Plant Health Problems
These trees are not related to chestnut trees (Castanea) and are not affected by chestnut blight disease.

Diseases caused by Fungi:

Leaf blotch, Guignardia aesculi.
Large reddish-brown blotches on the leaves late in the season are surrounded with yellowed tissue. Curling of infected leaves is common, and the fruiting bodies of the fungus appear as black specks in the discolored areas.

Raking and disposing of fallen leaves will reduce the chance of infection in the following season. Since this disease is usually not a serious problem for the health of the trees, chemical controls are usually not necessary.

Anthracnose, Glomerella cingulata.
Leaves and new growth appear blighted early in the season, and wood may be killed back for a short distance. Pustules formed on dead leaves and wood produce pink masses of spores.

Control is by pruning and removing infected twigs. Since this disease is usually not a serious problem for the health of the trees, chemical controls are usually not necessary.

Diseases caused by Physiological/Environmental Factors:

Leaf scorch, physiological.
Symptoms can be similar to those associated with leaf blotch and anthracnose but are caused by drought injury and poor tree vigor. Water trees thoroughly during dry periods in the summer and fall, and fertilize in the early spring to improve tree health.

Insect Problems:

Japanese beetle, Popillia japonica.
This beetle, in the adult stage, feeds upon a great variety of trees and plants, including horsechestnut. The beetle is ½" long, bright, shining green, with copper colored wing covers. Two white spots on the tip of the abdomen show beyond the ends of the wing covers, and there are five white spots formed by patches of white hairs on each side of the abdomen. The beetles begin emerging in late June but their greatest abundance is usually about the middle of July. During drought periods, populations of Japanese beetle are drastically reduced.

Japanese beetle traps containing floral and sex attractant lures are readily available at home and garden centers. These traps may help reduce the overall number of beetles. However, beetles often land and feed on plants close to traps, so traps should be placed well away from valued plantings. Feeding on horsechestnut foliage can be prevented by using sprays of acephate, azadirachtin, carbaryl or malathion, which are among the products registered for use against this pest in Connecticut. Imidacloprid, applied early in the season as a soil drench, will provide season-long systemic control. Consult the labels for dosage rates and safety precautions. Larvae develop in turf, so treatment of lawn areas should reduce the numbers of beetles. See Lawns, oriental beetle.

Whitemarked tussock moth, Hemerocampa leucostigma.
This insect has two generations each year, and spends the winter in frothy white egg masses on the trees. Eggs hatch in late May and the caterpillars mature about July 1. They make their gray cocoons on the trees. Two weeks later, the moths emerge and females usually lay egg masses on the old cocoons. The second generation larvae hatch in July and mature in August. The caterpillars reach a length of about 1-1/2". They are striped lengthwise with brown and yellow, and are hairy, with four upright white tufts on the front half, two long black hairs near the head, and a similar one on the tail. There is a bright red spot just behind the head. The female is ash-gray without wings. The male has prominent feathery antennae and ash-gray wings with darker gray markings. It has a wingspread of about 1-1/4". This caterpillar can be controlled by sprays of malathion, carbaryl, spinosad or Bacillus thuringiensis var. kurstaki, which are among the compounds registered for use against this pest in Connecticut. Consult the labels for dosage rates and safety precautions.