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Over the Fence

Where gardeners come to find out what's happening out in the yard.

Scale, Scab, Rust and Dormant Oil

Posted by Richard Hentschel -

Down the Garden Path

Richard Hentschel, Extension Educator

All the recent rains have reminded home orchardists and homeowners that the time to treat for our two most common leaf diseases is upon us. It is when the weather is really not very nice outside that fungicide sprays for Apple Scab and Cedar Apple Rust need to be started. The indicator is something called "green tip", when the buds have broken and just the very tip of the new green emerging growth has emerged from the protective scales on our apple and crabapple trees. We are not there yet, but will be soon. These diseases impact fruit trees a bit more than crabapples since we do not eat that fruit. Both diseases cause damage to the leaves and that means less food production leading to a poorer quality fruit.

Apple Scab is definitely more troublesome as it will cause the leaves to fall to the ground before mid-summer. Getting a good apple at that point is very difficult since the leaves are producing the energy that will fulfill the fruit. With ornamental crabapples, the issue is really of appearance, a tree that is mainly without leaves a great deal of the season. The spores like the cool wet weather since they can travel farther before drying out and dying. The Apple Scab spores are coming from the infected leaves just below the trees and the Cedar Apple Rust spores will be floating back from their evergreen hosts around the home or neighborhood.

If you are buying fruit trees this spring make sure they have resistance to both these diseases. If you have trees already and they are prone to either of these foliar diseases, then be prepared to make several treatments per the label instructions. One cultural management practice that will help with Apple Scab is to rake up and remove last year's leaves to lessen the potential for disease in 2013. There is still time to do that before growth resumes this spring.

This time of year is also when we will treat our fruit and ornamental crabs for overwintering insects and insect eggs with a dormant oil spray. The common insect that we treat is the scale insect that is naturally protected at most other times of the year. The key to good success with dormant oil is getting great coverage. The product works by smothering eggs and insects, so the better the coverage, the better control. Since this material is mixed with water, temperatures must remain above freezing for at least 24 hours and applied before we see any green tip. This is the reason we monitor the weather for a few days to be sure it will be ok to put this treatment on our trees. Treatments for both the diseases and a dormant spray for insects is a great way to start the growing season.

 

 



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