Corn plant

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.

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.

Immobile, brown or yellowish shields (1-4 mm) attached to stems and the underside of leaves, especially along the veins. Easily mistaken for growths on the plant. They suck sap, causing yellowing and weakening, and excrete sticky honeydew. Common on figs, orchids and plants with tough leaves. The collective name covers two groups with different biology. Armoured scales (Diaspididae) have a hard waxy shield separate from the body, which can be prised off - beneath it emerges a tiny, legless and wingless female; they do not excrete honeydew. Soft scales (Coccidae) have a convex, cup-shaped cover fused with the body, and it is they that excrete it abundantly. Sticky honeydew - the domain of soft scales and other honeydew-producing scale insects, not of armoured scales - settles on the leaves and attracts sooty mould fungi (sooty mould, sooty blotch) as well as ants, which defend the colony against predators. The only mobile feeding stage is the crawler - a flat, tiny larva which, after hatching from beneath the mother's shield, disperses over the plant, then settles, inserts its stylet into the conducting tissues and loses the ability to move; adult females remain immobile for the rest of their lives. Males pupate and, as tiny, winged, short-lived insects, do not feed. In heated interiors the pest breeds all year round, producing several overlapping generations; outdoors usually 1-3 generations, and fertilised females or eggs hidden beneath the shield overwinter. Feeding symptoms are yellowing and premature leaf drop, dieback of shoots, stunting and weakening of the plant, and secondarily a black coating of sooty mould on leaves covered with honeydew. On fruit trees infested by San Jose scale (Comstockaspis perniciosa), reddish discolouration appears around the feeding sites, and heavily colonised branches die back. Because the shields are immobile and their colour blends into the bark and veins, an infestation is easily overlooked until honeydew appears or the plant is noticeably weakened - which is why regular inspection of the leaf undersides and shoot axils is key.

Insects covered in a white, cottony, waxy coating, gathering in leaf axils, at the base of shoots and on the underside of leaf blades. They look like tufts of cotton wool. They suck sap, weaken the plant and excrete sticky honeydew, on which black sooty mould develops. A common pest of houseplants and succulents. Adult females are oval, soft-bodied insects about 2-5 mm long, with a pinkish or grey body hidden under wax, often bearing waxy filaments along the sides and at the tip of the abdomen. Males are tiny, winged and do not feed. In most species eggs are laid in white, cottony ovisacs (cocoons), from which mobile larvae (crawlers) hatch and spread across the plant, carrying the infestation to neighbouring specimens; some mealybugs (e.g. Pseudococcus longispinus), however, are ovoviviparous and give birth to live crawlers without a distinct ovisac. In the warm, dry conditions of homes and glasshouses mealybugs develop all year round, in several overlapping generations - which is why a single treatment is rarely enough. Feeding symptoms are stunted growth, yellowing, deformation and premature leaf drop, and under heavy infestation dieback of shoots; the black sooty mould on the honeydew further reduces photosynthesis and disfigures the plant. A separate form is the root mealybug (Rhizoecus), forming white, waxy clusters on the roots and inside the root ball - the plant weakens and wilts with no visible pest on the above-ground parts. Mealybugs are often attended by ants, which eat the honeydew and protect the colonies from natural enemies.

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.