Mossy saxifrage

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.

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.

Weevils 8-12 mm long, black or dark brown, with a short, broad snout, flightless and active at night. Their larvae are white, legless, C-shaped, with a brown head, and live in the soil. Symptoms: the adult beetles notch characteristic semicircular bites into leaf margins at night (box, rhododendron, bergenia, strawberry, houseplants), while the larvae gnaw at roots, bulbs and the base of shoots, causing the plant to wilt suddenly and die. The most common is the black vine weevil (Otiorhynchus sulcatus), more rarely the strawberry root weevil (O. ovatus) and related species. A key factor behind their numbers is parthenogenetic reproduction (virgin birth): populations consist almost exclusively of females, and a single female lays several hundred eggs into the soil without a male, so an outbreak builds up quickly. Life cycle: the beetles overwinter mainly as larvae (some as adults) in the soil; the adults emerge in late spring and early summer and for about 3-4 weeks carry out maturation feeding on the leaves before they begin to lay eggs, while the larvae feed on roots from summer through autumn and again in early spring. The larval stage is the most dangerous - the larvae chew the roots and gnaw the root collar at the base of the shoot (girdling), causing the plant to wilt suddenly despite moist soil and to lift easily out of the substrate; adult feeding (semicircular notches on leaf margins) mainly disfigures the plant and rarely kills it. Plants in containers are particularly at risk, where the limited root ball is quickly destroyed. The beetles are flightless, so they spread by walking across the substrate and - above all - with infested plants and substrate, which makes newly bought potted plants the main route of introduction.

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.