Hybrid tea rose

A white, powdery coating on leaves, shoots and buds. The leaves turn yellow, become deformed and drop prematurely. The coating develops on both sides of the leaf blade, is often clearly visible from above, and can easily be rubbed off with a finger — it is the superficial mycelium of an ectoparasite that draws nutrients from the epidermal cells by means of feeding structures (haustoria). At first single, round, white spots appear, which in time merge and cover the entire leaf surface and the young growth. Young leaves and the current season's shoots are the most susceptible; heavy infection causes them to pucker, stiffen and stop growing, and on buds it leads to deformation or death of the flowers. On the fruit of some species (gooseberry, apple, cucurbits) the coating leaves russeting or a corky skin. Towards the end of the season, small, dark fruiting bodies (chasmothecia, formerly called cleistothecia) — the pathogen's overwintering structures — appear within the coating. The disease affects many plants, including roses, oak, privet, lilac, cucurbits, cereals, grapevine, apple, gooseberry, phlox and coneflowers, with each of these hosts being infected by a specialised species of fungus.

Black to dark-brown spots with ragged margins on rose leaves, surrounded by yellowing of the leaf blade. Infected leaves drop in large numbers, weakening the plant and reducing flowering. The disease intensifies in wet seasons and where air circulation is poor. The spots are usually round, from a few millimetres to about 1-1.5 cm across, and their most important diagnostic feature is the radially fringed, feathery margin - the mycelium spreads beneath the leaf cuticle in all directions, producing the characteristic 'star-shaped' edge (hence the German name Sternrußtau). Under heavy infection the spots merge into larger, irregular patches, and the surrounding yellowing spreads across more and more of the blade until the whole leaf turns yellow and drops. Small, dark conidial fruiting bodies (acervuli) appear on the upper surface of the spots, while the underside of the leaf remains free of any coating. Symptoms appear first on the lower, oldest leaves and gradually move up the shrub, because the spores are carried by water splashing up from the soil and from lower leaves. Small, purplish-red discolorations and blotches may appear on young, green shoots. Prolonged wetting of the leaves for several hours is required for the spores to germinate and infect, which is why the disease breaks out after warm, rainy spells. Repeated defoliation exhausts the shrub, limits flowering and worsens winter hardiness.

Rusty-orange to brown, powdery clusters of spores on the underside of leaves, matched on the upper side by yellow spots. Heavily infected leaves turn yellow and drop prematurely, and the plant weakens. It affects roses and mallows among others, and some rusts have an alternate host (for example pear rust and juniper). The pustules (clusters of summer spores, the so-called uredinia) break through the leaf skin and release an orange-rust powder that rubs off onto the finger - this is the most reliable identifying feature. In high summer they appear mainly on the underside of the blade, along the veins and on the leaf stalks, and in some species also on green shoots and sepals. In late autumn the clusters darken to brownish-black - these are the overwintering spores (telia) by which the fungus survives winter on plant debris. Rusts are highly host-specific: a given rust usually infects only one species or genus of host, which is why the rusts of rose, mallow and marsh mallow, bean, leek and garlic, mint, pelargonium or lawn are different, non-interchangeable fungi. The general symptom - yellowing and premature leaf drop - weakens the plant and limits flowering and yield, though it rarely kills it within a single season.

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.