Butternut squash

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.

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.

A characteristic mosaic appears on the leaves — irregular, interweaving light and dark green or yellow patches and streaks. The leaves are often deformed, wrinkled, curled and smaller, and whole plants become stunted and fruit poorly. The symptoms are most visible on young leaves, and the fruit of cucumber, courgette or tomato may be blotchy and misshapen. The disease affects many vegetables, perennials and ornamental plants. The disease picture depends on the specific virus, the host species and the temperature — the same pathogen produces different symptoms on different plants. A distinguishing feature is that the mosaic is systemic (it affects the whole plant and develops from the youngest leaves) rather than localised like fungal leaf spots — there are no clearly delimited spots with a margin, no mycelial coating and no fruiting bodies. Besides the mosaic, there is vein clearing (a lightening of the tissue along the veins), striping and blister-like bulges of the leaf blade. Severe narrowing and thread-like tomato leaves (the so-called 'shoestring' symptom) is associated mainly with CMV (cucumber mosaic virus), occasionally with tobamoviruses, whereas TMV/ToMV mostly produce mosaic and mottling of the leaves. On the fruit you can observe blotching, concentric rings, wart-like bulges (cucumber) and uneven ripening (tomato), and on the flowers of ornamental plants — colour breaking, i.e. streaks and flecks of discolouration on the petals. In high temperatures (above ~28–30°C) the symptoms may become masked and temporarily disappear, only to return once it cools down — this does not mean the plant has recovered.

Dark, sharply defined, sunken spots appear on the leaves, stems and fruit, with collapsed, rotting patches on the fruit. In wet weather, pinkish, slimy masses of spores can be seen in the centre of the spots, often arranged in concentric zones. The spots enlarge and merge, the leaves die, and the fruit of beans, peppers, squash or strawberries rots; the disease intensifies in warm and humid summers. The most distinctive feature is precisely this salmon-pink, slimy coating of conidia filling the moist spots - these are the fungus's spore-producing clusters (acervuli), not a dry, powdery mould. On bean pods and fruit the spots are round, deeply sunken and sharply edged with a dark brown, sometimes reddish border; on leaf veins and stalks elongated, brownish-black, sunken streaks appear. On strawberry the disease attacks the fruit (hard, brown, sunken spots) as well as the crown and the stalks, leading to the dieback of whole plants. On cucurbits the young spots may be watery and translucent, later turning brown and cracking. Infected fruit is not suitable for storage, because the disease continues to develop after harvest.

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.

Slugs and shelled snails that feed at night and after rain. Symptoms: irregular holes in the leaves and young seedlings eaten off completely, silvery slime trails on leaves and soil. The greatest damage occurs in damp, shaded spots — particularly on lettuce, strawberries and hostas. The most damaging in gardens are the slugs (which lack an external shell): the Spanish slug (Arion vulgaris, formerly described as A. lusitanicus) and the grey field slug (Deroceras reticulatum). The leopard slug (Limax maximus) can be morphologically similar to them, but it feeds mainly on decaying organic matter and fungi and may even prey on other slugs, so it is of little importance as a pest of living plants. Shelled forms — grove snails (Cepaea) or the protected Roman snail (Helix pomatia) — usually cause less damage. The body is soft, elongated and permanently covered in slime; on the head there are two pairs of retractable tentacles — the longer upper pair bears the eyes, the shorter lower pair serves a tactile and olfactory function. Slugs and snails are hermaphrodites: they lay spherical, transparent or pearly eggs in clusters, hidden in damp soil, under stones and in leaf litter. A single Spanish slug can lay several hundred eggs; both eggs and young slugs overwinter, and feeding lasts from spring to late autumn, intensifying on warm, damp nights. The damage has smooth edges and often appears in the middle of the leaf blade, not only from the margin; seedlings and transplants are sometimes eaten down to bare soil, and on strawberries and ripening vegetables the slugs gnaw hollows into the fruit. The decisive diagnostic sign is the dried, silvery glistening slime trail — no insect leaves one.

Tiny (0.3–0.5 mm) arachnids, hard to spot with the naked eye, feeding on the underside of leaves. Symptoms: fine, pale speckling (feeding punctures); over time the leaves turn grey, yellow and dry out. Under heavy infestation a fine webbing becomes visible. Warm, dry air favours their development — a common problem for houseplants in winter near radiators. Spider mites are not insects but mites of the family Tetranychidae — adults have 8 legs (larvae only 6). The most common in cultivation is the two-spotted spider mite (Tetranychus urticae): yellowish-green females carry two dark spots on the sides of the body, while overwintering females turn orange-red. In orchards the fruit tree red spider mite (Panonychus ulmi) is frequent, whose red eggs overwinter in clusters on the bark of the shoots. Feeding mechanism: the mites puncture the cells of the epidermis and mesophyll on the underside of the leaf and suck out their contents, and the emptied cells fill with air — hence the characteristic, silvery-grey, fine speckling visible from the upper side of the blade (resembling a mosaic discolouration). At high numbers the leaves turn brown, curl and drop prematurely, and the whole plant weakens and loses its growth. Dense webbing spun between the leaves and shoot tips appears only under an advanced attack and is a sign of the risk of the pest spreading to neighbouring plants. The development cycle is very fast in warmth and drought — at a temperature of 25–30°C a single generation can be completed in 1–2 weeks, hence the numerous generations per season and the rapid, exponential build-up of the population. Females lay spherical, transparent eggs on the underside of the leaves. Fertilised females overwinter (in bark crevices, leaf litter and plant debris) or — in the fruit tree red spider mite — the eggs on the bark. Early infestation is easy to miss; diagnosis is helped by shaking a leaf over a white sheet of paper (moving specks become visible) or examining the underside of the leaf with a magnifying glass.