Powdery mildew and downy mildew are cruel diseases in the organic flower garden. After the gardener has ushered his tender seedling babies into adulthood, fought off aphid attacks with the blast of a hose, and suffered through late frosts that threatened early blooming lilacs, stealthy fungal diseases can catch one off guard.

Identifying Garden Mildew

Mildew diseases thrive in conditions of high heat and humidity, so they most often appear in the peak of summer.


In the earliest stages, the garden fungus that causes mildew may look like a bit of benign white dust, perhaps a circular spot or patch where some water dried and left a bit of residue. As the disease spreads, the leaves of affected plants may curl, wither, or drop off altogether.

Many plants can experience mildew, but some are notoriously susceptible. Roses top the list, especially hybrid tea roses. Lilacs, phlox, zinnias, and bee balm commonly show signs of mildew around the summer solstice. As if the insult to the flower garden wasn’t enough, mildew affects several vegetable garden favorites, including melons, spinach, lettuce, and cucumbers.

Cultural Control

The first tool in the organic gardener’s mildew arsenal is a strategy that denies the fungus an environment that promotes the growth and spread of the spores. Cultural control of garden mildew starts with planting, as gardeners must observe proper spacing requirements to promote air circulation. In bushy shrubs like roses and lilacs, you must use your pruning tools judiciously to thin weak or crossing branches that do little to contribute to the beauty of the plant, but prevent drying breezes from reaching its interior.

Other mildew prone flowering shrubs that benefit from pruning include crape myrtle, spirea, azaleas, and rhododendrons.

If you have a severe mildew outbreak in the garden, remove and destroy the plants. You may manage minor outbreaks by pruning away the infected foliage. Clean up all dropped leaves at the end of the growing season, as the spores spend their winter dormancy in your garden debris, waiting to shoot into the air and reproduce anew in the spring.

Water Wise

Observe smart irrigation practices in the garden. Drip irrigation not only saves water by delivering it directly to the root zone where it’s needed, but it also keeps the leaves dry. Water your gardens in the early morning, so the sun can hasten foliage drying.

Combine watering, organic fertilizing, and mildew control into one smart step by using compost. Place a shovelful of compost into a 5-gallon bucket of water overnight, strain, and spray the resulting tea onto flowers commonly affected with powdery mildew, including snapdragons, zinnias, dahlias, wisteria, and delphiniums.

Organic Fungicides

Baking soda is an old-fashioned natural remedy for garden mildew, with varying rates of efficacy. Researchers have improved on the simple sodium bicarbonate found in our pantries by combining it with surfactants and horticultural oils. You can look for these products under the brand names of Remedy or Kaligreen to fight a number of garden molds and mildews.

Choose Mildew Resistant Varieties

If you’ve shied away from planting certain flowers in your garden due to the inevitable white patches that appear in hot, humid weather, give these plants a second chance by choosing new mildew-resistant cultivars. Here are five typical mildew victims, with mildew resistant strains to try:

• Bee Balm: ‘Cambridge Scarlet,’ ‘Marshall’s Delight’
• Crape Myrtle: ‘Fantasy,’ ‘Osage’
• Phlox: ‘Delta Snow,’ ‘Natascha’
• Hybrid Tea Rose: ‘Mister Lincoln,’ ‘Tiffany’
• Zinnia: Any ‘Profusion’ series