Grey Mould (Botrytis cinerea) is probably the most common disease of vegetables, ornamentals, tubers, corms, bulbs, fruit, as well as many arable crops, and is almost certainly the most common disease in glasshouses. Predominantly a blossom blight and a fruit rot, it can also cause damping-off, stem canker, leaf spots and root rot. Infection often spreads to other adjacent produce after harvest. Small yellow-tan, depressed leaf spots enlarge eventually coalescing to cover the leaf, then sporulate as a light grey felt on the dying tissue. 

Plants are rarely infected directly by conidia, but indirectly though decaying petals or leaves that stick to healthy leaves. After diseased fruit or fleshy stems develop soft and watery pale brown rots, grey mould sporulates profusely on them as they split open. Squat black sclerotia may form, submerged within the tissue as it eventually wrinkles and dries. Seedling infection is worse in cold damp soil. Bulbs, corms and tubers are often infected while still in the ground. Lesions generally start at the crown or base. These appear soft and watery then enlarge, turn brownish spongy or corky, lose weight. Chrysanthemums develop purple spots on their leaves and mummified flower buds. Any attack is at the least disfiguring so prevention is best.

Generally, Botrytis cinerea overwinters in the soil on plant debris as mycelium. Scierotia or infected plant debris can be spread with seed. Growth and sporulation during cool damp weather favours release and germination of conidia, and establishment of infection through wounds, or via mycelium, on old flowers or dead foliage.

Impact/effects on chrysanthemums
Predominantly a blossom blight and a fruit rot, it can also cause damping-off, stem canker, leaf spots and root rot. 

Controls
Professional products: 
Iprodione - Rovral 
Carbendazim. 

For amateur gardeners:  Cheshunt Compound.