A whiteish, dusty, flour like coating on the leaves?
Most likely Powdery Mildew. Many have had good results controlling PM with either a mixture of 1 teaspoon Baking Soda in 1 quart water or a 50/50 mixture of fat free milk and water sprayed as needed to control it.

Here is a link that might be useful: About Powdery Mildew

that will not make it disappear though...

you can not...

you can only prevent it ... and to do such.. full fall cleanup .. get rid of the spores... cut plants to the ground.. and remove from property ...

thin plants to increase night air movement.. since that is when mildews thrive.. hot dank damp nights...

and NEVER water after dinner... and let leaves go into darkness wet ... and if you simply like watering in the evening.. get a water wand and only water the soil ...

and the last.. buy resistant varieties ... many of us have gotten rid of plants that are magnets for such ... think of removal as an opportunity for something new ....

its entirely cosmetic... and it will not kill a plant.. so the last alternative.. is to ignore it.. lol ... [though it might make it so ugly.. you want to kill it... lol]

ken

Thanks to both replies.

It's nice that we have plenty of rain this year, but rainfalls always hit later in the evenings this summer here..

Contrary to myths perpetuated by some Powdery Mildew does not form on wet leaves and water is one good way to rinse the spores that cause the disease off plant leaves. This is from the article I linked above, "Wind carries powdery mildew spores to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, water on plant surfaces for extended periods inhibits germination and kills the spores of most powdery mildew fungi. Moderate temperatures of 60ð to 80ðF and shady conditions generally are the most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and leaf temperatures above 95ðF may kill the fungus."

Ken...you keep preaching misinformation about PM. You've been corrected many times and not just by me. I'm thinking that you do no reading of followup posts and zero research on your own.

Here's how wrong your answer is, ken:

PM spores can be rinsed from leaf surfaces with a quick spray from the hose. Rinsing with plain water is always a good option. I have one susceptible crape myrtle that shows signs of this disease early every summer. I keep a watch for it and start syringing the foliage once or twice a day for about a minute and can avoid a serious infection by doing so. I also give my squash plants a rinse which works to stall serious infections.

HUMIDITY is required for PM to thrive, not standing water. Your suggestion of watering the soil raises the humidity. PM thrives when humidity levels are high, days are warm, and nights are cool.

PM can certainly kill plants, ken. Annual plants can be wiped out by an unchecked outbreak. Ignoring it is not an option. Woody plants can recover over time, but even for them, the stress of losing leaves and having to replace them is very challenging and debilitating Once a leaf has been fully infected, it dies.

PM spores, if allowed to remain on the plant without intervention, will germinate and send their haustoria into the cells! That's how parasitic fungi work, Ken! Your suggestion that it is merely a matter of aesthetics is ignorant.

The only useful bit of information you offered, ken, is to select varieties that are described to be PM resistant. Chisue, that takes a tiny bit of Googling before you purchase.

Ken, please try to learn about PM. It's not that complicated. At the very least, try to restrain your impulse to post responses.