Powdery Mildew: Causes and Effective Remedies

Several species from the same fungal group cause powdery mildew, and each type of infection has a preference for certain plants. Flowers, trees, vegetable plants, shrubs, and lawn grass are all potential hosts. Young plants are most vulnerable.

Without living plant tissue, powdery mildew won’t survive. Its life cycle begins with the development of fungal tissue called mycelium, which grows in thin layers on the affected plant. The powdery growth consists mostly of spores — some types look like tiny trees. The wind spreads these spores to new plant hosts, but splashing water and insects can also spread them. Unfortunately, the fungi finds no shortage of grasses, weeds, and other year-round hosts to keep it thriving.

This determined invader attaches itself to plant parts and, because it has a source of food, continues to thrive throughout the winter months. It even survives on plant debris, such as fallen leaves. During the winter months, the fungus produces resting spores, which survive during cold temperatures. Once the weather starts to warm up again, and as new shoots emerge on plants, the spores and mycelium invade them, and the powdery mildew’s cycle starts over again.

Powdery mildew loves warm temperatures between 60 and 80 degrees Fahrenheit. The disease occurs in overcrowded planting areas as well as in heavily shaded spots and those with poor air circulation. High humidity levels during the day are prime spore-formation times. As humidity levels and temperatures drop, the fungus releases its spores. The mildew is less common during the high temperatures of summer. Frequent rains or irrigation that keeps leaves wet seems to hinder its growth because water on the leaves prevents the spores from moving. Luckily, powdery mildew is host-specific. That means that if it pops up on one plant, it isn’t a threat to all of your plants.

The infection first appears as white, powdery spots on the plant’s surface, similar to a felt-like mat, and soon latches on to new shoots or buds. Sometimes the mildew looks similar to sprinkled baby powder. There are other signs of infection, however:

• Leaves may look as though tiny cobwebs cover their surface.
• Infected leaves may buckle, curl, twist, discolor, or fall off.
• Onions, artichokes, peppers, and tomatoes develop yellow patches on the leaves, but no powdery growth.
• Some types of powdery mildew turn leaves purple or red.
• Orange or black balls may form inside the spots.

Botanists say this fungus slows or lessens a plant’s photosynthesis abilities and makes it susceptible to other infections, but doesn’t kill it. A stressed plant may lose its hardiness during the winter months. Trees may leaf out later if they were infected the season before. Plants infected with powdery mildew earlier in the season can suffer serious damage, while those that aren’t infected until late in the growing season may not. The disease interferes with the plant’s flowering and can significantly impede fruit and vegetable plants’ yields. An infestation may not only affect the appearance of fruits and vegetables but can alter their taste as well.

Disease resistance is a genetic trait. Different types of plants are more prone to powdery mildew than others and certain species within a genus may have a higher resistance. The pastel-flowered prairie bee balm, for example, is more susceptible to the disease than the bog bee balm that has red flowers. Plant breeders have created strains that are resistant to this fungal infection. Powdery mildew loves plants like zinnias and phlox. Planting disease-resistant options not only reduces the severity of an infestation but also cuts down the need for fungicides.

Plants that need sunlight should get at least six hours each day, so sow them in places with access to plenty of it to minimize the risk of infection. If tree limbs or shrubs are blocking the sunlight for some plants, trim them back to reduce the amount of shade. For species that don’t require direct sunlight, provide ample space between plants to minimize infections. Adequate circulation is essential in preventing powdery mildew and avoiding over-fertilizing.

If you discover powdery mildew on a plant or a group of plants, remove and destroy all infected parts using plant clippers. Do not compost these infected plants and plant parts. The fungus will likely survive in the composted materials. Clean the clippers with alcohol wipes and wash your hands afterward.

An organic fungicide is an option for controlling powdery mildew outbreaks. Read the instructions provided with the fungicide to ensure it is safe for the species of plant you plan to apply it to. There are several other home-remedy options as well that you can try.

• Baking soda mixed with horticultural or liquid soap: Most effective when applied in the early stages.
• Potassium bicarbonate: A contact fungicide that kills powdery mildew after it infects the plant
• Mouthwash: Mix one part ethanol-based mouthwash and three parts water to control the mildew.