Powdery Mildew

Powdery Mildew (*order Erysiphales*, primarily families *Erysiphaceae*), a highly widespread and recognizable fungal disease belonging to the class *Leotiomycetes* of the phylum *Ascomycota*, is a major threat to agricultural, greenhouse, and home garden crops. Caused by diverse obligate biotrophic fungi, this disease lives on the surface of host leaves and stems, sending specialized feeding tubes called haustoria into the plant cells to drain nutrients. Over time, it severely reduces the plant's photosynthetic capability, stunts vegetative growth, and lowers crop yields.

Identifying powdery mildew is simple due to its highly distinct, white-to-light-grey dry powdery coating on host leaves, stems, flower buds, and sometimes fruit. The infection starts as small, circular white spots on the upper or lower surface of leaves. As the disease spreads, these spots expand and merge, making the foliage look as though it has been dusted with flour. Under high disease pressure, leaves puck, twist, turn yellow, and drop prematurely, while flower buds fail to open.

Powdery mildew has an extremely broad host range, affecting hundreds of plant species.

The powdery mildew fungus overwinters as dormant mycelium or small, black overwintering structures called chasmothecia in buds and leaf debris. In spring, conidia are produced and carried by wind currents to establish new infections. Unlike most other fungal pathogens, powdery mildew conidia do NOT require free-standing water on the leaf surface to germinate; they can infect plants in relatively dry weather as long as the relative humidity is high (above 70%). It thrives in warm, dry days (20°C to 30°C) followed by cool, humid nights, and in shaded, overcrowded garden beds. For organic management, prune and discard crowded shoots to open up the canopy, and apply foliar sprays of wettable sulfur, potassium bicarbonate, or neem oil to establish a protective barrier. A classic, highly effective home remedy is a spray consisting of 1 tablespoon of baking soda and 1 teaspoon of liquid soap in 1 gallon of water. Biological control is achieved by using *Bacillus subtilis* to colonize leaf surfaces. Chemical treatments include systemic strobilurin or triazole fungicides like myclobutanil or triadimefon. Prevent outbreaks by planting disease-resistant cultivars, placing plants in full sun, and avoiding broad-spectrum chemical sprays that kill natural biocontrol agents. For complete support, browse our Plant Disease Identifier Hub, or explore related guides on Downy Mildew, Black Spot, and Aphids.