Sweet cherry

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.

Sunken, necrotic wounds (cankers) form on the trunk and branches, from which – especially in spring – amber gum oozes (gummosis). The bark over the wounds dies and cracks, and infected buds and short shoots fail to break or die in spring, so whole sections of the crown dry out. Small, brown, watery spots may appear on the leaves and fall out of the blade ('shot-hole'). The disease affects stone-fruit trees – sour cherry, sweet cherry, plum, peach and apricot – and when it girdles a branch it kills it. The cause is two bacterial pathovars — Pseudomonas syringae pv. syringae and pv. morsprunorum — with similar symptoms that cannot be told apart with the naked eye. A characteristic early symptom is the dying of buds and flowers in spring (so-called blossom blast): buds turn brown and fail to open, and individual branches suddenly wilt and dry out, holding their browned leaves for a long time. The gum oozing from the cankers is glassy to amber and appears most abundantly in spring and early summer, after the sap starts to flow; in high summer the ooze usually stops. When the bark is cut at the edge of a canker, the tissue beneath is often watery, rusty-brown, with a sour, fermenting smell (typical of pv. syringae). The leaf spots are at first small, angular, watery and surrounded by a lighter halo; their dead centres crumble away, producing a 'shot-hole' effect. The symptoms are easily confused with frost damage to the bark and with fungal shoot dieback (brown rot) and fungal shot-hole of the leaves (Wilsonomyces carpophilus) — the key then is the season of peak activity and the presence of fungal sporulation. The disease intensifies in cool, wet weather in spring and autumn; in summer, in warmth and drought, the bacterium is least active.

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 small fly about 4 mm long with a black body, a yellow scutellum on the back and transparent wings marked with characteristic dark bands. The females lay their eggs singly in ripening, colour-changing fruit of sweet and sour cherry. The white, legless maggots feed in the flesh around the stone and cause the fruit to rot — the typical "maggoty" cherries that soften and drop prematurely. Late varieties, which ripen at the peak of the flies' flight, are the most at risk. The cherry fruit fly has one generation per year and overwinters as a puparium (pupa) in the soil, a few centimetres deep beneath the tree canopy; some pupae undergo a prolonged diapause and emerge only after two winters, which makes it difficult to eliminate the pest completely. The adult flies usually emerge from the turn of May and June, and their appearance is linked to the warming of the soil and the colour-change phase of the fruit. After emerging, the females mature sexually over several to a dozen or so days (supplementary feeding), and then lay their eggs beneath the skin of fruit changing colour from green to yellow and red. The hatched larva feeds in the flesh around the stone for about 3–4 weeks, then chews an exit hole, leaves the fruit and drops to the soil, where it pupates. The first symptoms are hard to spot: the flesh around the stone turns brown and rots, soft, darkening, slightly sunken spots appear on the fruit, and when it is cut open a white maggot and decomposed flesh are visible. Yellow sticky traps are used to determine the start and intensity of the flight.

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.