Pest Diagnostic Clinic, Factsheet on pest & diseases of horsechestnut

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PESTS AND DISEASES OF HORSE CHESTNUT
(Aesculus hippocastanum)


The horse chestnut, a medium-sized tree belonging to the botanical family Hippocastanaceae, is widely used for shade and ornamental purposes in streets, parks, campuses, and other large urban spaces. It is easily recognized by its palmate compound, opposite, and long-petioled leaves, which have 5 to 7 abruptly pointed, wedge shaped, irregularly toothed leaflets. It's large; 15 to 30 cm high, showy, chandelier-like white flower clusters are tinged with red and appear shortly after the leaves unfold in the spring. Unlike the true chestnut (Castanea sp.), the fruits of horse chestnuts, which are large, round, spiny capsules containing 1 or 2 large, shiny, brown nuts, are not edible.

Horse chestnuts can be transplanted balled and burlapped into moist, well drained soil where they can reach heights of 30 metres. They prefer full sun or light shade. This species should not be planted on extremely dry sites.

The Ohio buckeye (Aesculus glabra), a native species to the Southeast and Central United States closely related to the horse chestnut, is sometimes also planted as an ornamental in Ontario.

The following is a brief description of the more common disease and pest problems encountered on horse chestnut and Ohio buckeye in Ontario.
LEAF BLOTCH

The fungus Guignardia aesculi causes leaf blotch on horse chestnut. This pathogen overwinters in diseased leaves. Fungal spores produced on these leaves initiate infections the following spring. Reddish-brown blotches, usually with a yellow border, appear on the leaflets and leaf stalks by mid summer. Infected leaves turn brown and may fall early. Numerous, pinpoint, black fruiting structures of the fungus develop in the blotches. These fruiting bodies serve to distinguish leaf blotch from scorch although both diseases may be present on the same tree. During wet periods in summer, the fruiting bodies called pycnidia, produce spores which initiate more infections and cause further spread of the disease. For this reason leaf blotch tends to be more severe in seasons with frequent rains.

Control: The impact G. aesculi has on the overall health and vigour of an infected tree is minimal since symptoms rarely develop before mid-summer, after the trees have put on the season's growth. The disease however reduces their aesthetic value. Gathering and removing fallen leaves will provide good control. The leaves can be composted, buried, burned or otherwise disposed of. There is no fungicide in Ontario with a domestic registration for the control of leaf blotch on horse chestnut. If leaf blotch remains a problem and control is necessary, consult a licensed tree care company.
LEAF SCORCH

Leaf scorch is a physiological disorder caused by unfavourable weather or soil conditions or a combination of both. Tips and margins of the foliage become brown and papery, and the leaves become curled, giving the tree an unsightly appearance. Leaf scorch has been reported to be severe following both hot and dry as well as wet weather. It is particularly common on horse chestnut growing close to pavement where the soil is often compacted. Heat reflected from pavement and buildings, and root restrictions (eg. by foundation walls), compounds the problem.

Control: Watering may be helpful in dry seasons. If the soil is compacted, it should be aerated to improve water percolation. Providing favourable growing conditions for the horse chestnut will minimize leaf scorch development.
ANTHRACNOSE

Anthracnose caused by the fungus Glomerella cingulata is a common leaf disease on horse chestnut. The fungus is particularly aggressive toward plant parts already weakened by environmental stress, improper nutrition, or natural senescence. On affected leaves, the petioles, midribs, and veins turn brown. The association with the vein of the leaf browning differentiates anthracnose from leaf blotch where the browning covers large irregular areas of the leaf. Unlike leaf blotch, terminal shoots infected by G. cingulata can become blighted down to several centimetres below the bud. Diseased tissue is shrunken and ruptured. Acervuli, fruiting structures of the fungus, which contain spores, are formed on diseased leaves and bark.

Control: Since the fungus overwinters on fallen leaves and on infected buds, twigs and branches simply raking and disposing of the leaves in the fall does not prevent re-infection the following spring. When practical, prune out and destroy infected twigs and branches, especially on young trees. Lime sulphur can be applied when buds are dormant, and a registered protectant fungicide can be applied during the growing season. It is generally uneconomical and difficult to spray large trees with fungicide. Comparable results may be realized through fertilization and watering to encourage re-growth. Though defoliation may be severe, most trees will re-foliate and survive even if anthracnose is a recurring problem.
OTHER DISEASES

A few other minor diseases can also be found on horse chestnut. A powdery mildew, caused by the fungus, Uncinula flexuosa, is sometimes found covering the underside of leaves with a white mould. A small brown circular leaf spot caused by Septoria hippocastani is an occasional problem on horse chestnut. To control these diseases follow the measures suggested for leaf blotch.

Canker and other stem disease fungi along with various wood rots can also colonize horse chestnuts. The removal and destruction of affected limbs and a good program should control these problems.
SCALES AND MEALYBUGS

The walnut scale (Quadraspidiotus juglansregiae), an armoured scale, sometimes attacks the bark of horse chestnut, causing injury to the phloem, or living bark tissue. The walnut scale is tan or grey-brown, saucer-shaped with a raised point in the centre, and is less than 2mm in diameter. The Comstock mealybug (Pseudococcus comstocki), a small, elliptical, waxy-covered insect, sucks the sap from the leaves of its hosts. For both these insects and other sap sucking ones, weather conditions and natural parasites usually provide sufficient control, making the use of insecticides unnecessary.
DEFOLIATING INSECTS

These insects damage horse chestnuts by consuming the leaves. The whitemarked tussock moth (Orgyia leucostigma) larva, a caterpillar, first skeletonizes and then eats the entire leaf except for the main vein and petiole. The full-grown larva is about 31 mm long with a reddish orange head and yellowish body tufted with distinctive black and white hair. The insect overwinters in the egg stage. Control can be achieved by removing and destroying egg masses in the fall and winter. The conspicuous frothy white egg masses are found on empty cocoons on twigs, branches, or main trunks. During the days when they are feeding, the caterpillars can be controlled with Bacillus thuringiensis, a biological agent or other registered insecticides.

Adult Japanese beetles (Popillia japonica) skeletonize the leaves of more than 300 hosts including the horse chestnut. The beetle overwinters in the soil as a partially grown grub. Adults emerge from the soil in late June through mid-July and feed actively in sunny locations for 30-45 days. For this reason damage tends to be more severe at the top and on the south side of trees. If control is deemed necessary, registered insecticides can be applied from mid- to late July when adult beetles are active. For further information consult Ontario Ministry of Agriculture and Food (OMAF) Factsheet 92-105 "Japanese beetle in nurseries and turf".

When using pesticides follow label directions to ensure the safe use and efficacy of each product


M. Sabourin/M.D. Dykstra

PDCF-018 (Revised 2003/11/07)

Contact Information



PEST DIAGNOSTIC CLINIC
Laboratory Services Division, University of Guelph
95 Stone Rd. West
Guelph, Ontario N1H 8J7

Phone: (519) 767-6256
Fax: (519) 767-6240
E-mail: pdc@lsd.uoguelph.ca