Question
Dear Mr Camper,
I am writing from Canada in a zone 3-4 climate. My above named tree is in its 5 growing year, this year the powdery mildew has taken its toll and alot of the new formed leaves are infected and drying up. Now that we are leaning toward fall I am wondering if it would be beneficial to apply a winter dormant oil. Also if this would be a safe practice for a Maple tree. Last winter was the first winter that this tree had some dieback on the south part of the upper tree...weird! I pruned it in hopes that new growth will begin next spring . I also have this tree in the middle of a large bedding area with shrubs and perenials beneath same. Your expertise would be appreciated. Our summers here are hot and a little cool at night just so you are aware.
Thankyou Karen

Answer
Well Karen , the problem is an old one--the fungus disease powdery mildew (Uncinula circinata and Phyllactinia corylea). I have seen it often on various types of maples.It is generally not considered 憇erious?but if it is ruining the look of the tree then you could not be criticized for being concerned. Recently, arborists have drastically reduced the number of tree diseases and insects for which they will spray, due to various restrictions and criticisms from so-called environmentalists. There are a number of chemicals that could be used including Funginex, which I would want to spray on as soon as the leaves are fully developed. But, it is an expensive chemical, and you may have difficulty getting an arborist to spray it. A good 憃ld? alternative is wettable sulphur.
Control Measures
Gardening Practices
High relative humidity at night and low relative humidity during the day with temperatures of 70 - 80F (conditions that prevail in spring and fall) favor powdery mildew. Maintaining conditions that favor rapid drying of foliage will help reduce disease incidence. Locate susceptible plants in open areas where they will not be crowded. Plants in shade are more prone to mildew than are those growing in sunlight. Prune to thin out any dense foliage; this will increase air movement and favor rapid drying of foliage. Avoid nightly sprinkling during August and September; instead, soak the soil as needed.

Figure 1.Powdery mildew on lilac
Common Name                       Trade Name
----------------------------------------------------------
myclobutanil                 Immunox
propiconazole                     Banner
thiophanate-methyl                Cleary's 3336, Domain, Fungo-Flo
thiophanate methyl + mancozeb     Zyban, Duosan
triadimefon                       Bayleton, Strike
triforine                         Triforine, Funginex
----------------------------------------------------------
Figure 2. Fungicides for powdery mildew control
Powdery mildews are generally most severe on young. succulent growth which excessive nitrogen promotes. A balanced fertilization program is advisable.
In the fall, clean up and dispose of all mildew-infected plant debris. This will help reduce the amount of disease next year.
Chemical Control
Powdery mildew can occur any time throughout the growing season; spray susceptible plants with an appropriate fungicide at the first sign of mildew. Cover the upper and lower surfaces of the leaves thoroughly. Repeat the treatment at intervals of 7 to 14 days or as often as necessary to protect the new growth.
Using Milk to Control Powdery Mildew - Garden Pest Tip  Less than 3 years ago, researchers in South America discovered a new alternative to controlling powdery mildew. Wagner Bettiol, a scientist from Brazil, found that weekly sprays of milk controlled powdery mildew in zucchini just as effectively as synthetic fungicides such as fenarimol or benomyl. Not only was milk found to be effective at controlling the disease, it also acted as a foliar fertilizer, boosting the plant's immune system.
Powdery mildew in the cucurbit family is caused by the organism Sphaerotheca Fuliginea. It is a serious disease that occurs worldwide. For decades, organic gardeners had to rely on making a spray from baking soda to control the disease. Now, instead of measuring out the baking soda and combining it with a surfactant (a "sticking" substance) of either oil or soap, gardeners need only head for their refrigerators.
In his experiments with zucchini plants, Bettiol found that a weekly spray of milk at a concentration of at least 10% (1 part milk to 9 parts water) significantly reduced the severity of powdery mildew infection on the plants by 90%. While some gardeners may be tempted to increase the concentration of milk for more control, Bettiol found that once concentrations rose above 30%, an innoccuous fungus began to grow on the plants.
Fungicide
?  Sodium Bicarbonate
1. as a spray for mildew
5g (about a teaspoonful) Sodium Bicarbonate (baking soda) to 1 Litre water
2. to prevent blackspot and mildew on roses
3 teaspoonsful (3 x 5ml spoonsful) of Sodium Bicarbonate
1 heaped teaspoon of soluble fertilizer
a few drops of washing-up liquid
Mix and dissolve in a cupful (200ml) of water, then make up to a gallon (4.5l) in a watering can to be watered over the foliage every two weeks, starting in early spring and continuing throughout the growing season.
?  The enzymes in a 10 to 30% solution of fresh milk sprayed on plants will attack mildew.
A stronger solution will result in a foul smell as the milk goes rancid.
I know this is kind of long but I tried to present you with host of options. I took excerpts from a number of my files.  Hope it helps,Bill