Dogwood Tree Leaf Blight
I have a dogwood tree with purple splotches on the leaves. How to I treat it? The tree is about 15 feet high. The soil is clay like. It has had dying twigs which I pruned off up to 6-8 feet. So a lot of daylight shows through. The lower leaves have purplish spotting on their upper surface and it seems to be spreading to the higher level leaves. The underside of the leaves has some spotting but not severe. I can prune the lower leaves but not the upper leaves. Last Spring I gave it some fertilizer recommended by a local nursery, But it does, it seem to help. I have been watering it during the current hot weather. This leaf spotting has been going on for the last several years, so the fertilizer did not cause it. I have been getting suckers off the main trunk and will be happy if they turn into branches in a few years to fill out the tree. Also, the bark on the tree trunk and branches has lime green spotting of various sizes and the bark on the lower tree trunk has yellow ochre spotting. Is there a spray I can use? or a special fertilizer? We really love the tree and it is still beautiful when it blooms in the Spring.
Baltimore County Maryland
Based on your description you may be dealing with powdery mildew, a fungal disease. Older infected leaves can develop purple blotches that progress to dead areas. See our publication on management and control. The best recommendation is to plant powdery mildew resistant varieties in the future. Pruning for air circulation may help. You will have to monitor the plant in the future and decide if you want to spray fungicides or horticultural oil products listed for powdery mildew. This disease does not kill the plant but can weaken it.http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG12_IPM_Series_Dogw...
The green bark and branches may have lichens. They grow harmlessly on tree trunks and no control is necessary. They may grow on mature trees and are not a reason for decline.
Slime molds can grow on the mulch and may climb the trunk. No control is necessary. Google images to see if this is what you are referring to. If not, you can send us digital photos.
Best recommendation is to Keep the tree well watered during dry periods and make sure mulch is no thicker than several inches and keep away from the base of the trunk. Woody shrubs and trees rarely require supplemental fertilizer.