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MASTER GARDENERS: Be on the lookout for tree diseases

This is the time of year when many homeowners notice "something wrong" with a tree or shrub on their property.

Indeed, this is the time when pests and diseases are at a peak. Learning to recognize those that do not compromise the health of the tree or shrub and those that are more serious can be tricky.

Leaf spot diseases on trees and shrubs show up on leaves as small brown spots, sometimes with a transparent center. Maples will have black spots on the leaves (tar spot). Other trees will have blotchy brown patches on the leaves. In most cases these lesions are caused by fungi.

Hot humid weather, cool nights and overhead watering all promote the growth of the various fungi that cause these diseases. In most cases these do not affect the overall health of the tree or shrub unless the infection is so severe that many leaves are lost several years in a row.

To prevent the pathogen from coming back every year, sanitation is important. Rake up leaves before the first snowfall and remove them. Prune trees and shrubs to improve air circulation. Water shrubs at the base, not overhead. Reduce overcrowding. All these will reduce fungal infections.

Look for cankers on the bark around shoots or branches that are dying or showing signs of stress. Cankers form on the trunk and on infected branches causing them to crack and often ooze sap or black tarry liquid. Infected shoots or branches need to be pruned out. Infections in the trunk are serious and can compromise the whole tree.

Leaf rusts are also common. They appear as yellow or orange powdery spots on leaves that are easily rubbed off. The most common hosts to these diseases are apples, roses, currants, gooseberry and poplars. Some are devastating to other trees like white pine blister rust and cedar apple rust. Currants, gooseberries and white pines are not good neighbors for this reason.

Downy mildew and bacterial leaf spot are two other infections that show up this time of year. These infections are usually seen as small brown spots or fine white filaments on the surface of leaves. Most do no permanent harm to trees and shrubs.

People have also noticed different colors of bumps or little skinny projections all over the leaves especially on oaks, maples, hackberry, and elms. These are galls and are caused by insects such as gall wasps, aphids or others feeding on the leaf. While ugly, they do not harm the tree.

Far more serious is oak wilt and emerald ash borer damage. Usually this is seen as dead tips on the trees initially and eventually kills the entire tree. Professional assistance is needed to verify and recommend isolation or removal of these trees.

If you grow apples, crabs, pears, cotoneaster, or raspberry watch for fireblight, so named because the affected branches look as if they have been scorched. It is a bacterial infection that can be very devastating. It can be managed if caught early by pruning out the affected branch well back of the infection. Disinfect tools between each cut and bag or burn the infected branches.

The University of Minnesota Extension website has been updated to make finding information like this much easier. Simply type "tree diseases" in the box when you open the Extension page. For broader information go to "Learn About" and click on "Yard and Garden."