Expert speaks of fruit disease management after wet year

AFP Correspondent

BEDFORD, Pa. (March 24, 2015) — In her presentation at the Appalachian Tree Fruit Meeting on March 3, Penn State pathologist Dr. Kari Peter asked the growers, “Which diseases are influenced by wet weather?”
The answer, of course, is almost all of them. Powdery mildew is a notable exception.
As the growers look forward to the coming spring, Peter offered suggestions on how to cope with apple scab and fire blight particularly.
The wet season, coupled with the recent unusually cold weather, presents challenges.
Apple scab must be controlled early in the season, Peter advises. “If not controlled early, you’ll fight it the entire season,” she said.
To defeat apple scab, one must know how it develops.
It needs organic matter to survive the winter. Sanitation, including removal of leaves from the orchard floor, is essential in controlling overwintering spores.
The previous year’s leaves produce ascospores which infect new tissue. These primary infections result in lesions which produce the conidia spores.
Splashing rain and wind spread those secondary phase conidia spores, reinfecting the leaves and the developing fruit.
Peter illustrated the importance of the time period of wetness and temperature on apple scab spread. According to the Mills table, only six hours of wetness are required for the disease to develop in nine to 10 days at 61 to 75 degrees Fahrenheit. At temperatures above 80 degrees Fahrenheit, Peter noted, the spores are not viable.
Since rainy seasons obviously are conducive to apple scab, Peters told growers to reapply fungicides in rainy weather. In addition, she stressed that good coverage must be achieved, equipment calibrated and adherence to management principles followed.
Fungicide recommendations with recommended timing and rates are contained in the “2015 Spray Bulletin for Commercial Tree Fruit Growers,” available for ordering and download at
It includes information for Maryland, Virginia and West Virginia.
Peter pointed out that cherry leaf spot is scab of cherries. Consequently apple scab strategies can be used for cherry leaf spot as well.
The broad leaves of tart cherries are more susceptible to cherry leaf spot infection. This pathogen, too, is more troublesome in wet weather, and can defoliate trees.
Fire blight also requires moisture but does not function the same way as apple scab. It overwinters in the living tissue surrounding cankers in trees. These cankers ooze bacteria which attract insects. Although the insects do not cause the disease, their movement spreads it.
The optimum temperature for fireblight bacterial population spread is 65 degrees Fahrenheit. The presence of moisture is essential also.
Rain and dew wash the bacteria from the stigmas to the nectaries. The bacteria then enter the plant and move systemically through the plant. This causes shoot blight.
Since canker blight occurs regularly once established, management becomes imperative.
In the dormant season, thorough pruning to remove blighted limbs, shoots and cankers will reduce the number of sites for subsequent year infections. At the green tip/pre-bloom period, use copper sprays to prevent colonization of tree surfaces by the bacteria before bloom.
Peter urged providing proper fertility, but cautioned against excessive nitrogen which increases susceptibility. Thus, avoid legume cover crops. Also avoid other tree stressors such as nematodes and poor drainage.
In the bloom/petal fall stage, keep in mind that blossom sprays only protect open flowers. Just before or just after an infection event is the preferred time.
Peter said that streptomycin is still effective, but she cautioned that the label must be followed. The Virginia Tech spray bulletin mentioned above gives the appropriate state recommendations for the above periods. Post bloom to terminal bud set and late season management strategies are included.
In the later periods, Peter advised regular monitoring for infections and pruning when spotted. Sucking insects should be controlled as well.
Although challenging, well-timed management can limit damage from these diseases.