Like all fungi, powdery mildew has a defined life cycle, producing millions of airborne spores throughout its course. In favourable conditions, spores germinate on the leaf surfaces forming structures called appressoria. Germination occurs between 2°C and 30°C with an optimum range of 15°C to 20°C. The appressorium forms an infection peg and then hyphae penetrate the plant tissue, invading cells and parasitising the plant. A feeding organ, haustorium, is formed.
Sustained by the plant the fungus is able to develop and grow, forming conidia which in turn produce conidiospores. Growth can occur from slightly negative temperatures up to 30°C, the optimum range again being 15°C to 20°C. Conidiospores then break off and are dispersed by the wind ready to produce a new infection.
Sexual reproduction occurs when conditions are unfavourable for conidia production. The hyphae of different fungal strains fuse on the surface of plant tissue to produce a sexual structure which grows into a resting spore. This spore contains asci which in turn contain ascospores. When conditions become favourable, these ascospores infect growing plants in the same way as conidiospores. These two methods of reproduction mean that host plants are under constant threat from mildew.
When conditions are favourable epidemics can occur as powdery mildew can progress through its life cycle stages in just three to four days (20°C). Under less favourable conditions this latent period, the time between infection and the development of visible symptoms, can take longer e.g. 12 days at 10°C and 30 days at -2°C.