Powdery mildew is a fungal disease that is caused by many different species of fungi such as Erysiphe, Podosphaera, Oïdium, Leveillula.
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.
Powdery mildew symptoms are fairly similar in all plant-pathogen combinations. The symptoms are white fluffy colonies, first and foremost on the upper side of the leaf. In severe epidemics in cereals, the ears may also be infected.
In pepper, the fluffy colonies occur mostly on the underside of the leaf, because the fungus infects the plant by entering through the stomata and there are more stomata on the underside of the leaf. On the upper side, yellow spots are noticeable. In case of a severe epidemic, the fluffy colonies also appear on the upper side of the leaves. Many sweet pepper cultivars react to powdery mildew infection by dropping leaves that are not infected yet. It depends on the cultivar how soon after infection this happens.
In apple, pear, outdoor grown rose and other Rosaceae, the fungus causing powdery mildew is extremely sensitive to water and ‘hides’ on the underside of the leaves. Therefore, this is where the colonies mostly occur in these crops. In covered roses, the colonies appear first and foremost on the upper side of the leaves.
In most crops, the white colonies later turn brown or grey. Severely affected leaves may turn yellow, curl up or drop off.