Showy stonecrop

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.