What is Mildew?

“Spoiled meal” is how the Middle English word “mealdew” is translated. We now know it as “mildew”. We also know it as the foul-smelling mold that can invade our kitchens and bathrooms as it moves in on our moist surfaces causing discoloration and stains. It colonizes as a growth of threadlike hyphae produced by several species of fungi.  It can spread quickly on damp surfaces and travel from its source to new colonies by releasing airborne spores.


Mildew is commonly seen on vegetation. We’ve all discovered some kind of over-ripe fruit with the familiar white powder eating at its surface. It lives on organic matter such as plaster, wood, wood, paper and those sweaty workout clothes you left in your gym-bag last week. Mildew is a  kind of mold usually black or white in color. It is caused by molds that thrive in dark, damp and warm environments, often where air is poorly circulated . Basements, crawl spaces, closets, shower curtains, and yes, damp clothes are favorite habitats. Humid summer weather frequently provides a climate that is mildew-friendly.

New houses are not immune because often moisture is found in building materials when construction has been done in wet or humid circumstances. Mildew  can deteriorate many kinds of building materials, from framing wood to drywall.  It can also likes paint (especially latex). It can penetrate though paint pores and begin to deteriorate the absorbent plaster or wallboard paper underneath.

Heat is known to accelerate  the growth of mildew (explaining why it is so at home in steamy bathrooms) but some species actually thrive at temperatures just above freezing.

Two Families of Mildew

powdery mildew left downy mildew right

Powdery mildew left. Downy mildew right.

The two varieties of mildew are Erysiphaceae, also known as  powdery mildew, and Peronosporaceae,  the downy mildews. Both species of fungi are parasitic on living or decaying vegetation.

Identifying Powdery Mildew

Powdery mildews get their name from the powdery, spiderweb-like masses that their white spores produce on the leaves of green plants — especially shade plants grown in humid areas.  Powdery mildew infestations cause the withering of leaves and will inhibit new growth. Powdery mildews attack almost 1,500 varieties of flowering plants including roses, apple, grape, lilac, gooseberry, pea, cherry and peach.

Downy mildew

Downy mildew goes after vegetation in cooler humid regions. A single fertilized spore is all that is needed to start a full-on fungal infestation. Downy mildew can be identified by light-green to yellow or brownish areas on the top part of a leaf. Small spores come out of the breathing pores of the leaf’s underside. White, blue, grey or violet patches can be seen on the  leaf’s underside in damp weather. Effected leaves often wilt and wither with an early death. The plant’s stems, fruits and flowers can also be infected. Cabbage plants, cucumber, onion, grape and lettuce are often victims.

To prevent downy mildew, avoid over-watering and over-crowding plants, keep planting beds free of weeds and make sure soil is well-drained. Immediately remove diseased parts if they become infected. A solution of copper sulphate and lime in water can be sprayed on some plants as an anti-fungicidal solution.Some plants now have mildew resistant varieties.

To slow the spread of mildew on plants suggests spraying the tops of leaves with a mixture of 1 part milk to 9 parts water. Spray again after rainfall.