QUESTION: The leaves of my apple tree and roses are drying up. The leaves have a white-grayish fuzzy material coating them. What is this, and what can I do?

ANSWER: Powdery mildew is affecting your apple and roses. Powdery mildews are common, widespread, and on many crop and ornamental plants. The total loss by these organisms each year probably surpasses the losses caused by any other single type of plant disease. There are many species of powdery mildew. Your apple tree was probably infected by Podosphoera leucotricha and your roses by Spaerotheca pannosa. These fungi are common and cause serious problems in cool and warm humid areas, but are even more of a problem in warm dry climates like Arizona. Powdery mildew spores (fungal "seeds"), can be released, germinate, and cause infection when the relative humidity in the air is fairly high, but there is no film or water on plant surface. This spring, with abnormally high rainfall, has favored the growth and spread of powdery mildew. Once infection has begun the fungus spreads on plant surfaces regardless of the moisture conditions in the atmosphere. These organisms send haustoria (feeding organs), into the epidermal cells of plants to obtain nutrients. Powdery mildews are obligate parasites; meaning they cannot be cultured on artificially growing media in the laboratory but must grow on their specific hosts. Powdery mildew seldom if ever kills its host, but utilize their nutrients, reduce photosynthesis, increase respiration and transpiration, impairs growth, and can reduce yields as much as 20-40 percent.

Control: When planting apples or roses or other susceptible plants, place them in a location with good sunlight and air flow, i.e. not up against the house. Prune plants properly to open them up to ensure air flow through the plant. A rose or euonymus planted on the shady north side of a house will surely have powdery mildew problems. Many new rose varieties have disease resistance to powdery mildew. This resistance will help, but may not completely eliminate the need for chemical control. Chemical controls include spraying or dusting sulfur. Care must be taken not to apply sulfur on hot days because plant tissue can be burned. Other chemical controls include benomyl (Benlate), tridimefon (Bayleton), and triforine (Funginex). Source: Plant Pathology, George N. Agrios. Third Edition, 1988, pp. 337-342.