Disease Cycle
Powdery mildew is a polycyclic disease (one which produces a secondary inoculum) that initially infects the leaf surface with primary inoculum, which is conidia from mycelium, or secondary inoculum, which is an overwintering structure called a cleistothecium. When the disease begins to develop, it looks like a white powdery substance.
The primary inoculum process begins with an ascogonium (female) and antheridium (male) joining to produce an offspring. This offspring, a young clesitothecium, is used to infect the house immediately or overwinter on the host to infect when the timing is right (typically in spring). To infect, it produces a conidiophore that then bears conidia. These conidium move along to a susceptible surface to germinate. Once these spores or conidia germinate, they produce a structure called a haustoria, capable of "sucking" nutrients from the plant cells directly under the epidermis of the leaf. At this point, the fungi can infect leaves, buds and twigs that then reinfect other plants or further infect the current host. From this point, you see more white powdery signs of powdery mildew, and these structures produce secondary inoculum to reinfect the host with mycelium and conidia, or use the mycelium to produce primary inoculum to another plant.
For germination to occur using a cleistothecium, the cleistothecium must be exposed to the right environmental conditions to rupture the structure to thereby release spores in hope that they'll germinate. Germination of conidia occurs at temperatures between 7 and 31°C and is inhibited above 33°C. Germination is greatest at 30-100% relative humidity.
Read more about this topic: Uncinula Necator
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