Common bean

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.

On the stems, at the base of the plants and on parts in contact with the soil, a watery, soft rot appears, covered with a dense, white, cottony fungal mycelium. Within the tissue and on its surface, black, hard resting bodies (sclerotia) the size of a grain form, which are a diagnostic feature. Infected plants wilt, snap and die; the disease attacks many species of vegetables and ornamentals. The infection usually begins as a pale, watery, almost translucent spot that quickly enlarges into a soft rot; in humid air it becomes overgrown with the characteristic, cotton-white mycelium. As the disease progresses, the infected stem becomes girdled and constricted and loses its rigidity, so that the plant above the point of infection wilts and dies, while the tissue, once dried out, bleaches and turns dry, stringy and frayed. The inside of affected stems (the pith) is often hollow and filled with black sclerotia the size of cereal grains — this is the most reliable feature distinguishing this disease. Sclerotinia infects a very wide range of hosts (including lettuce, beans, cucumbers, carrot, brassicas, sunflower, oilseed rape and numerous perennials and ornamentals), and the rot also develops after harvest, in stored roots and heads. In spring, small, cream-brown, cup-shaped fruiting bodies (apothecia) grow from the sclerotia lying in the soil and release ascospores, which infect plants most often through spent petals and weakened tissues.

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.

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.

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.

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.

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.