Powdery Mildew: The Unwanted Guest

As I sit down to write this, I can see one of the peony plants in my backyard. I have no idea when it was planted or how old it might be. It, along with many other perennial flowers and ornamentals plants, came as a package deal when I bought the house. Over the last five years, I’ve grown quite fond of nearly every plant in my yard, and the robust purple flowers blooming from this peony bring me soft spoken joy. However, each year, this plant encounters the same tragic fate as the year before. It succumbs to the devastation of powdery mildew (PM) because it resides in an area of almost constant shade with poor air circulation.

I think it’s safe to assume that pretty much every grower or gardener has run into the menace that is PM. It’s like that one person no one likes that somehow finds their way into the party they were never invited to. With its nearly unmistakable grayish-white, powder-like appearance, PM is a common problem affecting a wide range of host plants. When it starts to rear its ugly self in your garden, remember that it isn’t the end of the world; and when the proper steps are taken, it can be almost always be prevented.

What is Powdery Mildew

Powdery Mildew is a fungal disease caused by several different species in the Erysiphales order, the most common being Sphaerotheca fuliginea. It is known to affect a wide variety of plants including roses, lilacs, peonies, zinnias, pumpkins, zucchini, and cucumbers. Regardless of the plant species being attacked, the symptoms will always look similar. Most often starting from the lower, older leaves, PM will present itself as a greyish-white powdery coating that resembles talcum powder. The leaves will be the most noticeably affected, but it can also spread to the stems and flower buds.

PM infection is rarely fatal to the host plant; but if left unchecked and allowed to reach the more severe stages, it can result in leaves that turn brown and become brittle and withered. When PM reaches this stage, it can begin to clog the plant’s stomata and restrict transpiration and water flow, effectively starving the leaf of nutrients. PM spores reproduce asexually and can spread quite rapidly under the right condition.

Powdery Mildew Is Host-Specific

The numerous PM fungi types that cause this disease are almost always host-specific. This means that each species of PM carrying fungi prefers a specific plant type in which to infect. The PM fungus that is attacking my peony plant is different from the one that would infect my rose bushes that are only 15 feet away. Likewise, the species that attack lilacs will have no effect on zinnia plants.

Conditions for Powdery Mildew and Treatment

The conditions in which PM is most prevalent are more or less the same for all of the species that cause it. It is quick to inoculate and grows best when the relative humidity is high and soil moisture is low. In areas with temperate climates, this is usually in the mid to late summer months of July, August, and September. PM also thrives in areas of heavy shade and poor air circulation.

The best way to handle PM is by taking the proper steps to prevent an infection from occurring. Avoid planting PM susceptible plants in areas that are close to large structures or buildings that can block or restrict the natural flow of the breeze. Also, avoid planting underneath or too close to large trees that cast a large amount of shade throughout the day. These types of areas are almost always the most active sights for PM infections.

PM fungal spores can travel easily through both water and air. Plants should be watered at ground level and not from above to prevent PM infections from occurring and spreading. If there is a small portion of a plant that is in the beginning stage of infection, watering from above the plant can certainly cause it to spread. When planting several plants in a group or rows, make sure to give them plenty of space, so the air has plenty of room to flow through and around the plant foliage. Pruning and thinning plants often will also help increase air circulation and will let the surfaces of the plant dry out faster when they do get wet.

Any plant debris, whether infected or not, should be cleaned up regularly to prevent the fungal spores from populating there. PM fungal spores can even lay dormant throughout the winter and once again become active in the spring. Keep the areas underneath and around the plants as clean as possible to avoid a nasty surprise. For another level of preventative protection, foliar applications of calcium carbonate and silicon supplements can help strengthen and fortify the plant cells making them less susceptible to a PM attack.

Moderate to Severe Powdery Mildew Attack

When a PM infection does become a moderate to severe problem, there are additional steps that can be taken to eliminate the problem and stop it from spreading. The first step is to remove any infected areas of growth immediately. Do not add the infected plant parts to a compost bin or pile. Most home compost piles do not reach the high temperatures needed to kill the spores properly, and once the compost is added to the ground soil, the spores can easily infect nearby plants.

Chemical fungicides are by far the most effective tool for eradicating and preventing PM infections, but be sure to read the labels carefully since there are many that should not be used on food crops. Safe handling of chemicals is always of extreme importance, so make sure to use the suggested safety precautions when going this route. For the organic gardeners out there, potassium bicarbonate and liquid elemental sulfur products offer a safer, more natural way of dealing with a PM outbreak. Organic methods are not as strong and reliable as chemical fungicides, but they are much safer to use. Postpone any planned fertilizing until the PM infection is completely dealt with. PM promoting fungi like to attack younger, more susceptible growth.

Indoor and Greenhouse Powdery Mildew Attacks

PM can also wreak havoc when allowed to enter an indoor growing space or greenhouse. Be sure to pay close attention to the environment of the growing space. Keeping relative humidity lower than 70% is a great way to prevent an infection. Also, always try to water from the bottom of the plant to avoid getting unneeded moisture on the leaves. Only do foliar applications when completely necessary and make sure that there is plenty of air circulation to properly dry the plants off, so no residual moisture accumulates. Keep plants properly spaced and well-pruned so that they aren’t touching each other, and the air can freely flow around and through the canopy. And always, ALWAYS keep the growing area as clean as humanly possible by immediately disposing of plant debris and mopping up any standing water from the ground.

More often than not, PM fungal spores are brought into a greenhouse or indoor garden from the outside. Practice good sanitation before entering the growing area. If you have come in to contact with a plant that may be infected with PM, wash your hands with soap and perhaps even change your clothes. PM spores can easily spread. If the building or growing room has an intake fan that draws fresh air from the outside, make sure there is a good filter in place and that it is changed or cleaned frequently. Larger indoor growing areas and greenhouses often incorporate a sulfur burning furnace that can help prevent or minimize an occurrence of PM.

To be quite honest, anyone that grows a substantial amount of plants will eventually run into a persistent and severe case of PM. Even when taking great care to avoid such a situation, it can still catch you by surprise. Implementing preventative measures that address the key environmental issues that lead to PM is an extremely important step that should always be taken. Working hard to prevent an infection of this type is the only way to truly limit the chances of it occurring on a larger scale. Remember: Your garden is a party and you are the bouncer- always working to keep the unwanted guests out!

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