University of Vermont Extension 
Department of Plant and Soil Science

Anytime News Article


TREE FRUIT DISEASES
 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
 
Apple scab, brown rot, and black knot are common fungal diseases on tree fruits in New England.  Being ready for these if you have crabapples, flowering cherries, and fruit trees, and knowing cultural controls, will help you have better fruit with the least harm to the environment.  A New England website of Extension services (pronewengland.org) provides some photos, information, and further resources on these and other garden problems.
           
Apple scab disease causes velvety brown lesions on apple and crabapple leaves in spring and summer.  It causes corky scars and splitting on fruit in late summer and fall.  There are similar scab diseases which attack peaches and pears.  Watch for the most apple scab after rainy, cool spring weather.

Since infected leaves drop and produce spores the following spring, which spread the infection, one method of control is to keep such leaves raked in the fall and destroyed.  There are fungicides that may be used, but if doing so be sure and follow all label directions.  Perhaps the best control if you are considering planting apple trees, or replacing old ones, is to plant varieties resistant to this disease.  These, and highly resistant crabapples, can be found in a Cornell University leaflet (plantclinic.cornell.edu/Factsheets).

Brown rot disease kills blossoms of peach, cherry, plum, and other stone fruits in spring. It causes a soft, watery rot on fruits in summer.  These infected fruits eventually dry (called “mummies”) and produce spores for the following spring.   So one of the best controls is to prune such dried fruits from trees and burn or bury them deeply.  Insects create wounds on fruits for the disease to enter, so controlling insects in summer helps to minimize damage.  Since most stone fruits are susceptible to brown rot to some degree, fungicides often have to be used for optimum control.  Check your local garden store for these, and follow all label directions.

Black knot disease is found on plum and cherry trees.  Spring infections lead to an inconspicuous swelling on current year growth during the fall.  These swellings turn into green and soft knots the following spring.  By the second fall these knots turn hard and black, from a half inch to over a foot long.  Since this disease takes so long to develop, it is often overlooked until it is hard to manage.

Prune out branches with such knots during winter, before spring bud break.  Make sure to cut the branch at least six inches below where you see the knot, in order to remove all the disease.  Remove such knots in any wild plums or cherries that may be nearby as well.  Pruning, planting resistant varieties, and not planting trees on sites with the problem (abandoned orchard or infected wild trees adjacent) usually will keep this disease under control without fungicides.

Other diseases to be aware of on apples include cedar-apple rust, fireblight, and powdery mildew.  Other diseases on stone fruits include botrytis blight, brown rot, cytospora canker, and powdery mildew.  More on all these can be found at the above Cornell factsheet website. 

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