Apple tree

A white, powdery coating on leaves, shoots and buds. The leaves turn yellow, become deformed and drop prematurely. The coating develops on both sides of the leaf blade, is often clearly visible from above, and can easily be rubbed off with a finger — it is the superficial mycelium of an ectoparasite that draws nutrients from the epidermal cells by means of feeding structures (haustoria). At first single, round, white spots appear, which in time merge and cover the entire leaf surface and the young growth. Young leaves and the current season's shoots are the most susceptible; heavy infection causes them to pucker, stiffen and stop growing, and on buds it leads to deformation or death of the flowers. On the fruit of some species (gooseberry, apple, cucurbits) the coating leaves russeting or a corky skin. Towards the end of the season, small, dark fruiting bodies (chasmothecia, formerly called cleistothecia) — the pathogen's overwintering structures — appear within the coating. The disease affects many plants, including roses, oak, privet, lilac, cucurbits, cereals, grapevine, apple, gooseberry, phlox and coneflowers, with each of these hosts being infected by a specialised species of fungus.

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.

On pome and stone fruit, brown, enlarging rot spots appear, on which concentrically arranged, grey-beige cushions of spores form. Infected fruit rot, dry out and mummify, often remaining on the tree until the next season. In spring the fungus also infects blossoms and short shoots, causing them to die back and killing the branch tips. The disease has two distinct phases. Spring blossom and twig blight (most often Monilinia laxa on stone fruit — sour cherry, sweet cherry, plum, apricot) begins on the blossoms, which turn brown, wilt and dry out without dropping; the infection enters through the stigma and style into the short shoot, girdles it and causes whole shoots to die back, with withered leaves hanging as if scorched — hence the misleading resemblance to frost damage. On stone fruit, infected shoots often exude gum (gummosis) and develop small, cracked bark cankers. The summer-to-autumn brown rot of the fruit (mainly Monilinia fructigena on apple and pear) develops on ripening fruit: the rot spot quickly engulfs the whole fruit, the flesh turns brown and softens, and characteristic, concentrically arranged rings of cream-beige cushions of conidial spores grow out on the surface. In cool conditions (including in storage), the fruit may take the form of hard, black glossy mummies lacking sporulation (so-called black rot). Mummified fruit hanging on the tree and Monilinia cankers on the shoots are the main source of spores in the following season.

Rusty-orange to brown, powdery clusters of spores on the underside of leaves, matched on the upper side by yellow spots. Heavily infected leaves turn yellow and drop prematurely, and the plant weakens. It affects roses and mallows among others, and some rusts have an alternate host (for example pear rust and juniper). The pustules (clusters of summer spores, the so-called uredinia) break through the leaf skin and release an orange-rust powder that rubs off onto the finger - this is the most reliable identifying feature. In high summer they appear mainly on the underside of the blade, along the veins and on the leaf stalks, and in some species also on green shoots and sepals. In late autumn the clusters darken to brownish-black - these are the overwintering spores (telia) by which the fungus survives winter on plant debris. Rusts are highly host-specific: a given rust usually infects only one species or genus of host, which is why the rusts of rose, mallow and marsh mallow, bean, leek and garlic, mint, pelargonium or lawn are different, non-interchangeable fungi. The general symptom - yellowing and premature leaf drop - weakens the plant and limits flowering and yield, though it rarely kills it within a single season.

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.

The disease mainly attacks apple and pear. Sunken, concentrically cracked and gradually enlarging wounds, known as cankers, form on the shoots and branches, and within them the bark and wood die back. Around the wound the tree produces ridges of callus tissue in an attempt to heal it over. In autumn and on damp days, small, brick-red fruiting bodies of the fungus appear along the edges of the cankers. Branches above the canker die back, while young shoots become infected through the leaf scars and quickly perish. A canker is a perennial wound — the fungus overwinters in the infected bark and each year widens the margin of necrosis, so that the edges of the wound form characteristic concentric rings that alternate between dying tissue and ridges of callus (an image resembling a 'target'). The fungus produces two types of spores: in summer, in moist conditions, creamy-white, slimy cushions of conidial spores appear on the surface of the canker, while in autumn and winter the aforementioned brick-red, spherical fruiting bodies (perithecia) with ascospores form. Infection occurs mainly through injuries and natural points of entry: fresh scars left by fallen leaves, stipular scars and lenticels, as well as wounds from pruning and fruit picking, frost cracks and damage from woolly apple aphid feeding. A shoot or branch completely girdled by a canker dies back above the wound. The same fungus can also cause fruit rot — most often developing from the calyx end and only becoming apparent in storage.

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 nocturnal moth with a wingspan of 15-20 mm; the forewings are grey-brown with a characteristic coppery, glossy patch at the tip. The caterpillar is whitish to pinkish with a brown head, grows to about 18 mm and tunnels right into the core of an apple or pear (less often quince or walnut). Symptoms: 'maggoty' fruit, entry holes with brown, sawdust-like droppings, and premature fruit drop. The caterpillar has five pairs of abdominal prolegs - the key feature distinguishing it from the larva of the apple sawfly, which has six or more. The codling moth overwinters as a full-grown caterpillar in a dense cocoon hidden in bark crevices, under loose flakes of bark, in the soil at the base of the trunk or in fruit crates; in spring it pupates, and the moths usually emerge from the end of apple blossom (May-June). Flights take place at dusk and at night, when the air temperature rises above about 15C. Females lay eggs singly on the fruit and on leaves nearby; after about one to two weeks the caterpillars hatch and, after a short wandering, bore into the fruit and head for the seed cavity, feeding on the flesh and the seeds. Under Polish conditions there is usually one full generation, and in warmer years and in the south of the country a partial second one whose feeding reaches harvest. Damaged fruit rots, drops prematurely and is unfit for storage.

Weevils 8-12 mm long, black or dark brown, with a short, broad snout, flightless and active at night. Their larvae are white, legless, C-shaped, with a brown head, and live in the soil. Symptoms: the adult beetles notch characteristic semicircular bites into leaf margins at night (box, rhododendron, bergenia, strawberry, houseplants), while the larvae gnaw at roots, bulbs and the base of shoots, causing the plant to wilt suddenly and die. The most common is the black vine weevil (Otiorhynchus sulcatus), more rarely the strawberry root weevil (O. ovatus) and related species. A key factor behind their numbers is parthenogenetic reproduction (virgin birth): populations consist almost exclusively of females, and a single female lays several hundred eggs into the soil without a male, so an outbreak builds up quickly. Life cycle: the beetles overwinter mainly as larvae (some as adults) in the soil; the adults emerge in late spring and early summer and for about 3-4 weeks carry out maturation feeding on the leaves before they begin to lay eggs, while the larvae feed on roots from summer through autumn and again in early spring. The larval stage is the most dangerous - the larvae chew the roots and gnaw the root collar at the base of the shoot (girdling), causing the plant to wilt suddenly despite moist soil and to lift easily out of the substrate; adult feeding (semicircular notches on leaf margins) mainly disfigures the plant and rarely kills it. Plants in containers are particularly at risk, where the limited root ball is quickly destroyed. The beetles are flightless, so they spread by walking across the substrate and - above all - with infested plants and substrate, which makes newly bought potted plants the main route of introduction.

The caterpillars roll, fold and glue leaves together with a webbing of silk threads, creating shelters in which they feed hidden from view. When disturbed they wriggle violently backwards and drop on threads, which aids their identification; the adults are small moths with a bell-shaped outline of the folded wings. The host plants are fruit trees (apple, pear, plum), ornamental shrubs and roses. Symptoms: the caterpillars chew leaves, flower buds and fruit set and damage the skin of ripening fruit, reducing its value. The name covers many species of the family Tortricidae with a similar way of life - among them the summer fruit tortrix (Adoxophyes orana) and the rose tortrix (Archips rosana). The caterpillars are slender, up to about 1.5-2 cm long, usually light or olive green with a darker head and prothoracic plate, and very active. The moths have a wingspan of about 15-25 mm; at rest they fold the forewings roof-like, giving the characteristic bell-shaped (trapezoidal) outline, and their pattern is greyish-brown with a darker cross-band. The life cycle depends on the species: leaf-rollers overwinter as eggs laid in patches on the bark or as young caterpillars hidden in bark crevices and under bud scales; in spring they bore into the swelling buds, then move onto the leaves and fruit set, and one or two generations develop over the season. A diagnostic feature on pome fruit is the attachment of a leaf to the fruit and the shallow grazing of the skin beneath it, which leaves irregular, corky scars while the flesh deeper inside remains intact. On roses and ornamental shrubs the symptom is rolled young leaflets bound with webbing and hollowed-out buds.

A small beetle of the weevil family, about 4–5 mm long, brown-grey, with a pale, oblique V-shaped band on the wing cases and a long, curved snout. In early spring, before flowering, the females lay single eggs inside the developing flower buds; the hatched legless larva eats out the inside of the bud, which stops developing, turns brown and dries up, remaining closed as if "scorched" — this is the most conspicuous symptom. It feeds on apple and pear; under heavy pressure it can destroy a considerable part of the flower buds. The main and proper host is the apple — on pear, comparable bud damage is caused above all by other, closely related blossom-weevil species (among them the pear blossom weevil, Anthonomus piri), so infestation of pears can only occasionally be attributed to this species. The species produces one generation per year. The adult beetles overwinter in bark crevices, under lifting flakes of bark, in leaf litter and at the base of the trunks; in early spring, once the temperature rises above a few degrees (roughly at the bud-swelling and green-bud stage), they leave their hiding places and move up into the crowns. As they feed, they pierce the swelling buds with their snout, and droplets of sap flow out — a symptom known as "bud weeping." The female bores a hole in a flower bud with its snout and lays a single egg inside it. The larva develops in concealment, eating out the stamens and pistil, then pupates inside the browned, undeveloped bud — the dried petals form a characteristic brown "cap" over it. The young beetles of the new generation emerge at the turn of spring and summer, feed briefly on the leaves, and then withdraw to their shelters for summer dormancy and overwintering. The harmfulness depends strongly on the abundance of flowering: in years of lush flowering, the destruction of some buds acts like natural thinning and may be immaterial to the yield, whereas with weak flowering the loss of fruit set is severe.

An aphid covered with dense, white, waxy fluff resembling cotton wool or mould, beneath which brownish-red insects hide. It forms compact colonies on the trunk, main branches, in bark cracks, on pruning wounds and also on the roots. At the feeding sites the bark grows abnormally, producing knotty swellings and cracks that serve as an entry point for rot and canker. It attacks mainly apple, less often pear. A distinguishing feature is the rust-red fluid released when a colony is crushed — it stains the fingers like blood, hence the German name Blutlaus (blood louse). The insects suck sap from the phloem; the honeydew they secrete may become coated with sooty mould. The woolly apple aphid overwinters as young larvae hidden in bark cracks at the base of the trunk and on the roots; in spring it migrates up the tree, colonising young shoots, pruning wounds, watersprouts and root suckers. In Europe it reproduces exclusively by parthenogenesis, producing several generations over a season — for it lacks its primary host (elm), which it uses in its native North America. Winged females appearing in summer carry the infestation to neighbouring trees. The root form produces knotty thickenings on the roots that weaken and deform young trees — it is especially dangerous in nurseries and new plantings.

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.