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The Greener View: Bacterial Leaf Spot

Jeff Rugg on

Q: We want to trim our trees, but we were told not to cut off any branches in the summertime. Why is that? The weather is warm, and we can be outside, and we don't want to trim the trees in the winter when it is cold. Is it really better for the trees to prune in winter?

A: There are several benefits if trees are not pruned in the summer. First, there is less landscape waste generated without the leaves on the branches. If you chip or shed the branches without the leaves, you can use the chips as mulch, but with the leaves, you will need to compost it first to break down the leaves.

Every pruning cut creates a wound. To prevent insects and diseases from getting into the tree, wounded tissue needs to dry up, and the remaining cells need to create a scar tissue that can heal across the wounded area. When the cut occurs during the winter, the cut wood can dry out and the scar tissue can begin forming in the spring before insects and diseases become available. If the wound is created when the weather is warm and potentially rainy, or at least humid, then bacteria and fungi can infect the open wound.

As you cut the wood, you will smell the fragrance of the fresh-cut wood. Insects can find the wound much more easily because they, too, can smell the cut wood. Research has shown that insects are attracted to fresh cuts on birch, elm, oak and pines. It wouldn't surprise me to find out the same is true for the emerald ash borer on ash trees.

So, insects and diseases are both more likely to attack trees pruned in the growing season than the dormant season. Does that mean we shouldn't do any pruning in the summer at all? If possible, I wouldn't trim any of the trees listed above, but the best time to prune small trees and shrubs for the promotion of more flowers is the month or two after they bloom, so late spring to early summer is the time that crabapples and fruit trees are pruned.

Any trees damaged by storms in the summer are better off getting proper care and pruning than letting them sit with large areas of damaged tissue. Any time dead stems or branches are noticed, they can be pruned off as long as the cut doesn't damage the live branch and is just cutting off the dead tissue.

Q: I have a plum tree with little holes in the leaves. This happened last summer and again this spring. Can you tell me what the cause could be and how to treat it?

 

A: Plums, apricots and peaches all get a bacterial disease called bacterial leaf spot. It is also commonly called shot hole disease, because it looks like someone stood back and shot the tree with a shotgun.

The disease spends the winter in the stems. The spring rain opens up lesions on the stems and splashes on to leaves, stems and fruit. On a leaf, it kills small areas of leaf tissue that drop out, leaving a hole. If the infection is bad, it can kill the whole leaf. If the tree loses a lot of leaves, it will decline in vigor, and other things may kill it, such as a very cold winter.

The infected fruits develop brown or black dots that eventually become pits and cracks. The fruit is still edible, but it will be unsightly.

The disease can be controlled with a weekly fungicide spraying in the spring, starting when the tree begins to leaf out. Use any fungicide that is labeled to treat bacterial leaf spot on plums, apricots or peaches.

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Email questions to Jeff Rugg at info@greenerview.com. To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

 

 

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