Tree, turf and ornamental plant update – Sept. 7, 2018

An update on strawberry root weevil adults, green stink bugs, tar spot on maples and leaf blotch on horse chestnuts.

Horse chestnut leaf blotch
Horse chestnut leaf blotch. All photos by Jill O'Donnell, MSU Extension.

The following is a summary of current plant development and pest activity for landscape professionals, Master Gardeners, nursery and Christmas tree growers.

Strawberry root weevil adults (Otiorhynchus ovatus) are a common home invader in late summer and early fall. The weevil is small, one-quarter inch long, dark brown or black, pear-shaped and has small, longitudinal grooves on its back that look like a series of tiny, indented spots.

We are finding larger populations of green stink bug [Chinavia halaris (Say)] in strawberries, blueberries, field crops and home gardens. Stink bug adults and nymphs can cause damage to plants with their piercing-sucking mouthparts. The nymphs are predominantly black when small, but as they go through different instars, they become green and yellow or red. Like other stink bugs, the adult green stink bugs are shield-shaped, solid light green and measure 0.5–0.75 inch long.

Signs and symptoms of maple tar spot are becoming more noticeable on maples in the landscape. Initially, the lesions are yellowish in appearance, and by the end of the season they take on a shiny, black appearance, hence the name tar spot. Several species of Rhytisma cause tar spot. The appearance of the spots is alarming to some and in some cases, severe disease will result in premature leaf drop. However, the health of large, well-established trees that are otherwise healthy will not be significantly impacted.

Rake and destroy infected leaves this fall to remove the overwintering fungus from the area. For more information, see the Maple Tart Spot Fact Sheet from Michigan State University Diagnostic Services.

Leaf blotch of horse chestnut is caused by the fungus Guignardia aesculi. Leaf blotch causes browning of the leaves, especially during years with wet springs. It is usually not of concern to the health of the tree although young trees and nursery stock may suffer due to complete defoliation. Rake and destroy infected leaves this fall to remove the overwintering fungus from the area.

Note: Horse chestnut is not edible. For information on the differences from sweet chestnuts, see “What’s the difference between horse chestnuts and sweet chestnuts?” from MSU Extension.

Close-up of brown spots with yellow margins caused by  leaf blotch

Closeup of brown spots with yellow margins caused by leaf blotch.

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