Purple crabapple

Olive-green, velvety, blurred spots appear on the leaves and gradually darken and turn brown; heavily infected leaves turn yellow and drop prematurely. Dark, corky spots form on the fruit and crack as the fruit grows, producing the characteristic fissures and deformations. Infected fruit stay small, store poorly and lose market value. The first spots are usually seen on the youngest, developing leaves and on the sepals of the flowers; at first they are poorly delimited, later taking on a distinctly dark-olive to blackish-brown colour with a velvety sporulating coating. The primary spring infections are caused by ascospores discharged during rain from the fruiting bodies overwintering in the fallen leaves; the resulting spots in turn produce conidia responsible for numerous secondary infections through the summer. Early infection of the fruitlets leads to one-sided growth and severe fruit deformation, and the cracks in the corky tissue become entry points for rot fungi and storage rots. On especially susceptible cultivars the disease also produces crusty lesions and cracks on the young shoots and twigs, which become an additional source of spores the following season. A separate form is the so-called storage scab - small, pinpoint spots that only show up in cold storage when the fruit was infected just before harvest.

Blossoms and young shoots suddenly turn brown and then black, looking as if scorched by fire — hence the name of the disease. The tips of infected shoots bend downwards into a characteristic hook resembling a shepherd's crook. In humid conditions a milky, sticky bacterial ooze seeps from the infected tissue, and the infection spreads rapidly down the shoot, killing whole branches. Infected leaves and fruit dry out but remain attached to the tree for a long time. Pear and quince are the most severely affected, followed by apple, and among ornamental and wild plants — hawthorn, cotoneaster, firethorn (Pyracantha) and rowan, which can be a dangerous reservoir of infection. On the bark of the branches and trunk, sharply delimited, sunken necroses (bark canker) form with cracking bark; when the bark is cut, the tissue beneath is discoloured reddish-brown, often with marbled streaks. The drop of bacterial ooze eventually turns amber and dries, and the presence of the bacterium can be confirmed by the so-called streaming test — from freshly cut infected tissue immersed in water, threads of a cloudy bacterial suspension flow out. Infected fruit turns black and mummifies, remaining on the tree, which distinguishes fire blight from ordinary wilting caused by water shortage.

Small (1–3 mm) soft-bodied insects, green, black or pink, feeding in clusters on young shoots and the underside of leaves. They excrete sticky honeydew. The diagnostic feature that distinguishes aphids from other small sucking insects is the pair of siphunculi (tubular projections) at the rear of the abdomen together with a tail-like cauda; a single colony usually holds wingless forms (parthenogenetic females) alongside winged migrants that fly onto new plants in summer. In summer aphids reproduce parthenogenetically and viviparously over many generations, so the population builds up explosively; in autumn many species produce a sexual generation that lays overwintering eggs on the host, and host-alternating species move between a winter host (often a tree or shrub) and a summer host (a herbaceous plant). Feeding symptoms include curling, crinkling and distortion of young leaves, stunted growth, yellowing, and the shedding of buds and fruitlets. Abundant honeydew coats the leaves, and the sooty mould fungus that develops on it forms a black film which limits photosynthesis; the honeydew is fed on by ants, which in return defend the colony against predators. Aphids are also important vectors of plant viruses (mosaics among them), transmitting them from plant to plant even during a brief probing puncture.

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.