Wahoo! You Have Powdery Mildew
It's a wonderful morning in a heretofore wonderful world. You walk out to your garden, only to discover to your shame and horror that some miscreant has confused it with a baby's bottom and showered Johnson's Baby Powder all over your plants.
Well, it looks like baby powder. There is white stuff are over the leaves and flower buds. How did it get there? More importantly, how can you get rid of it? Once again, the benevolent Grump is here to help.
First of all, it isn't powder. It's a lookalike fungus that's very common around this time of year called powdery mildew. There isn't just one organism that causes powdery mildew. There are hundreds. So the one that gets on crepe myrtle is different from the one that gets on roses is different from the one that attacks phlox is different than the one that plasters lilacs.
But the result is the same. Small, white spots form on the upper surface of the leaves and coalesce into a powdery film. The film causes flowers and flower buds to wither and die.
How Did the Mildew Get There? Powdery mildew spreads by spores. The spores are everywhere. They blow in on the wind and splash from plant to plant when it rains. When they land on a leaf and conditions are right, they germinate, grow, and make more spores.
What are the right conditions? Warm days, cooler nights, high humidity, still air, and some shade. Although powdery mildew is often associated with wet foliage, the funny thing is that spores can only germinate on dry foliage. If foliage stays wet too long, the spores die.
What Are Some of the Most Common Garden Plants Affected? 1. Bee balm (Monarda didyma)
2. Crepe myrtle
4. Flowering dogwood
9. Summer phlox (Phlox paniculata)
How Can I Prevent Powdery Mildew? The best way is to plant mildew-resistant selections if they're available. For bee balm, try 'Gardenview Scarlet' (red), 'Marshall's Delight' (purplish-pink), and 'Violet Queen' (purple). For crepe myrtle, choose any selection with a Native American name, like 'Natchez,' 'Sioux,' or 'Arapaho.' Most newer crepe myrtles, such as 'Red Rocket,' 'Dynamite,' 'Pink Velour,' and 'Burgundy Cotton,' also resist mildew. For summer phlox, try 'David' (white), 'Delta Snow' (white), 'John Fanick' (lavender-blue), 'Robert Poore' (pink), and 'Rosalinde' (pink). For dogwoods, try 'Cherokee Brave' (red), 'Cherokee Chief' (red), 'Cherokee Daybreak' (white), and 'Springtime' (white).
Control method #2 -- Deny powdery mildew the conditions it likes. Reduce shade around susceptible plants. Don't crowd plants -- leave space between them so that air circulates freely. Don't splash water onto the leaves.
Control method #3 -- Quickly remove any infected leaves, flowers, and flower buds and either burn them or throw them out with the trash. This controls the spread of spores. On crepe myrtles, cut off side branches affected with mildew and get rid of them too.
Control method #4 -- Spray the foliage according to label directions with an appropriate fungicide. Two natural products that work are Natria Disease Control and neem oil. The first suppresses powdery mildew and many other diseases using a beneficial bacterium, Bacillus subtilis, that is safe for people and wildlife. The second employs an extract from the tropical neem tree. It controls both insects and diseases. For the longest lasting control between sprayings, use a systemic fungicide called Immunox. It's absorbed into plant tissues and works for weeks.