One of the most common issues faced by gardeners – hobbyists and professionals alike – is powdery mildew.
This disease is easily recognized by its namesake powdery residue. Most commonly found on young leaves and shoots, powdery mildew is not usually fatal to the plant though it is a nuisance.
There are multiple causes that contribute to powdery mildew, and many ways to prevent the occurrence and spread, as well as control any outbreaks.
What exactly is powdery mildew?
Caused by fungi in the Erysiphales order, this disease is particular in that certain fungal species will only affect certain plants. This means that powdery mildew on roses, for example, will probably not spread to a nearby lilac plant.
Various fungal strains can affect wheat, barley and other cereals; legumes; grapes; onions; apples and pears; cucurbits such as gourds – especially squash and pumpkins, melons, watermelon, and cucumber; lilacs; strawberries; and some trees – especially ornamental trees.
Its white powdery appearance is caused by the large number of microscopic spores that form living chains on the host plant.
Unlike many other fungal diseases which thrive in dark, moist places, powdery mildew flourishes in warm, dry climates if the area around the plant is fairly humid.
Powdery mildew can also recur and can be an annual problem for some gardeners.
How is powdery mildew spread?
The fungi that cause powdery mildew can only survive on a living host; however, spores can be transferred through the air or can be carried to new plants by sucking insects such as woolly aphids.
Spores do not require water for either germination or infections. Depending on the fungal strain, new spores can be produced every 3 to 14 days.
In the fall, or when the fungus has reached maturity, round black specks may form within the mildew.
These are the cleistothecia – the sexual fruiting bodies of the mildew – which will crack open in the spring to release spore sacs and potentially infect nearby plants.
How to identify powdery mildew
The most common symptom is, of course, the presence of a white or greyish powdery residue on the upper surface of the leaves. It may appear to be dust or dirt, but if wiped away, will return. The powdery spots may also appear on stems or fruit.
Young, succulent foliage is particularly susceptible to powdery mildew and will often appear disfigured, as though it has twisted or curled, exposing the underside of the leaves, or the leaves may yellow and dry out.
If left untreated, the powdery residue will spread to cover most of the affected area, causing the leaves to appear almost completely white.
The impact of powdery mildew on plants
Though only severe cases will be fatal to their host plant, powdery mildew is a serious problem for many plants, impeding the growth of new leaves and shoots.
The disease can also, if a considerable portion of the leaves are covered in spores, impede photosynthesis as the sunlight cannot reach the plant.
This can be a particular problem when growing vegetables or fruit, as insufficient photosynthesis can decrease the amount of sugars produced within the plant and affect its flavour.
How to prevent powdery mildew
A well-planned garden will reduce the likelihood of a powdery mildew infestation: choosing cultivars with natural resistance to the disease, and keeping plants away from shady, humid areas will greatly decrease the potential for the fungi to grow.
When attempting to prevent powdery mildew, it’s best to choose plants that are naturally resistant to tolerant to the fungi that cause the disease.
Several cultivars of cucurbit, for example, have been developed to resist the Podosphaeraxanthii fungus – its main cause of powdery mildew.
Choosing well-drained, sunnier areas will reduce fungal growth. Avoid both overwatering and watering from directly overhead in order to decrease the relative humidity around the plant.
Do not crowd the plants. Also, selectively pruning overgrown areas will diminish the growth capacity as it eliminates overshadowed areas and permits better air circulation around the plant.
Pruning and staking plants will also improve air circulation and reduce the risk of powdery mildew.
Many gardeners choose to spray their plants with a bicarbonate solution to prevent fungal growth; using 1 teaspoon of baking soda in 1 quart of water, they thoroughly spray the plants.
This solution will only kill the fungus with which it comes into contact. Adding liquid soap or neem oil can increase the effectiveness of the solution.
How to treat powdery mildew
The first step to eliminating powdery mildew is to prune away any impacted plants, ensuring that any leaves or stems with fungal spores are removed.
These infected plant parts should be destroyed; do not compost any infected plants, as spores can survive temporarily within the composted materials and be spread by air to new hosts.
Next, clear any fallen debris from the area. Using a thick layer of mulch may prevent any diseased spores remaining on the ground from reaching the lower leaves. It’s also important to sterilize any tools that may have been contaminated, using one part bleach to 4 parts water.
Avoid fertilizing the affected areas until the problem has been fully corrected; powdery mildew prefers to establish itself on young, succulent growth.
Finally, treat the infected plants. Many treatment options are available; organic gardeners have a multitude of non-chemical choices, while several chemical treatments are available at garden supply stores.
As powdery mildew is a common problem, commercial fungicides are readily available in most garden supply stores or online. Many products will both eradicate existing mildew and prevent the growth of new fungi.
It’s important to rotate the use of chemical fungicides to slow down the development of fungal strains with resistance to the treatment.
Also, plants that have been treated with antitranspirants also seem less likely to develop powdery mildew infections.
Severally commercially produced organic fungicides are available online or at local garden supply stores. Most contain sulfur or copper, and some use potassium bicarbonate.
It’s also possible to introduce a hyperparasite such as ampelomycesquisqualis that reduces the growth of powdery mildews and may eventually kill them.
Indoor gardeners could introduce a sulfur burner or vaporizer, which creates a fine dust that changes the pH level of the leaf surface and inhibits spores from establishing themselves on the surface.
Some growers use milk to control powdery mildew – the compounds found in milk may have antiseptic and anti-fungal properties. A ratio of 1 part milk to 2 or 3 parts of water has proved effective on cucurbit plants.
Others have tried mouthwash; it’s also an antiseptic designed to kill germs. This should be used sparingly as mouthwash is very potent and may damage new plant growth; a solution of 1 part mouthwash to 3 parts water is recommended.