Blackcurrant

Yellow or angular (vein-bounded) spots on the upper leaf surface, matched underneath by a grey-violet or brown coating of spores. It differs from powdery mildew, whose white coating sits on the upper surface. It develops in humidity and cool nights and is dangerous to cucumbers, onions and grapevines. The spots are initially light green or yellowing, and their angular, mosaic-like shape arises because the pathogen's spread is halted by the larger leaf veins. The delicate, felt-like coating on the underside (hence the name 'mildew') is the sporulation of the oomycete — best seen in the morning or after a period of high air humidity. Over time the spots turn brown, dry out and merge, the leaf dies prematurely, which weakens the whole plant and lowers the yield. The disease is highly host-specialised: cucumbers and squashes are attacked by Pseudoperonospora cubensis, onion by Peronospora destructor, grapevine by Plasmopara viticola, and lettuce by Bremia lactucae — so infection of one species usually does not spread to unrelated crops.

On the young shoots, leaves and fruit of gooseberry and currant, especially blackcurrant, a white, mealy, delicate coating of fungal mycelium first appears. Over time the coating becomes felted and brown, forming a rust-brown, hard-to-rub-off layer on the fruit and shoot tips. Infected shoot tips become deformed, grow poorly and die back, while the fruit becomes unsightly, cracks and loses its market value. The coating appears first on the youngest, apical parts of the new growth and on the upper side of young leaves, from where it spreads to the developing fruit. The distinguishing feature of American gooseberry mildew - setting it apart from the mild leaf mildews - is precisely this dense, felt-like crust, which turns from white to brown felt and adheres firmly to the fruit skin and the bark of the shoots; embedded in the brown layer are the overwintering fruiting bodies of the fungus (cleistothecia), visible as fine, dark dots. The affected growing points become twisted and stunted, the leaves at the tips are sometimes crinkled and curled under, and the bush forms clustered, deformed growth. The fruit beneath the coating can still be used once the layer is wiped off, but it loses its appearance and market usefulness; under heavy infection the shoot tips dry out completely. The symptoms build up from late spring and intensify in a warm, humid summer.

Numerous small, round spots appear on the leaves with a lighter, greyish centre and a darker, brown margin. Within the spots tiny black dots are visible – pycnidia, the fruiting bodies of the fungus, which distinguishes this disease from other leaf spots. The infection progresses from the lower, oldest leaves upwards; with heavy infection the spots merge, the leaves yellow, dry out and drop prematurely, weakening the plant. Affected plants include tomato, currant, phlox and many perennials. The spots are usually 2–5 mm across; the decisive identifying sign is the dark pycnidia at their centre – best seen through a hand lens, and in high humidity exuding whitish 'tendrils' of spores. Leaves densely peppered with spots look as if dusted. The disease is favoured by warm (about 20–25°C), humid weather and water lingering on the leaves for a long time (rain, dew, overhead watering). On tomato the infection is confined almost exclusively to the leaves and leaf stalks – the fruit usually stays healthy – but the loss of the photosynthetic apparatus leads to smaller fruit and exposes it to sunscald.

The larvae strongly resemble the caterpillars of butterflies and moths, but have more pairs of abdominal prolegs (six and more) and single ocelli on the sides of the head, by which they can be told apart. They often feed gregariously, skeletonising the leaves or eating them down to the veins, and can strip a gooseberry or currant bush bare within a few days. The host plants are gooseberry, currant, rose, plum and pine. Symptoms: besides the defoliation of leaves, the larvae of fruit-dwelling species (e.g. on plum) bore into the young fruit set and cause it to drop. Adult sawflies are small (a few millimetres) hymenopterans without a distinct 'wasp waist' — the abdomen is broadly joined to the thorax, which distinguishes them from wasps and bees; the females have a saw-like ovipositor (hence the name 'sawflies'), with which they cut into the tissue of a leaf or fruit set and lay their eggs inside it. The larvae are most often green, matching the colour of the leaf, in some species with dark warts or spots; their prolegs lack the grasping crochets that the caterpillars of moths have, and when disturbed the larvae of many species raise the rear end of the body into a characteristic 'S' shape. The most damaging groups are: the gooseberry sawfly (Nematus ribesii) on gooseberry and currant, stripping the bush from the inside out; the slug sawflies (genus Caliroa) on pear, cherry and plum, whose slimy, slug-like larvae graze away the upper skin of the leaf (windowing), so that the leaf turns brown as if scorched; the fruit sawflies of the genus Hoplocampa on plum and apple, tunnelling galleries into the young fruit set and leaving moist, foul-smelling droppings; and the conifer sawflies (Diprion, Neodiprion) on pine, feeding gregariously on the needles. Life cycle: the adults emerge in spring, the larvae feed for several weeks, after which they descend into the soil or leaf litter, where they pupate in a cocoon and overwinter as a larva (prepupa); depending on the species, 1–3 generations develop in a season.

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 microscopic mite (eriophyid, about 0.2 mm), invisible to the naked eye, that colonises the inside of buds. Infested buds swell abnormally, take on a rounded shape and die off instead of opening — a symptom known as "big bud". The pest is a vector of blackcurrant reversion, a serious virus-like disease that deforms leaves and causes loss of cropping. It attacks mainly blackcurrant, less often redcurrant and gooseberry. The mite's body is elongate, worm-like and spindle-shaped, whitish, bearing only two pairs of legs at the front — a feature typical of the whole eriophyid family (Eriophyidae). A single infested bud may overwinter anywhere from several hundred to several tens of thousands of individuals, so a single "big bud" is a focus of mass invasion. Infestation is easiest to recognise in winter and early spring, when the bush is leafless: healthy leaf buds are narrow, pointed and pressed against the shoot, whereas colonised ones are round, distinctly swollen and open, resembling miniature cabbage heads. The mites feed hidden under the bud scales for most of the year. Migration coincides with bud burst and flowering (usually April–May): the individuals leave the dying old buds and move — passively on the wind and on the bodies of insects, and actively along the shoots — to young, forming buds, which they colonise for the rest of the season. Direct feeding distorts and weakens the buds, but the most dangerous effect is the transmission of reversion: its symptoms are simplified leaves (fewer lobes and shallower marginal serration), with sparser venation, lacking the characteristic aroma of blackcurrant, and deformed, often reddened flowers, leading to a gradual loss of the bush's cropping.