Grey mould fungal spores (Botrytis cinerea). Grey mould is a fungal disease also known as 'Botrytis bunch rot' in horticulture. It is caused by either consistant wet, humid conditions, or by dry conditions following wet weather. The mould's asexual spores, known as 'conidia' are dispersed by wind and rain. Grey mould affects many plants, in particular strawberries, which are inedible once infected.