Large-flowered clematis

The first symptom is a sudden wilting and drying of leaves on individual branches or on one side of the crown, while the rest of the plant appears healthy. A cross-section of an infected shoot reveals a brownish or greenish discolouration of the wood — the vascular bundles clogged by the fungus, which can no longer conduct water. The disease progresses over successive years, affecting an ever-larger part of the crown and leading to the gradual death of the plant. Maples (especially Japanese maple), catalpa and many other trees, shrubs and perennials are particularly susceptible. The disease takes two forms: an 'acute' form, in which a branch or the whole plant dies rapidly within a few weeks, and a 'chronic' form — a slow decline spread over several seasons, with smaller, paler leaves, marginal scorching of the blades (resembling sunburn) and premature autumn colouring on infected shoots. The sectoral, one-sided distribution of symptoms is the hallmark of Verticillium wilt above all on woody plants — the part of the crown supplied by the clogged vessels dies off. The discolouration of the wood is best seen just under the bark as longitudinal streaks, and in cross-section as an interrupted ring; in maples it is often olive-green. Symptoms intensify in heat and during drought, when the demand for water exceeds the capacity of the damaged bundles. On herbaceous plants (dahlia, aster, chrysanthemum, tomato, potato, strawberry) one observes wilting and yellowing of the lower leaves, stunting and one-sided (half-blade) yellowing of the leaf — on these species, however, the symptom picture alone is unreliable for differentiation, because one-sided yellowing may also be caused by Fusarium wilt. The vascular discolouration by itself is not conclusive — a definitive diagnosis requires laboratory isolation of the pathogen.

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.