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Do your trees have apple scab?
This article has been updated. Please check our website for the most recent story.
July 13, 2006
CORVALLIS - Apple scab on crabapples and eating apples is in full swing in western Oregon, according to Melodie Putnam, plant diagnostician with the Oregon State University Plant Clinic.
What does apple scab look like? OSU's "On-Line Guide to Plant Disease Control" shows photos of infected leaves and fruit. On the Web, go to: http://plant-disease.ippc.orst.edu/
On leaves, apple scab shows up as regular to irregular velvety olive green spots on the leaf surfaces of apple and crabapple trees. On some apple trees, the spots are a dark olive to almost charcoal gray. These spots are often associated with the small secondary and tertiary veins (secondary veins are those that branch off the main veins, while tertiary veins branch from the secondary veins).
As the infection becomes more severe, the affected portions of the leaves may turn yellow, and eventually the entire leaf may become yellow and drop off. Leaves may also drop while still green. Very susceptible trees will drop a lot of leaves during the summer, so that by mid-summer they may have only half as much foliage as they started with.
Plants affected earliest in the season are those that are most susceptible, trees that have some resistance showing damage later than susceptible varieties.
During fruiting season, the infection also shows up as scabby-looking areas on the apples. Infected fruit may have one or more small lesions or may have large dark lesions with small cracks in them that distort their appearance. The fruit may eventually crack, allowing fruit-rotting organisms into the apple. Late season infection may not show on fruit until they are in storage, where rough, black, circular lesions result. To combat apple scab, planting resistant trees is really the best option for home gardeners, recommended Putnam.
Also, it helps to remove the fallen leaves of infected trees and dispose of them by burning, or put them in the garbage, as the leaves harbor the infection. Avoid watering fruit trees by overhead sprinklers.
Since 1985, OSU has tested scab-immune and scab-resistant apple varieties at their experimental farms in Corvallis. The trees received no fungicide applications. The following eating apple varieties showed no symptoms of apple scab infection, plus had good flavor:
- Redfree - a medium-small crisp, sweet red apple that matures in late August;
- Priscilla - a small, part red apple that matures in mid-September;
- Liberty - a medium red very firm apple that matures in early October. Rated as having the best flavor of all the apple-scab immune varieties.
For more information on "Scab-immune apple varieties for new orchards," EC 1334, visit our on-line catalog. Our publications and video catalog at: http://extension.oregonstate.edu/catalog/ shows which publications are available on the Web and which can be ordered as printed publications.
Source: Melodie Putnam