Calibrachoa

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 symptom affects acid-loving plants — hydrangea, rose, rhododendron, blueberry and others — grown on alkaline or calcareous soils. The leaf blade turns yellow while the veins remain distinctly green, producing the characteristic interveinal chlorosis. The symptoms appear first on the youngest leaves at the shoot tips, because iron is not very mobile within the plant and is not translocated into new growth. In extreme cases the leaves are almost white and their margins dry out. Crucially, the cause is usually not a lack of iron in the soil but too high a pH, which blocks its uptake by the roots. It is worth noting that not all susceptible plants are equally acid-loving — the rose prefers a slightly acidic to neutral reaction (about pH 6–7) and also develops iron chlorosis on strongly alkaline, calcareous soils, although it is not acid-loving in the way rhododendron or blueberry is. The pattern of yellowing is very regular: not only the main vein stays green but the whole fine network of veins, so that the leaf looks like green lace on a yellow-green background — this is the most reliable diagnostic feature, distinguishing interveinal chlorosis from the uniform, whole-surface yellowing typical of nitrogen deficiency. The symptoms intensify with each successive flush: older leaves at the base of the shoot may still be green or only slightly paled, and the younger the leaf, the paler it is, up to almost white, lemon-yellow tips. Under a long-lasting deficiency the margins and tips of the youngest leaves turn brown and necrotic, shoots grow more weakly and become smaller, and in blueberry the yield and fruit quality decline. Iron chlorosis is not contagious — it does not spread from plant to plant but appears simultaneously in all specimens growing on the same, overly alkaline site, which also helps to distinguish it from infectious diseases. Its severity can vary through the season: cold, wet and compacted soil, poor root aeration, excess calcium, a high bicarbonate content in the watering water, and over-fertilization with phosphorus additionally hinder iron uptake and aggravate the symptoms even when the element is not lacking in the soil.

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.

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.

Very small (1–2 mm), slender, elongated insects with fringed wings, coloured yellow, brown or black; the larvae are pale yellow and wingless. They suck sap from the cells of leaves, petals and buds. Symptoms: silvery to silvery-grey streaks and blotches on the leaves, tiny black specks of excrement, and distortion of young leaves, buds and flowers. They are especially damaging under glass and on houseplants, and some species transmit plant viruses. The silvery sheen appears once the pest has sucked out the contents of the epidermal cells — the empty, air-filled cells reflect light like metallic streaks; they are accompanied by glistening black specks of excrement, which distinguish thrips feeding from that of most other sap-sucking pests. The mouthparts are piercing-sucking and asymmetric (only a single mandible), so the insect first rasps the tissue and then sucks up the exuding sap. On flowers, feeding produces white or brownish discolouration and distortion of the petals — severe on roses, chrysanthemums, orchids and cyclamens. The life cycle is short and, in warm conditions, the generations overlap: the females lay eggs inserted into the plant tissue, the larvae (two instars) feed on the leaves and then drop to the soil or litter, where they pass through the immobile prepupal and pupal stages. Among the most damaging are the western flower thrips (Frankliniella occidentalis) — a key greenhouse pest and vector of tospoviruses (including tomato spotted wilt, TSWV) — and the onion thrips (Thrips tabaci). Only the larvae acquire the viruses, while the adults transmit them, so reducing the larvae also lowers the risk of viral infection.