Does a July application of dormant oil spray have any value in fighting apple...
Does a July application of dormant oil spray have any value in fighting apple scab disease?
Cuyahoga County Ohio apple scab
Hi. No, that will not do anything for apple scab. Oils are effective against some insects, and apple scab is a fungal infection. Also, dormant oils must be applied when the plant is dormant (i.e., before it leafs out in the spring) and should not be applied in the summer. Here is a very nice column from the U. of Illinois on dormant oils, what they treat, and how they are used. Plants are generally infected with the apple scab fungus in the spring, and once a plant is infected, there is no treatment. This is true with the vast majority of fungal infections; they can only be prevented, not cured. One way to try to prevent infection is to use disease-resistant varieties. Another is to apply fungicides before plants are infected (in the spring). If you do use a fungicide, be sure to follow the label directions. This page from the U. of Minnesota discusses these and other management strategies for apple scab that you can use.