Life cycle and appearance of Powdery mildew

The fungi that cause powdery mildew (Erysiphe, Podosphaera, Oïdium, Leveillula) are all biotrophic fungi, meaning they feed on living plant cells and barely survive in the absence of a living crop. They survive as ascospores or perithecia, structures containing ascospores. Ascospores have not been found yet for all powdery mildew fungi and especially in greenhouse crops they don’t play a role in the epidemics.

Fungal spores germinate on the leaf surface and the germ tubes grow and branch out on the leaf surface. Small structures are produced, the haustoria, from which the fungus penetrates the plant cell and takes up nutrients from the epidermal layer of plant cells. Most of the fungus remains on the outside of the plant surface. On the mycelium on the plant surface, new conidiophores are formed, structures which contain new spores, the conidia. These conidiophores are the fluffy fungal growth so typical of powdery mildew. Only in pepper, infection (by Leveillula taurica) is through the stomata and the fungus grows inside the leaf. Later, the conidiophores in this case also protrude from the stomata.

Conidia are the main means of dispersal. They are dispersed by wind, as are the ascospores. In general, powdery mildew fungi are not very resistant to water so rain often limits the epidemics and spore dispersal by rain is negligible, since spores explode when in contact with water for no more than a few hours.

Powdery mildew spores, either ascospores or conidia, don’t need nutrients or water for germination. In general, the development rate of the epidemic is fastest at 18-25 °C. High relative humidity (RH) promotes germination of spores but inhibits spore production, so the net effect of the RH is very small. Wind and draught in greenhouses promote disease. In greenhouses, dispersal of spores over small distances (limited time) mostly happens via workers’ clothing.

How to prevent Powdery mildew

• Use resistant cultivars whenever available
• Extra Silicon and/or Calcium nutrition hardens the cell walls and makes it harder for the fungi to enter the leaves
• To reduce chances of powdery mildew in greenhouses, it is advised to first ventilate by only opening the windows on the lee side. Please note, this is not relevant for other diseases that are inhibited by reducing the RH
• In fruit trees, pruning should be done early in the season
• Do not provide excess nitrogen since this promotes the disease

Prevent plant diseases by optimizing plant potential and crop resilience.