Netted iris

The disease develops in overly wet, poorly aerated substrate and attacks the roots and the base of the stem. The roots turn brown, rot and lose their bark, which slips off easily, and a dark, watery rot appears at the stem base. The above-ground part wilts, yellows and dies back despite moist soil – a characteristic symptom, because the damaged roots no longer take up water. Especially at risk are houseplants, succulents and cacti (from overwatering), and in the ground conifers, rhododendrons, ericaceous plants and strawberries on waterlogged sites. The process begins inconspicuously at the root hairs and root tips, which darken and disappear, after which the rot advances towards the framework roots and the root collar. Healthy, pale, active roots waste away — what remains is a dark, rusty-brown or blackish, watery mass, and the outer layer of the root bark slips off like a glove, exposing the discoloured central cylinder. In trees and shrubs, infection of the collar and the base of the trunk (stem-base rot) shows as a dark, often streaking patch, sometimes with oozing, and the bark there is moist and sunken; in conifers and rhododendrons, browning and dieback are seen from the bottom or from the centre of the crown while the soil is still moist. Characteristic is the nested (patchy) occurrence of symptoms in the bed — where water and the pathogen accumulate locally. It is worth distinguishing the two causal oomycete genera: Pythium mainly attacks seedlings and fine roots, causing damping-off and root rot in cool, wet substrate, whereas Phytophthora also attacks older, woody plants, framework roots and the collar, more strongly in warmer, water-saturated soil. Key for diagnosis is the contradiction of the symptoms: the plant wilts and looks 'dry', although the substrate is wet — this results from the destruction of the roots, not from a shortage of water.

A grey-brown, fluffy coating of mycelium on flowers, fruit, leaves and shoots, preceded by watery, rotting spots. It attacks weakened and damaged tissue and develops rapidly in humidity and cool conditions and where air circulation is poor. Common on strawberries, begonias, peonies and roses. The coating is characteristically 'mousy' — fluffy and ashy grey, and when the infected tissue is knocked a visible cloud of spores (conidia) puffs up, which is a diagnostic feature of this disease. Infection usually starts in the weakest tissues: fading petals, wounds, hail marks or pest feeding damage, as well as pollen residue settled on the petals. Small, brown, watery spots appear on the flowers and quickly engulf the whole bloom ('flower blight'); from the infected flower the fungus often grows down into the shoot, causing it to constrict and die back. On soft fruit (strawberry, raspberry, grapes, tomato) a watery, browning rot develops, covered with a thick layer of grey, fluffy felt — the berry softens and 'dissolves'. On seedlings and cuttings Botrytis causes basal stem rot and the sudden collapse of plants. In older, dying mycelium and on plant debris hard, black, irregular lumps form — sclerotia (resting bodies), by which the pathogen overwinters in the soil and in plant debris. The disease also shows up after harvest, as a rapidly advancing storage rot of fruit and vegetables.

The soil-borne fungus infects the root system and enters the vascular bundles, blocking the flow of water within the plant. The plants wilt, first on hot days and often on one side or from the lower leaves, then turn yellow and die despite moist soil. A cross-section of the stem reveals a characteristic brown discolouration of the vascular bundles. The disease intensifies in warmth — the pathogen develops best at high soil temperatures (about 25–30°C), which is why the first symptoms usually appear in midsummer. In the early phase the plants wilt during the hot midday and temporarily recover at night or after watering; over time the wilting becomes permanent. Yellowing progresses from the oldest, lower leaves upwards and is often distinctly one-sided — sometimes only half a leaf or one side of a shoot is affected, on the side along which the blocked vascular bundles run. The plants are stunted (growth is inhibited), and in seedlings fusarium wilt appears as damping-off and toppling of the transplants. Brown discolouration of the vascular bundles is the most reliable diagnostic feature — in fusarium wilt it is usually intense and reaches deep into the stem and the roots. Under moist conditions, especially at the base of the stem or on the cut surface, a delicate pink-salmon or white bloom of fungal sporulation may appear. Fusarium oxysporum occurs in specialised forms (formae speciales), each of which infects only a particular species or genus of plant (e.g. tomato, cucumber, pea, carnation, banana), which helps to distinguish it from pathogens with a wide host range.

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.

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.

Small rodents with a stocky body, short tail and blunt muzzle, about 9–13 cm long, active all year round. They dig shallow tunnels just below the soil surface. Symptoms: gnawed roots and bark at the base of trees and shrubs (girdling of trunks under the snow in winter), eaten-out bulbs and tubers, dying plants that pull out of the ground easily, and small mounds of excavated soil with numerous shallow burrow openings linked by nibbled runways in the grass. In garden practice the name refers above all to the plant-eating field voles of the genus Microtus. Their fur is grey-brown to reddish-brown, and their small but clearly visible eyes and ears distinguish them from the mole. They breed exceptionally fast — a female produces several litters of several young per season, which mature within a few weeks — so numbers can rise sharply in multi-year cycles (outbreak years), when the damage becomes severe. They do not hibernate; the worst losses occur in late autumn, winter and early spring, when green food is scarce and they switch to roots, bark and bulbs overwintering in the soil. Under the snow cover, sheltered from predators, they gnaw the bark around the base of young trees (girdling), which after the thaw shows up as sudden wilting or death of the tree. Most at risk are young fruit and ornamental trees (especially apple trees on weak-growing rootstocks), tulip and crocus bulbs, tubers, root vegetables, perennials, and plants stored in clamps and compost heaps; daffodil bulbs, being poisonous, are usually left alone. Characteristic signs are the even, smooth tooth marks on roots and bark, roots cut off just below the soil surface, and a network of shallow runs with numerous open exits — unlike the mole, whose mounds have no entrance hole.