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I decided it might be a good time to review some of the research on powdery mildew. We spent a lot of time last year fighting with the disease on various crops. First,we could not make it happen no matter what we tried, even on Gerber daisy. Then whenwe did have the right weather, we could not seem to get any control even with the bestproducts. Some of our most recent trials were on roses and they really do seem to be oneof the toughest crops to control powdery mildew on. We also tested Gerber daisies, crapemyrtle and zinnias in this time frame.
Results of national trials for chemical control of powdery mildew are summarized in the table below. It is interesting to see what all of the researchers are testing as well aswhat specific results are. Eight researchers from Michigan, New York, Alabama andCalifornia performed trials on powdery mildew of rosemary, aster, miniature rose,sedum, poinsettia, phlox, Monarda, crape myrtle, azalea, hydrangea, salvia, Gerber daisy,dogwood, Scabiosa, verbena, zinnia and Ranunculus. While this list does not include allornamentals that get powdery mildew it certainly does show an effort to work on a widerange of crops.
The fungicides that are colored the same belong to the same chemical class. You can see that a few chemical groups are very good to excellent for control of powderymildew. The sterol inhibitors (in blue), including Banner MAXX, Bayleton, Immunox,Rubigan, Strike, Systhane (now Eagle) and Terraguard, are a key chemical class forfighting powdery mildew. In addition, the strobilurins (in pink), including Compass O,Cygnus, Heritage and BAS500 (an experimental compound from BASF Corporation) arealso very effective and have become standards for powdery mildew control on someimportant crops at least.
These two large chemical groups do not even scratch the surface of products for powdery mildew control. Copper products like Camelot and Phyton 27, oils like Triactand Sunspray and potassium bicarbonates like Kaligreen and Milstop also give goodresults.
One interesting development is the continued use of thiophanate methyl products (in green) for powdery mildew control. These products were excellent for control foryears but rapidly developed resistance if they were overused. The results for Fungo,Spectro, Zyban and 3336 show some of this condition. At times they may give excellentcontrol but if resistance to thiophanate methyl has developed for a particular powderymildew fungus they will not be effective. As this illustrates, rotation for resistancemanagement is critical. Make sure you use products from at least two chemical groups inan alternating pattern or in a tank mix.
Many ornamental plants are attacked by powdery mildew fungi we have a very large arsenal of chemicals for their prevention. In many cases, we can even eradicate aninfection once it is found instead of spraying preventatively. There are a few plantspecies that are so badly affected by powdery mildew that eradication is not usuallyeffective. For these crops, like some rose cultivars, you should probably spraypreventatively when the conditions are favorable for disease.

Source: http://www.chaseagriculturalconsultingllc.com/resources/pdfs/articlesPdf/42PowderyMildewChemicalControlUpdate.pdf