Box elder 'Flamingo'

The first symptom is a sudden wilting and drying of leaves on individual branches or on one side of the crown, while the rest of the plant appears healthy. A cross-section of an infected shoot reveals a brownish or greenish discolouration of the wood — the vascular bundles clogged by the fungus, which can no longer conduct water. The disease progresses over successive years, affecting an ever-larger part of the crown and leading to the gradual death of the plant. Maples (especially Japanese maple), catalpa and many other trees, shrubs and perennials are particularly susceptible. The disease takes two forms: an 'acute' form, in which a branch or the whole plant dies rapidly within a few weeks, and a 'chronic' form — a slow decline spread over several seasons, with smaller, paler leaves, marginal scorching of the blades (resembling sunburn) and premature autumn colouring on infected shoots. The sectoral, one-sided distribution of symptoms is the hallmark of Verticillium wilt above all on woody plants — the part of the crown supplied by the clogged vessels dies off. The discolouration of the wood is best seen just under the bark as longitudinal streaks, and in cross-section as an interrupted ring; in maples it is often olive-green. Symptoms intensify in heat and during drought, when the demand for water exceeds the capacity of the damaged bundles. On herbaceous plants (dahlia, aster, chrysanthemum, tomato, potato, strawberry) one observes wilting and yellowing of the lower leaves, stunting and one-sided (half-blade) yellowing of the leaf — on these species, however, the symptom picture alone is unreliable for differentiation, because one-sided yellowing may also be caused by Fusarium wilt. The vascular discolouration by itself is not conclusive — a definitive diagnosis requires laboratory isolation of the pathogen.

The disease affects maples, especially Norway maple and sycamore maple. In summer, large, round, raised, glossy black spots up to 1–2 cm across appear on the upper side of the leaves, deceptively resembling drops of tar or hardened lacquer and often surrounded by a lighter, yellowish halo. The damage is mainly aesthetic, although with heavy infection the leaves drop somewhat earlier. Interestingly, the spots appear mainly where the air is clean — the fungus is sensitive to sulphur dioxide pollution and is therefore regarded as a natural indicator of good air quality. The symptom develops in a characteristic way: the first sign, visible as early as late spring or early summer, is a blurred, pale-green to yellowish patch of discolouration, still without any black structure. Only within it, in the second half of summer, does a thick crust build up — at first matt, later glossy black (the fungal pad, the so-called stroma), slightly raised, with a faintly radial or wrinkled surface. The spots are hard and permanently fused with the blade — they cannot be rubbed off with a finger or washed off with water, which is the key feature distinguishing them from a superficial, sooty film. On the underside of the leaf, opposite the spot, the tissue is sometimes lighter, yellowish. The number of spots ranges from single to a dozen or more per leaf; at high density they may almost touch. The disease is monocyclic — no secondary infections occur during the season, and the mature fruiting bodies (apothecia) form in the black stromata only on the fallen leaves over winter and in spring discharge ascospores that infect the young, developing leaves. Infection does not permanently weaken the tree — even with numerous spots, growth and overall condition remain undisturbed.

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 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.