New York aster

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.

Numerous small, round spots appear on the leaves with a lighter, greyish centre and a darker, brown margin. Within the spots tiny black dots are visible – pycnidia, the fruiting bodies of the fungus, which distinguishes this disease from other leaf spots. The infection progresses from the lower, oldest leaves upwards; with heavy infection the spots merge, the leaves yellow, dry out and drop prematurely, weakening the plant. Affected plants include tomato, currant, phlox and many perennials. The spots are usually 2–5 mm across; the decisive identifying sign is the dark pycnidia at their centre – best seen through a hand lens, and in high humidity exuding whitish 'tendrils' of spores. Leaves densely peppered with spots look as if dusted. The disease is favoured by warm (about 20–25°C), humid weather and water lingering on the leaves for a long time (rain, dew, overhead watering). On tomato the infection is confined almost exclusively to the leaves and leaf stalks – the fruit usually stays healthy – but the loss of the photosynthetic apparatus leads to smaller fruit and exposes it to sunscald.

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.