Disease: powdery meldew

Powdery mildew Podosphaera macularis sp. humuli; formerly: Sphaerotheca humuli on hops

The term "mildew" derives from the floury appearance of the diseased areas, which is caused by the cotton-like mycelium lying on the surface of the plant, but above all by the multitude of produced conidia. The infestation site looks like powdered (powdery mildew).

Classification: Ascomycotina, class Plectomycetes, order Erysiphales, family Erysiphaceae.

Development:
The powdery mildew fungi are obligate parasites on higher plants and widespread worldwide; due to their mass occurrence they cause great economic damage. All green parts of the plant can be affected, with the whitish-mealy spots forming more often on the top of the leaves than on the undersides of the leaves. The later flat fungal structures can take on a dirty-grey-brown hue. In the genera Erysiphe, Microsphaera, Podosphaera, Sphaerotheca and Uncinula parasitic parasitism occurs by the formation of appressories (a kind of attachment organs) on the surface mycelium, with the help of which the epidermis of the host plant is crossed by the hyphae. The house stories are formed almost exclusively in the epidermal cells. Minor deviations, the penetration of hyphae and the occasional formation of house stories in crevices, are exceptions to the rule.

From simple, unbranched, upright spore carriers (conidiophores) the single-chamber spores are separated individually or in chains. Ascus development takes place in closed fruiting bodies, the Kleistothecia, which only tear apart after spore maturity. They develop towards autumn as small, black-brown grains recognizable to the naked eye. To identify and classify the genera of the family Erysiphaceae, the appendices of the cleistothecia and the number of Asci in these fruit bodies are used. In Sphaerotheca and Erysiphe (see pictures) the appendages are mycelial and originate in Erysiphe at the base of the Kleistothecium. Erysipheus has several Asci, while Sphaerotheca has only one Ascus. Some mildew fungi form another ("secondary") mycelium at an advanced stage of infestation, which can be very pronounced in Sphaerotheca and consists of thick-walled, brownish hyphae. Grass mildew also sometimes develops a reddish-brown, felt-like layer of mycelium.

(After: Wittmann, Wolfgang, Atlas of Ornamental Plant Diseases, Blackwell Wissenschaftsverlag Berlin, 1995)


Fig. 1: Real mildew on the shoot


Fig. 2: True mildew on the top of the leaf