Apply the oil only when temperatures are below 90 degrees Fahrenheit. Powdery mildew thrives in temperatures 50-65 degrees Fahrenheit with humidity levels of 80-90 percent. This disease creates a powdery white substance on the surfaces of leaves that resembles mildew. 4 Chrysanthemum powdery mildew is uncommon Table 1 Powdery mildew diseases occurring on cut flower crops in the UK: causal fungi, occurrence and symptoms 5 Small, dark-coloured spore bodies (ascocarps) may develop within the mildew as the crop reaches maturity They key identifier is that downy mildew has the white powder on the under side of the leaves only whereas powdery mildew has a white powder on both the underside and top of the leaves. Each type of phlox disease has a different type of "cure." greenhouse crops, powdery mildew can be active throughout the year. Powdery mildew is best treated with a wettable sulfur spray every 2 weeks. I found a suggestion online to deal with powdery mildew on phlox – milk! Moreover, downy mildew is a disease that appears in cool, wet conditions and is generally stopped by warm, dry, windy weather. Powdery mildew is far more common compared to downy mildew but it is important to recognise the difference between the two if you plan to treat either of them. Powdery Mildew. 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. Powdery mildew can also cause the infected areas to … It usually occurs at fall, and it will cause the growth of phlox to be retarded and will disfigure the plant's buds, flowers and leaves. Prevention/Solution. Powdery mildew infestations start with some mildew spores growing on the phlox plant. No plants are safe from this disease, but it most commonly affects edible plants including apples, squash, cucumbers and ornamental flowers such as lilac, zinnia, rose and phlox. Of all the garden diseases, powdery mildew is one of the most widespread and easily recognised. You use 1 part milk to 9 parts water, put it in a squirt bottle and spray it on all the parts that are affected. According to Garden Crossings, powdery mildew is the most common disease for phlox. To prevent powdery mildew from forming in the first place, avoid low-temperature, high-humidity environments. As the mildew reproduces, more spores spread all over the plant and the other plants in your garden or yard. Preventing the spread and/or severity of powdery mildew is the most cost-effective way of dealing with the fungus. Spray the plant with a horticultural oil, completely coating the phlox, before a powdery mildew infection becomes severe.