Powdery mildew is a fungal disease that is nearly universal, in that it is found almost anywhere and can infect almost any species of plant. The disease is caused by many pathogens, the most notable of which include Podosphaera fuliginea (also known as Podosphaera xanthii) and Erysiphe cichoracearum. While sometimes confused with downy mildew—largely due to the similarity between names, rather than any visual resemblance—powdery mildew is easily identified by the fuzzy white growths which appear on the leaves and stems of infected plants.
Even though powdery mildew is caused by multiple species of fungi, the fuzzy white growths for which the disease is known are a universal and readily recognized symptom.
The early symptoms of powdery mildew are easy to miss. The first signs of infection appear on young leaves in the form of small, raised blisters, which cause the leaves to curl and expose undersurfaces.
As the disease progresses, the namesake symptom becomes evident: round, pinpoint powdery white spots dusting the upper surfaces of leaves, as well as stems and occasionally fruit. As symptoms worsen, the spots will become larger, and more interconnected and irregular in shape. Over time they progress from younger to older leaves, and the undersides of leaves as well. However, despite the advance of symptoms, mature leaves are usually much less severely infected than new growth. If the white patches—which have a granular, powdery texture—are wiped away, the growths will return in a matter of days. Severely infected leaves will yellow, dry out and drop from the plant. Buds and growing tips of shoots can also become infected, eventually becoming distorted and stunted.
Powdery mildew is caused by fungi which thrive in warm, dry climates.
When it comes to plant diseases which produce visible growths on plant tissues, growers are often conditioned to fear the cool, moist climates favored by the water molds responsible for diseases like damping off.
Much as with water molds, the spores of Podosphaera fuliginea, Erysiphe cichoracearum, and other powdery mildew species are found nearly everywhere. But these fungi only require moist conditions to germinate. Once germinated, the fungi prefer warm weather—temperatures of 70 to 80 degrees Fahrenheit—during the day, with more humid conditions at night which allow for continued spore germination.
Powdery mildew can thrive in conditions where humidity is as high as 90%, but struggle when leaf surfaces are wet due to rainy weather. Consequently, powdery mildew is seen less often in years where cool, rainy weather continues late into spring.
But the most ideal conditions for powdery mildew are often found in greenhouses. Shaded, damp areas with poor air circulation provide conditions that are perfect for the initial germination of spores. After germination, the typical warmth and humidity of greenhouse air allows for the explosive spread and growth of powdery mildew.
However, the fungi which cause powdery mildew can only grow on specific hosts. The aforementioned Podosphaera fuliginea, for instance, favors cucurbits. On the other hand, soybeans are infected by Microsphaera diffusa, and the vineyards of Napa by Uncinula necator. This means that even in the case of pervasive powdery mildew infections, only certain crops will be impacted.
Powdery mildew is not a serious disease, in large part because the fungal growths do not penetrate into plant tissues, instead only rooting in the very top layer (epidermis) of leaves and branches.
Very serious infections can impact photosynthetic efficiency, increase the vulnerability of affected tissues to sunburn, and ultimately reduce yields and crop quality. But the disease is very rarely fatal, and readily treatable.
As with many fungal diseases, effective treatment of powdery mildew requires a dual prevention/management regimen.
The disease is particularly common in greenhouses because of the combination of warm air, dry plant surfaces, and high relative humidity. Indoor grow areas can be made significantly less hospitable to the disease by turning the heat up and opening up vents for one evening every 3 to 5 days. This helps to interrupt the cycle of germination, growth, and spread.
Some greenhouses take the additional step of spraying plant surfaces when conditions are otherwise dry, as standing water inhibits germination, and in some species kills powdery mildew spores. But this step should only be taken with caution, as wetting plant surfaces can make plants more vulnerable to other water mold-borne diseases.
Because powdery mildew spores commonly overwinter in debris piles in outdoor environments, in the winter all infected debris should be removed from fields and burned or otherwise disposed of. Tilling under and composting of infected debris should be avoided, as this will only serve to spread the disease to the next year’s planting.
In both indoor and outdoor environments, growers should avoid use of nitrogen fertilizer in the late summer, as this encourages the growth of the young, succulent plant tissue favored by powdery mildew. Growers should also ensure that plants are spaced far enough apart to ensure proper air circulation and reduce humidity. Avoid overhead watering, as this tends to produce the higher humidity levels needed for spore germination. Where possible, implement the use of drip irrigation or other ground-based watering systems.
Trimming and removing infected plant tissue is an effective means of controlling the spread of powdery mildew. All infected debris should be removed and destroyed. But this approach is usually only feasible at smaller scales, or if the disease is caught very early. Otherwise, there are a variety of standard sulfur-based fungicides which are effective in treating powdery mildew. For organic growers and growers of plants that do not tolerate applications of sulfur, heavy applications of potassium bicarbonate-based sprays have been shown to be effective in controlling powdery mildew.
However, chemical-based treatment regimens will only show limited effectiveness if environmental and nutritional issues are not also addressed. Ensure that balanced nutrition is provided, avoid irrigation practices which make the environment more hospitable to powdery mildew, and remove all infected material as soon as possible to break the cycle of reinfection.