Powdery mildew spores overwinter on perennial crops such as grapes, raspberries, strawberries and fruit trees, or in plant debris left from last year. In fact, powdery mildew spores can’t germinate or grow when foliage is wet, so overhead watering is sometimes recommended as a preventative on highly susceptible crops. Powdery mildew thrives when foliage is dry and the weather is warm wind spreads the spores to other plants. Moreover, downy mildew is a disease that appears in cool, wet conditions and is generally stopped by warm, dry, windy weather. Downy mildew develops spores only on the undersides of leaves, whereas powdery mildew will appear on both sides of leaves as well as shoots, buds and sometimes flowers. Both produce light-colored masses of spores on foliage. Identification The first thing to know about powdery mildew is that it is quite different from downy mildew, despite some similarities in appearance. Nine other species of powdery mildew fungus affect cole crops, peas, eggplant, pepper, tomato, strawberries, beans, black-eyed peas, grapes, and tree fruits. Flowers - chrysanthemum, begonia, dahlia, phlox, sunflower and zinnia.Vegetables - cucurbits, cucumbers, endive, lettuce, melons, parsley, poumpkins, and potatoes.The main species is Erysiphe cichoracearum, which affects composites, including: Several species of fungus cause powdery mildew, each affecting different groups of plants. Without treatment, it will spread to shoots and buds and eventually kill the plant. Powdery mildew, common on zinnias and other crops in mid-summer, starts with just a few spots on the lower leaves, as shown in this photo. But most powdery mildew can be avoided or cured with inexpensive, homemade remedies that have been proven to work as well as or better than commercial fungicides. Powdery mildew is a common summer problem on many types of vegetables and cut flowers. Check the label and make sure the product is labeled for controlling mildew on the plants you are treating.As warm, dry weather settles in this month, be alert for the development of powdery mildew on crops. The organic fungicide Neem and other commercial products can also be used. Cornell University discovered a weekly application of one Tablespoon baking soda mixed with 1 teaspoon insecticidal soap or light weight horticulture oil (these act as spreader stickers) in a gallon of water will suppress powdery mildew. You can use an eco-friendly fungicide as soon as the disease is spotted and throughout the remainder of the season. Further reduce the risk of powdery mildew on susceptible plants by removing ¼ of the stems in spring to increase light penetration and air flow. Grow the plants in full sun with proper spacing to increase the health and vigor of the plants and reduce the risk of disease. Or replace this plant with a more mildew resistant variety like Raspberry Wine, Blue Stocking, Marshall’s Delight, or Violet Queen. Planting something a bit shorter in front of the bee balm will help mask the damage while allowing the colorful flowers to show through. Unfortunately, they look pretty ugly by the end of the season. The good news is the plants will survive. This is a common fungal disease on lilac, garden phlox, zinnia, squash and other vine crops and lilacs. The leaves of my bee balm plant are covered with a white substance.
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