Sweet marjoram

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.

A grey-brown, fluffy coating of mycelium on flowers, fruit, leaves and shoots, preceded by watery, rotting spots. It attacks weakened and damaged tissue and develops rapidly in humidity and cool conditions and where air circulation is poor. Common on strawberries, begonias, peonies and roses. The coating is characteristically 'mousy' — fluffy and ashy grey, and when the infected tissue is knocked a visible cloud of spores (conidia) puffs up, which is a diagnostic feature of this disease. Infection usually starts in the weakest tissues: fading petals, wounds, hail marks or pest feeding damage, as well as pollen residue settled on the petals. Small, brown, watery spots appear on the flowers and quickly engulf the whole bloom ('flower blight'); from the infected flower the fungus often grows down into the shoot, causing it to constrict and die back. On soft fruit (strawberry, raspberry, grapes, tomato) a watery, browning rot develops, covered with a thick layer of grey, fluffy felt — the berry softens and 'dissolves'. On seedlings and cuttings Botrytis causes basal stem rot and the sudden collapse of plants. In older, dying mycelium and on plant debris hard, black, irregular lumps form — sclerotia (resting bodies), by which the pathogen overwinters in the soil and in plant debris. The disease also shows up after harvest, as a rapidly advancing storage rot of fruit and vegetables.