Plant Protection

Diseases

Endothia Canker (Endothia parasitica [Murr.] Anders.) is the most serious disease of chestnut. The pathogen infects the branches and trunk, destroying the phloem and cambium. Its orange-red fruiting bodies develop through the bark. Infections can occur through wounds, but also in bark creases at forked branch junctions, even without any visible injury. During summer, the disease is indicated by dried leafy branches in the crown, which remain on the tree throughout the winter. The disease mostly spreads downward in older branches and the trunk. Above the infected areas, the wood dries, the bark cracks, and the crown declines. Vigorous shoots often emerge below the infection site; these shoots usually succumb to the downward-spreading pathogen in the following year. In forests or scattered trees, infected chestnuts display a characteristic appearance with dry, broom-like branches.

The pathogen is of East Asian origin and initially spread worldwide through the introduction of Asian chestnut species. American and European chestnut species did not develop resistance to it, making its destruction of these species unique on these continents. The disease was first observed in Hungary in 1969 (Körtvély, 1970). It has been classified as a quarantine pathogen since 1971, so any infection must be eliminated in strict compliance with official regulations. In Hungary, the permanent risk of the pathogen establishing itself comes from spring bird migration. The pathogen, which became widespread in Italy from 1940 and in Yugoslavia from 1949, is spread northward by birds. In commercial orchards, continuous preventive chemical protection is necessary to prevent establishment. In the case of an established infection, adherence to quarantine regulations is mandatory.

Another pathogen, the fungus Melanconis modonia, also plays a role in chestnut canker in Hungary. Its occurrence in the country was first reported by Hausz (1972). The fungus has two conidial forms, of which Coryneum perniciosum is more common on dead chestnut branches. The fungus infects through wounds and, according to our observations, causes damage in nurseries affecting graft success. On older trees or vigorous seedlings, its damage is not significant. The fungus occurs on all dead, woody parts of the chestnut, and for a long time it was mistakenly considered the causal agent of chestnut ink disease; this error continues to appear repeatedly in the literature.

On young, smooth-barked trees, Melanconis infection resembles the symptoms of Endothia, so distinguishing between them is important. The bark affected by Melanconis modonia also becomes sunken and turns reddish-brown, but on its surface, instead of orange-red fruiting bodies, black conical stromata or black, lens-shaped conidial structures develop through the bark. The conidia are dark and multicellular. Maintaining chestnut trees in good vigor can prevent damage, and preventive measures against Endothia also provide protection against Melanconis canker.

Among the diseases causing chestnut decline, the oldest known is ink disease caused by Phytophthora, although its pathogens were only identified at the beginning of the 20th century. Chestnut ink disease has two causal agents, Phytophthora cambivora and Phytophthora cinnamomi, both of which produce similar symptoms. These pathogens are not known to occur in Hungary.

In Europe, Phytophthora cambivora has been recognized as the causal agent of chestnut ink disease since 1917. Phytophthora cinnamomi was first detected on chestnut in the United States in 1932, and its spread has since been confirmed in most European chestnut-growing countries. This latter pathogen also occurs on numerous other plant species. Distinguishing between the two pathogens is difficult because they do not produce reproductive structures during their parasitic stages.

The disease affects the roots and root-collar region of chestnut trees, sometimes extending 10–20 cm above the soil, destroying the phloem and cambium through so-called wet rot. It is called ink disease because the tannins exuded from infected chestnut wood oxidize to a black color. However, the black tannin exudate is not a definitive symptom, as other rots or mechanical injuries can also cause sap to oxidize to black, and some Phytophthora infections do not produce tannin exudate. A general symptom of the disease is yellowing and premature leaf drop, small fruit, and nuts falling prematurely from the burrs, which dry on the shoots and remain on the tree through winter. In advanced stages, root destruction causes the trees to die. The disease poses a threat to chestnuts on moist, heavy soils.

Since 1971, the introduction of these pathogens has been prevented in Hungary through regular inspections by quarantine authorities. Based on foreign experience, the possibilities for chemical control of these pathogens are very limited. If the disease occurs in Hungary, destruction can only be prevented in at-risk areas by using resistant rootstocks.

Mycosphaerella Leaf Spot (Mycosphaerella maculiformis) is the most widespread leaf disease of chestnut trees. The fungus’s summer conidial stage is known as Cylindrosporium castaneae, which is why the disease was formerly referred to in older literature as cylindrosporium leaf spot. In Hungary, the occurrence and biology of the pathogen were studied by Krenner (1944).
The symptoms usually appear on the leaves in June as small, whitish spots that enlarge and turn brown. After bud break, the pathogen—having overwintered on fallen leaves—infects the developing foliage during the summer via conidia produced on the leaf spots. By late summer, the spots merge, and the leaves turn yellow. In wet, humid weather with wide temperature fluctuations, the infection is followed by heavy defoliation. In contrast, in a dry, warm August, infected leaves curl, the main vein twists, and the dead leaves usually remain dried on the tree until leaf fall.

The disease occurs every year, but the extent of damage varies depending on the season. Because infections recur annually, control of Mycosphaerella leaf spot is the key factor in developing the spray program for chestnut orchards. Chestnut varieties differ in their susceptibility to the disease. Among Hungarian-bred cultivars, Iharosberényi 2 is highly resistant, while Kőszegszerdahelyi 2, Kőszegszerdahelyi 29, and Nagymarosi 22 show moderate susceptibility to Mycosphaerella leaf spot.

Among the less significant foliar diseases in Hungary, oak powdery mildew (Microsphaera alphitoides) also infects chestnut (Körtvély, 1974). Its damage is considerable in nurseries or on young trees, as it causes short internodes, delayed lignification, and increased frost sensitivity. On older trees, the fungus usually infects only basal shoots. The pathogen overwinters in buds and infects the leaves from there. The powdery coating becomes noticeable only by mid-summer. The fungus develops on the upper surface of the chestnut leaves. Infected leaves show stunted growth, contracted interveinal areas, and a wavy surface. In Hungarian chestnut varieties, the disease has been observed in nurseries primarily on the Kőszegszerdahelyi 2 cultivar, which therefore requires particular attention for preventive protection.

PESTS

Among the insect pests, the buds and foliage of chestnut trees are damaged by the larvae of several polyphagous moth species known from other host plants. The most common are the winter moth (Operophtera brumata) and the mottled umber (Erannis defoliaria).

The oak leaf-rolling weevil (Attelabus nitens) rolls the chestnut leaves into a barrel shape after oviposition, and its larvae develop inside these leaf rolls. The adult beetles swarm from late April to mid-June and, during their maturation feeding, also damage the flower buds. The injury caused to the foliage is not significant.

The oak leaf miner moth (Tischeria ekebladella) larvae create whitish, translucent mines inside chestnut leaves. The moth lays its eggs in the leaves during May and June, and the larvae feed within the leaf tissue, consuming the inner layers of the blade. This feeding activity causes the characteristic white spots on the leaves.

The chestnut aphid (Myzocallis castanicola) feeds by sucking sap from the young shoot tips and leaves of chestnut trees. This aphid species, native to both Europe and North America, is also common in Hungary. Its feeding does not cause the leaves to curl, but it can significantly reduce shoot growth, especially in nurseries, scion wood plantations, and young orchards. Insecticidal sprays applied during the period of active shoot growth can effectively limit its damage.

The chestnut weevil (Curculio elephas) is one of the most significant pests of chestnut fruits. In Hungary, adults typically begin to emerge and fly around August 20 (Sifter, 1971), with the flight activity peaking on sunny days around midday. The females lay their eggs either inside the chestnut burrs or around the peduncle base.
The larvae consume the nut kernel entirely, leaving behind only frass and feeding debris. When the chestnuts ripen, the larvae bore out of the nuts and burrow into the soil, where they pupate and remain until the following July.
The species can also develop in the acorns of sessile oak (Quercus petraea), meaning that oak forests near chestnut plantations increase the risk of infestation.
For mature chestnut stands, Sifter and Bürgés (1971) developed a hot aerosol chemical control method. However, in intensively managed, closely planted commercial orchards, the importance of this pest is expected to decline, as regular soil cultivation disrupts its life cycle.
Effective mechanical harvesting and timely collection of chestnuts can also help reduce larval populations, since the larvae only leave the fallen nuts. The weevil tends to thrive where nuts are not regularly collected or where numerous small, unharvested nuts remain on the ground among uncultivated undergrowth. After thorough harvesting, larvae typically exit the nuts during storage, making chemical control feasible mainly on smaller, young trees in managed orchards.

Several moth pests also damage chestnut nuts. In Hungary, the chestnut moth (Laspeyresia splendana) and its subspecies are present (Bürgés, 1973; Gál, 1973). The damage caused by its grayish-white larvae is similar to that of the chestnut weevil. Infested nuts can be identified by the presence of larval frass mixed with fine silk threads, which the larvae spin within the nut.

The chestnut moth causes 5–41% damage in scattered chestnut stands in western Hungary (Bürgés, 1973). Its importance does not diminish in plantations either, making regular control measures necessary.

Treatments

Chestnut plant protection is primarily determined by preventive sprays against Endothia canker, as well as control measures for Mycosphaerella leaf spot and the pests affecting the nuts.

Before bud break, a foliar wash with an inorganic copper-based spray is necessary to prevent Endothia canker infection. Chestnut bud break occurs relatively late, in the second half of April, at which time scattered ascospores of Mycosphaerella leaf spot also require control. This is done using mancozeb-based fungicides, supplemented with trichlorfon-based insecticides to protect against bud- and foliage-feeding insects. Between bud break and the flowering period in the second half of June, 2–3 additional sprays are needed, primarily targeting Mycosphaerella leaf spot.

In rainy weather, it is also necessary to protect against Mycosphaerella leaf spot during the period of summer shoot growth. From the first week of August, spraying against the chestnut weevil and the chestnut moth is required every two weeks, at least three times, using products containing methyl parathion, prolate, or trichlorfon as active ingredients. To prevent Mycosphaerella leaf spot and Endothia, it is recommended to spray the fallen leaves and tree branches with DNOC products after leaf fall.