scab (Venturia ineaqualis) is the most destructive fungal
disease affecting the home apple planting. It is most severe in
regions that are cool, humid and rainy. If left untreated, it will
cause lesions on the leaves and fruit. It can defoliate susceptible
varieties in mid-season, which will weaken trees, reduce yields,
and blemish fruit so that they crack and split, making them fit
only for cider.
in the spring, infected leaves from the previous season which have
been lying on the ground all winter produce sexual spores called
ascospores. Primary infections on new leaves develop when rain causes
the ascospores to be forcibly ejected. In mid-May to early June,
infections begin to develop on the underside of leaves, or fruit
spur leaves, as these are the sides first exposed when the fruit
buds open. The young spot or lesion has rather indefinite margins,
but within two to three weeks the boundaries become very distinct
and the entire lesion becomes velvet-green in color, indicating
spore production. These asexual spores, called conidia, are spread
by rain and wind and will infect the newly developing fruit and
other leaves throughout the rest of the summer and into the fall.
If infection occurs in late summer or early fall, rough black, circular
lesions may develop on the fruit in storage. These lesions are usually
very small, varying in size from tiny specks to 1/4 inch diameter,
and are know as "pin point" scab.
by R.Harrison, URI Plant Clinic
Control of apple scab is possible through resistance, sanitation
and/or fungicide treatments.
There are many scab-resistant varieties available through nurseries
and local garden centers. Choice of these varieties can reduce or
eliminate the need to apply fungicide sprays to control apple scab.
We have over 20 resistant varieties planted at our University research
farm, but after 10 years of evaluation the only varieties we recommend
for Rhode Island are Pristine, William's Pride, Redfree, Liberty,
Nova Easygro, Freedom and Enterprise. See GreenShare
Factsheet on Apple Scab Resistant Varieties for more information.
Maintaining a healthy and vigorous orchard through proper watering,
fertilization, and pruning should be an integral part of any pest
management program. Prune trees well in the dormant season and also
in late July to early August to thin out the canopy.
Sanitation should be used to enhance any chemical spray program.
Providing good air circulation will help reduce disease development
while allowing chemical applications to penetrate more effectively.
Raking and burning (or bagging and hauling to landfill) all fallen
leaves will help decrease the amount of early spring spores. Remember
to include any scab-susceptible crabapples in your sanitation program.
Do not compost infected leaves, as most homeowner compost heaps
do not reach temperatures high enough to kill pathogenic fungi.
Managing primary scab is the key to winning the battle against
this fungus. If controlled early, secondary infections will be far
less severe. Susceptible trees such as McIntosh, Cortland, Red Delicious
or Rome should be sprayed every seven days, from the time buds begin
turning green in the spring until mid-June. If a fungicide must
be applied, it is far better to spray early in the season before
apples are formed or when they are smallÜthe further into the season
that chemicals are applied, the more residue is likely to be on
a detailed spray schedule, call the URI Cooperative Extension Hotline
at 1-800-448-1011. See the "Home Tree and Small Fruit Pest Management
Guide" for an overview of home apple tree management and sources
of disease resistant fruit trees. For the latest information on
infection periods or pest outbreaks during the growing season, visit
the URI Apple IPM website at http://www.uri.edu/research/IPM
By David B. Wallace,
Plant Protection Specialist, and Heather H. Faubert, IPM Specialist