A question that is continually asked by indoor gardeners goes something like this: “I am controlling my humidity down to about 55% and the room temperature is maintained at about 75˚F. I have good air circulation and yet I still have a powdery mildew problem. Why?”

To answer this question, it is important to think small, very small. At the microscopic level small. In general, individual powdery mildew spores are not visible to the naked eye. If you see powdery mildew you are looking at thousands, or hundreds of thousands of spores piled up. Plant pathologist Dr. Ken Horst estimated that if the seven leaves of rose leaf were covered in powdery mildew, you would be looking at approximately 3 million spores. Now that we’ve got you thinking small, let’s think about what powdery mildew likes and needs to grow.

The environmental conditions that are most favorable for disease development in nature are cool night temperatures (about 60˚F) with high humidity (90-99%) and breezy, warm day temperatures (about 80˚F) with humidity of 40-70%. Humidity as it relates to powdery mildew growth is a relative term, in that powdery mildew could still thrive even if the air humidity is low. For example, an outdoor gardener who waters the garden in the evening is physically putting moisture on the leaf surfaces, so even if the air humidity is low, the necessary ingredient for fungal growth—water—has been artificially introduced.

Powdery mildew spores are transported through the air and land on plant surfaces and lay in wait for the perfect environmental conditions for reproduction. In general, spore germination occurs at the 60-64˚F range with high humidity, or leaf surface moisture. At this stage, mycelium (tube-shaped structures), grow out from the spores. Root-like structures called haustoria burrow into the plant surface and extract nutrients.

Stalk-like structures called conidia grow out of the mycelium and new spores stack up like loosely connected balloons. These new live spores need only a bit of warm wind and they burst off and float through the air. The ideal environment for powdery mildew spores to spread is warm, dry, breezy conditions.

So even the most perfect conditions of low humidity and warm, breezy air spreads existing mildew spores all over. In fact, what we think of as perfect conditions are actually necessary to spread spores, which then land on plant surfaces and wait for just the right temperature and moisture to start reproducing again. And herein lies the problem.

Often indoor growers who believe they are controlling their humidity and temperature will have a drop in temperature down toward the 60-64˚F range when they go to lights out. This is a problem that can be managed with the right equipment and control. However, the powdery mildew problem is more complex.

Leaves transpire moisture, so even if the air-measured humidity is low there can still be moisture at a microscopic level on the leaf surface. It is important to realize that the world in which fungi live is at the microscopic level, so undetectable moisture on a leaf surface and the micro-climate at that level might be the perfect breeding ground for powdery mildew, even though the temperature and humidity we monitor seems to be in control.

Even if a grower really is controlling their temperature at a steady level of 75˚F and appears to be controlling humidity at 55-65% or lower, the surface temperature at the micro-climate level on the leaf surface can be lowered basically by the wind chill effect. A surface will lose heat through convection. The insulating layer of warm air that forms at the leaf surface is disrupted by moving air, allowing cooler air to replace the warm air at the leaf surface.

Unfortunately, the practice of providing a constant breeze blowing across plant surfaces may actually be contributing to the growth of fungi by lowering the surface temperature and helping to create a perfect micro-climate for powdery mildew to germinate, and then the breeze itself spreads the live spores throughout the growing area.

Air circulation is important as it mixes the air and prevents temperature layering. It is also necessary to draw overall humidity away from plants and to move the excess heat generated by artificial lights. But rather than blowing directly across or through plants, it is my opinion that it should be moving air around the plants.

It’s worth considering that air movement should not be forcefully pushed directly through the plant canopy, but move the air around the plants. That circulation should be sufficient to draw minimal air movement through the plants without disrupting the leaf surface temperatures and causing a wind chill effect.

A grower needs to experiment with what constitutes the right amount of air circulation depending upon their lights, room size, number of plants, etc. Controlling overall room temperature and humidity is critical, but it is important to keep in mind what you can’t see or monitor right at the leaf surface.

To win the battle against powdery mildew, think small. Experiment with refining your air circulation around the plants and throughout the room. Employ good fungicidal solutions, because there are always spores in the air and on plants surfaces just waiting. Consider filtering air and of course control the environmental conditions you can monitor, but remember the micro-climate world you can’t see.