Horse chestnut

The fungus attacks the roots and stem base of trees and shrubs, causing their slow decline – the plants grow weakly, the crown thins, the leaves become smaller, and eventually they dry up. The diagnostic sign is white, fan-shaped sheets of fungal mycelium forming a dense layer under the bark at the stem base, with a distinct mushroom smell. In the soil and under the bark there are black or dark-brown, bootlace-like rhizomorphs, by which the fungus spreads to further plants. In autumn, clusters of fruiting bodies – honey mushrooms – grow at the base of infected trees. The above-ground symptoms are non-specific and easily confused with drought or overfeeding: some plants die suddenly (the whole tree turns brown within a single season, especially in hot weather), while others weaken over years – producing short growth, discolouring prematurely, and just before death setting an exceptionally heavy crop of flowers or fruit ('stress flowering'). In conifers, resin bleeding at the stem base is common. When the bark is exposed at the root collar, the wood of the roots and stem turns white and softly rots – honey fungus causes a white rot of the wood. The mycelial layer can be cream-coloured and leathery, spreading radially from beneath the dead bark. A biological curiosity, rather than a sign useful in garden diagnosis, is that the mycelium in moist, infected wood can glow very faintly in the dark (so-called 'cold light') – the effect is too weak and requires complete darkness, so in practice it cannot be used to identify the disease. The disease advances in foci – neighbouring plants die off one after another around an infected one.

A small moth of only a few millimetres' wingspan with rusty-brown, silvery-striped wings; the damage is caused by its larvae, which mine the leaves of the horse chestnut. Small brown mines form in the leaf blade, later expanding and lying lengthwise between the veins. Heavily infested leaves turn brown, dry out and fall as early as July and August, so that the trees look autumnal in the middle of summer. Repeated feeding over the season (several generations) weakens the trees and worsens their condition from year to year. An invasive species, described only in 1986 by Lake Ohrid in Macedonia (hence the epithet ohridella), it spread across the whole of Europe within about a dozen years. The main host is the horse chestnut (Aesculus hippocastanum); the red horse chestnut (Aesculus × carnea) is distinctly less susceptible. The mines lie on the upper side of the leaf blade and are best seen against the light — inside one can then make out the larva and black granules of frass, which distinguishes the damage from fungal spots. Under Polish conditions the insect usually produces 3–4 generations per season (even more in warm years), and successive generations overlap, so the damage builds up from June right through to autumn. It overwinters as a pupa in the fallen leaves, and the spring flight of the moths coincides with the leafing and flowering of the horse chestnuts. Feeding by the leaf miner alone rarely leads to the death of the tree, but the chronic premature defoliation repeated year after year weakens it, reduces its growth increments and worsens its vigour and ornamental value, increasing its susceptibility to other stress factors.

A moth with marked sexual dimorphism: the male has grey-brown, finely banded wings with a span of about 25 mm, whereas the female is practically wingless and flightless and therefore climbs up the trunk in autumn to lay its eggs in the crown. In spring green looper caterpillars with pale lengthwise stripes hatch, moving with the characteristic "looping" gait (arching the rear of the body) and eating the developing buds, young leaves and fruitlets, often webbing them together with threads. It feeds on apple, pear, plum, cherry and many deciduous trees and shrubs. The mature caterpillar reaches about 25 mm in length and is yellow-green with a darker dorsal line and pale stripes along its sides; as a typical looper (Geometridae) it has only two pairs of prolegs at the end of the abdomen, hence its looping gait. The eggs are laid singly near the buds and in bark crevices, where they overwinter, and just before the spring hatch they change colour from greenish to orange-red. The species has one generation a year: the moths fly in late autumn (October-December, which is the origin of the Polish name "przedzimek", meaning before-winter), on cool, frost-free evenings, and the flightless females then climb the trunks. The spring hatch of the caterpillars is closely synchronised with bud burst (usually April); after about 4-6 weeks of feeding the caterpillars lower themselves to the ground on threads and pupate shallowly in the soil, where the pupa spends the summer. The signs of feeding are holed and ragged young leaves, eaten flower and leaf buds, and deformed fruit covered with corky scars that develop from the damaged fruitlets; under heavy pressure the caterpillars can almost strip the crown bare.