Here is the site I looked this up. ? powdery mildew on phlox
Attracting butterflies with its fragrant blooms, the garden phlox (Phlox paniculata) is an herbaceous perennial that grows in U.S. Department of Agriculture hardiness zones 4 through 8. The garden phlox requires full to partial sun in well-drained soil. This lovely plant with colorful flowers is highly susceptible to the damaging fungus known as powdery mildew.
Unfortunately, the plants are often plagued by mold and mildew. The most common fungal problem is powdery mildew, which distorts foliage, weakens the plant and can reduce flowering. The mildew usually results in a white or gray powdery growth on leaves or flowers. Powdery mildew thrives in humid weather when temperatures are between 50 and 70 degrees Fahrenheit.
Powdery Mildew One of the oldest known plant diseases, powdery mildew is a wide destructive force that can affect a wide array of plant species. Typically, the first sign of a powdery mildew infestation is a white, powdery growth on the branches and leaves of the garden phlox. The leaves will begin to discolor, turning yellow or brown before dropping off prematurely. In some instances, the shoots and leaves may begin to twist. If left untreated, powdery mildew can severely damage the plant and is highly contagious, spreading to nearby flowers, bushes and crops.
Organic Treatment A natural fungicide treatment against the harmful powdery mildew is skim milk. The amino acids and calcium in skim milk help eliminate the fungus without harsh chemicals. To begin, mix 9 parts water with 1 part skim milk and pour the concoction in a clean garden sprayer or spray bottle. Apply the organic powdery mildew treatment once every other week during the morning hours. The baking soda (sodium bicarbonate) commonly used for recipes, cleaning and removing odors can also eliminate powdery mildew from your garden phlox. Dissolve 1 teaspoon of baking soda for every quart of water and transfer the mixture to a spray bottle or garden spray. Liberally coat the top and underside of the leaves as well as the branches once a day every three days with the mixture.
Prevention The best defense against the fungus that causes powdery mildew is proper prevention. Provide adequate spacing between plants to allow for air circulation. Air circulating between the garden phlox helps keep the plant’s foliage dry and reduces the growth of the fungus. Furthermore, refrain from wetting the plant’s leaves angularly remove decaying or dead branches and sticks from under the plant. Decaying matter is a food source for powdery mildew fungus. Keeping the plant dry this year has been the problem with all the rain we have had, and the high humidity.
1 Cut out infected leaves and plant stems as soon as the powdery mildew is noticed. Use clean shears to prevent the spread of the spores to healthy plants.
2 Remove all fallen leaves and dead plant material from the bed, but dispose of it rather than adding it to your compost pile. Old plant material can harbor powdery mildew spores.
3 Delay any scheduled fertilizer applications until the phlox has recovered from the powdery mildew infection. High nutrient levels in the soil can cause an increase in mildew growth.
4 Water phlox in the morning so that the excess moisture dries quickly. Light overhead watering can rinse spores off the plants, but wet foliage later in the day that doesn't dry quickly can increase mildew infestations.
5 Cut back the phlox to the ground in fall after the plants begin to die back. Remove all old plant material and mulch from the bed and dispose of it so that mildew spores don't survive over winter and reinfect the plants in spring.
Planting resistant phlox varieties can prevent most powdery mildew problems. Resistant varieties include “Natasha” (Phlox maniculata “Natasha”) and “David” (Phlox paniculata “David”). till next time, this is Becky Litterer from Becky's Greenhouse, Dougherty Iowa