Powdery mildew is not as lethal as black spot. It usually starts appearing from the top down on the newer growth, where as black spot starts from the bottom up on the older growth of the bush. Cold nights and warm days coupled with dry, rainless weather encourages the growth of powdery mildew. Wet weather is hard on PM, and can be counted upon to help control it. I know a rosarian who sprayed his PM afflicted roses twice a day, AM and PM, with water alone, no fungicide, and effectively halted the PM in its tracks. Spring and fall in our climate, NJ and PA, have very similar growing conditions, hot days coupled wth cool nights, are the powdery mildew seasons. PM, as stated earlier, is not as harsh a disease as is black spot. You can go for years with no outbreaks and almost overnight it can hit, not to return for several more years. Powdery mildew is sneaky, it appears subtly, some slighty misshapen newer growth that is mild, then bingo almost before your eyes it turns into a full blown case. It severely disfigures the foliage and buds, suppressing their development. An outbreak this fall does not mean your roses are from this day forward PM prone. PM is in the environment and really needs very specific conditions to manifest itself, conditions that are not present every spring or fall every year. I believe some rose varieties are more susceptible to PM. Those with dull, matte leaves being prime targets, with shiny leaved roses being more PM resistant. There are probably other susceptibility factors, too, but the foliage finish is the major one of which I know. Hybrid rugosas, as tough as they are, are PM susceptible, probably due to their dull finish foliage and rugose, pebble grained, texture. The nooks and crannies being perfect breeding grounds for the PM fungi. What to do? You have several choices: 1. Spray with a fungicide, organic or otherwise. 2. Spray twice a day, AM or PM, with plain water, wetting the foliage down. However this method may promote black spot! 3. Do nothing. It is now very late in the season, roses are slowing down and very soon to be dormant. The PM on the roses will not be a factor in the spring of 2017. Remember, PM is ever present in the environment, waiting for the perfect conditions to infest our roses. Fortunately the specific conditions it needs to be an outbreak are not easily met in our climate, thank God. One last thing, PM in the spring needs to be dealt with by some means. In the fall, the upcomming winter stops it dead in it tracks. Therefore, a hands off, do nothing approach, may be your wisest action at this time.

@gardengal48 (PNW Z8/9) I have made a lot of progress with this hedge, from its prior sorry state. Compost as a mulch made big changes, and then I discovered they were short on water so I added a manual drip line and do a weekly drench outside of the normal irrigation schedule. These changes resulted in fewer leaves showing signs of fungal infection, and dramatically increased growth of new leaves. Everything was going well, and as usual, a new disease pops up to replace the original one. Long-term, I agree the plants need replacing. So we have planted some Pittosporums underneath them, and as those reach maturity and fill in the hedge, we will selectively start removing Pittosporums. So it's a process and the endpoint will hopefully be the removal of the problematic Photinias. As usual, I appreciate your deep experience and opinions on these issues. I have a call into my arborist, and I guess we will use big gun fungicides. Will this be requiring repeated spraying to clear? In the short term, does it make any sense to spray water on the affected leaves?

When powdery mildew is rubbed, much of it rubs off. If it doesn't rub off, it's not powdery mildew. Powdery mildew spreads. Variegated patterns don't. Variegated patterns mostly follow the veins. Much as what I see here. Powdery mildew is in splotches.

I sometimes try to throw the cornmeal under that plant but it all lands in one clump. Organic fertilizer hates being in a clump, so when you drop it down through the middle of the plant there is a Plinko effect which randomizes the droppage through the leaves. Cornmeal is one of the ingredients in many organic fertilizers. When the beneficial microbes in the soil decompose it, in about 3 weeks the end result is plant food. That is why organic fertilizer is said to be slow release. Actually it is slow decomp followed by very fast uptake by the plants.