Asparagus

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.

The soil-borne fungus infects the root system and enters the vascular bundles, blocking the flow of water within the plant. The plants wilt, first on hot days and often on one side or from the lower leaves, then turn yellow and die despite moist soil. A cross-section of the stem reveals a characteristic brown discolouration of the vascular bundles. The disease intensifies in warmth — the pathogen develops best at high soil temperatures (about 25–30°C), which is why the first symptoms usually appear in midsummer. In the early phase the plants wilt during the hot midday and temporarily recover at night or after watering; over time the wilting becomes permanent. Yellowing progresses from the oldest, lower leaves upwards and is often distinctly one-sided — sometimes only half a leaf or one side of a shoot is affected, on the side along which the blocked vascular bundles run. The plants are stunted (growth is inhibited), and in seedlings fusarium wilt appears as damping-off and toppling of the transplants. Brown discolouration of the vascular bundles is the most reliable diagnostic feature — in fusarium wilt it is usually intense and reaches deep into the stem and the roots. Under moist conditions, especially at the base of the stem or on the cut surface, a delicate pink-salmon or white bloom of fungal sporulation may appear. Fusarium oxysporum occurs in specialised forms (formae speciales), each of which infects only a particular species or genus of plant (e.g. tomato, cucumber, pea, carnation, banana), which helps to distinguish it from pathogens with a wide host range.

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.

Small (2-10 mm), elongated true bugs, usually greenish or yellowish, which when disturbed nimbly jump and scurry sideways across the leaf. They suck sap mainly from the underside of leaves, causing fine pale stippling, discolouration and mottling of the blade. Many species are vectors of phytoplasmas and viruses, so the damage can be more serious than the feeding itself. They feed on perennials, vegetables, grapevine and ornamentals. The body is wedge-shaped, tapering towards the rear, and the wings are held roof-like over the abdomen at rest; the characteristic oblique, sideways gait and the ability to make long leaps are what give the group its name. The larvae (nymphs) are wingless, paler and smaller than the adults, cling to the leaf underside and also move sideways; they pass through several moults, and the white, transparent cast skins they leave stuck to the underside of the blade are one of the more reliable signs that leafhoppers are present. The feeding symptoms result from the emptying of mesophyll cells: fine, pale flecking appears first, then merges into marbled pale patches, and in some species (e.g. leafhoppers of the genus Empoasca) the leaf margins yellow, brown and curl, a condition known as hopperburn. Unlike aphids, leafhoppers usually do not coat the plant with abundant honeydew, so sooty moulds rarely develop with them. The eggs are laid singly into slits cut in the tissue - into veins, leaf stalks or young shoots; many species overwinter as eggs, some as adults in leaf litter or on evergreen plants, and one to several generations develop over the season. Their role as vectors is, however, the most important: leafhoppers transmit, among others, phytoplasmas (aster-type yellows - carried by leafhoppers of the genus Macrosteles, and on grapevine the golden yellows, flavescence doree, carried by the leafhopper Scaphoideus titanus) and viruses, so that even a small population can be dangerous in crops susceptible to these diseases.