Purple-leaf cherry plum

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.

The disease attacks plum, apricot and peach, along with other species of the genus Prunus. Pale, chlorotic rings, spots and streaks appear on the leaves along the veins, most clearly visible in spring during cool weather. Depressions, discolouration and deformations form on the fruit, and the flesh beneath is often discoloured and gummy with an impaired flavour. Infected fruit frequently drop prematurely, so the crop loses its market and eating value. The leaf symptoms also take the form of vein clearing along the veins and irregular mottling; they are strongest in spring, often fade (become masked) during hot summers and return the following season, which makes diagnosis difficult. The severity of symptoms depends on the strain of the virus and the susceptibility of the cultivar - ranging from almost symptomless forms to severe deformations. On plum fruit, dark, bluish rings and spots as well as depressions are visible; in apricot, characteristic diagnostic rings and spots also appear on the stone (endocarp), which is one of the more reliable identifying features. In peach, colour breaking of the flower petals (paler streaks) is additionally seen. Freshly infected trees can remain symptomless throughout the season, so reliable confirmation is provided only by laboratory testing (the ELISA serological test or molecular RT-PCR).

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.