wet condition leads to the spread of the disease. "Try to just water the base or use trickle irrigation instead," he said. "Also, if you do use sprinklers, turn them on in the evening so that the leaves can dry during the heat of the day."

Disease-resistant rose varieties include 'Charlotte Armstrong', 'First Prize', 'Granada', 'Pink Peace', 'Carousel', 'First Edition', 'Goldilocks Impatient', 'All that Jazz', 'Baby Betsy McCall', 'Gourmet Popcorn', and 'Polyantha'.

Volutella leaf blight of pachysandra causes leaves to develop brown blotches and eventual dieback of the plant. Chatfield said the disease is hard to control since pachysandra flourishes best in dense, shady areas where it constantly stays moist. "Good sanitation practices are important. Remove diseased leaves and stems," he said. "If the disease gets too bad, you can always mow the pachysandra bed to a height of about three inches. It won't rid the plant of the disease, but it will help control it."

Chatfield said the best thing people can do to control for any plant disease is to devise ways to stop its development at a source that can lead to infection. "There are three things that are needed for an infectious plant disease to occur: a virulent pathogen, a susceptible host and an environment conducive to disease," he said. "If you can find ways to halt the development of the disease at any one of those sources, then you can effectively control it."

For more information on apple scab, black spot and Volutella leaf blight and stem canker of pachysandra, log on to http://ohioline.osu.edu/, http://bygl.osu.edu/, or visit your local county Extension office.

Author(s): 
Candace Pollock
Source(s): 
Jim Chatfield