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LifestyleColumnistsJessica Damiano

Garden Detective: Diagnosing black spots on maple leaves, mimosa leaf drop

Maple tar spot is a fungal disease that

Maple tar spot is a fungal disease that affected a maple tree growing in Barbara Scherg's North Babylon yard. Credit: Barbara Scherg

DEAR JESSICA: I’ve noticed for the past two to three years that the maple leaves in the yard have these brown burn spots.  What is causing that?  — Barbara Scherg, North Babylon

DEAR BARBARA: Your trees are affected by maple tar spot, a fungal disease often affecting maple trees, particularly red, silver and Norway cultivars. The disease becomes evident in June as small, pale yellow spots appear on leaves. As the season progresses, the spots become larger and darker, finally developing into raised black splotches that look like wet tar. The bad news is the spots are really ugly. The good news is the ugly spots are the worst of it: The disease is typically not fatal to trees, although severe infestations can result in trees dropping their leaves ahead of schedule.

The fungal spores survive winter on fallen leaves and samaras (often called helicopter seeds, polly noses or whirlybirds) and reinfect trees just as new leaves appear in spring, starting the cycle again. To get ahead of this, it’s important to rake and clear the soil surface of leaves and plant matter in autumn.

Treatment is very difficult because leaves must be thoroughly coated with a fungicide, and even then, because the disease is widespread and spores are airborne, nearby trees can infect yours despite your best efforts. If you want to attempt such treatment, I recommend using a licensed tree care service.

DEAR JESSICA: My mimosa tree bloomed for the first time, then within days the leaves turned brown and fell off. The tree was dead except for some new shoots at the base of the tree. What happened? In addition, nothing seems to bother my hosta, but this year the blossoms were eaten, then the leaves were eaten, too. What's going on? — Al Lovett, Huntington

DEAR AL: Seems you’re getting hit from both ends, Al — pests and disease. Your mimosa tree, or silk tree, likely has been infected with mimosa vascular wilt, also known as fusarium wilt. The disease, caused by the Fusarium oxysporum fungus, causes leaves to yellow, brown, shrivel and fall, and entire branches to die. It usually proves fatal within a month to a year.

The beautiful small-to-medium fast-growing landscape trees are valued for their fernlike foliage and pink puffball flowers that bloom all summer long. To confirm diagnosis, look for the presence of brown tissue in the wood when a lower branch is cut. Unfortunately, once a tree is infected, there is nothing that will save it. In fact, it should be removed and not chipped for use as mulch, as that would be a sure way to spread the disease.

The damage to your hosta, on the other hand, is not disease-borne: Your photo shows a textbook example of deer munching.   

DEAR JESSICA: I have lived in Huntington most of my life, and have never seen these spiders before! We have seen of them this year in different locations in our yard. Can you tell us what type they are? — Mary Rech, Huntington

DEAR MARY: That looks like a nocturnal orb weaver. Considered beneficial, they prey upon many harmful insects. And although they might look scary, they don’t typically bite unless disturbed and, even then, their venom is of low-toxicity — no worse than a bee sting, unless one is allergic. My advice is to leave them be.

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