Diseases & Pests —
Disease of the Month: Guignardia on Aesculus Pest of the Month: White Grubs
by Heather Prince
Pest of the Month: White Grubs
Disease of the Month: Guignardia on Aesculus
One culprit of brown patches in a lawn is white grub, a common name for the larvae of June beetles, chafers, and Japanese beetles that feed on the roots of turfgrass. These grubs are found in the top eight inches of soil beneath lawn grass. They are white, C-shaped larvae, about an inch long and have six jointed legs attached close to their small brown heads. Excessive root feeding by white grubs leaves turfgrass poorly anchored to the soil and damaged areas can be pulled back like a rug. August is the best time to scout for young grubs when they are hatching and beginning to feed. Cut a one-foot square of turfgrass and roll it back. If you find 10 to 12 grubs or more, you have enough insects that treatment is warranted.
A common fungal disease on Aesculus trees, particularly horsechestnut (Aesculus hippocastanum), guignardia (Guignardia aesculi) leaf blotch begins to show its ugly face in mid to late August. Spots on leaves begin as rapidly enlarging, irregularly shaped, water-soaked areas that eventually turn red-brown with a yellow halo. These White Grubs then merge with the surrounding healthy green tissue, turning the whole leaf rusty brown and dried with distortion and partial shriveling. Tiny black fruiting bodies appear within lesions, which tell you it’s fungal and not environmental scorch. In severe cases, leaves will drop prematurely. The pathogen overwinters as fruiting bodies in fallen leaves. In spring, fruiting structures release spores into the air. An extended period of leaf wetness following spore landTreatment: ing will initiate germination and infection. For a systemic insecticide, neonicotinoids such Guignardia on Aesculus About 10 to 20 days after infection, infected as imidacloprid are effective and can remain within leaves can produce new fruiting bodies and initithe turfgrass plant structures for up to three months. However, ate secondary infections. neonicotinoids are broad-spectrum insecticides, so avoid applying them to flowering plants within lawns to prevent pollinator Treatment: exposure. Trichlorfon is an effective and short-acting treatment In most cases, cultural controls such as raking up infected leaves for white grubs that comes as a granular formulation that must be and pruning out twigs with cankers is recommended. Keep trees watered in to the turfgrass. Chlorantraniliprole is a more selecwatered and healthy so they can push out new growth. However, tive insecticide that can provide control for white grubs and some if anthracnose fungi have repeatedly hit a tree or a very young caterpillars that feed on turfgrass but has a lower risk of harming tree is involved, fungicides may be applied. Spray in the early pollinators like bees. GrubGone!® is a microbial product with spring when buds begin to swell and reapply every 7 to 14 days the active ingredient, Bt galleriae (Bacillus thuringiensis galwhen it is a rainy, wet spring. Fungicides listed for anthracnose leriea), a soil microbe that damages the gut of beetle larvae when include chlorothalonil, thiophante-methyl, mancozeb, propiconit is consumed. Turf allowed to go dormant in the heat of sumazole, and copper fungicides. mer is the least attractive place for beetles to lay eggs, but brown lawn may not be an option for clients.
University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php 217-333-0519
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Additional resources:
The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 The Landscape Contractor August 2020
Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972
