There are multiple reasons leaves turn brown and yellow. Are you sure the issue is powdery mildew ?

I'm pretty sure....I'll send photos in the morning. Lots of problems...mildew, brown, yellow, wilting, thinning out, but blooming.

Yes, pictures might help and so would info on what treatments you have tried as some symptoms are PM related but they could also be weather related. Important steps to take: make sure that you are not doing overhead watering, maintain year around levels of mulch and make sure you increased watering amounts/frequencies for the summer.

PM spores are probably having a good time in many parts where days are warm and nights are cool. Just to confirm, PM covers leaves and branches with a white/gray powderish growth that can make leaves wilt, yellow out, brown out and eventually fall. So PM is usually not a big deal. The PM spores may not be your fault; they could also originate from plant nurseries themselves, i.e., the place where you bought them or the wholesalers.

Plants should be spaced out to help increase air flow/circulation and minimize outbreaks. Again, never water the leaves; water only the soil early in the mornings (6am-ish say). Remove any fallen leaves/stems/dried out blooms to the trash and con$$$ider replacing the mulch if the infestation is great.

If used, fungicides should be applied to the stems and the underside of leaves too. Here is a list of fungicides for severe cases (cleared for the control of Powdery Mildew on hydrangea): azoxystrobin (Heritage; has the smallest application rate and largest repeat interval); fenarimol (Rubigan); fenarimol (Cleary's 3336). You can also use Sunspray Ultrafine Oil. These should be applied at the first sign of PM and again regularly (see the label but around every 7-14 days) while weather conditions are favorable. They perform best if applied before this happens so make a note for future years. Below are some cheaper treatments that others have suggested….

Sometimes, just waiting for the weather to change does the trick. Spores like warm days and cool nights (not sure where you are but, think of the days one expects in the northeastern states in June/July/etc; frequent Fall rains then halt spore development.

You mentioned that treatments were not working but did not say exactly what did not work. Note that treatments help control PM as opposed to eliminate it. Here are some treatment options in case that helps.

Some people use a weekly spray solution consisting of ½ tbsp. baking soda, ½ gallon of water and a couple of drops of dish soap (optional: some people also add a tsp of canola oil). This can be effective but only IF applied early. Other people use 3 tbsp. of grocery store bought apple cider vinegar and 1 gallon of water sprayed weekly. Milk in various
ratios has also been used, from 1 cup of milk to 9 cups of water sprayed weekly and on to higher milk concentrations. I used this once against PM in Crape Myrtles. Spraying weekly solutions of 1 part mouthwash to 3 parts water has also been suggested.

Note that once the problem has started, it is more difficult to get rid of it so do n-o-t expect “quick results”. I would try the non-fungicide solutions before resorting to expensive fungicides. For example, I would only use the fungicides IF the infestation is large (a lot of PM and a lot of PM on many plants) beginning next year; but this year, I would first try the other solutions to keep PM under control (notice I did not say to get rid of it). If the infestation is large then I would start fungicide treatments in 2018 about a month before you noticed it this year. I would also be careful to apply a-l-l treatments to any very small/new leaves.

Of course, realize that we are in the middle of the summer and heat stress and insufficient water can also cause some of the leaf yellowing symptoms. So what I am saying is that some symptoms, while common to PM, may not be PM related. Hard to tell in the summer.

Another example: if you see leaves that turn brown from the edges inwards, the leaves may just need more water. Generally, increase the amount/frequency of waterings this time of the year and water the soil early in the mornings if the soil feels dry when you insert a finger into the soil. Maintain 2-3” of organic mulch up to the drip line.

Since it sounds like these hydrangeas are new, they may pout a lot in their first summers in the garden. Another example: wilting could be due to PM too but is common when the shrub cannot absorb water thru the roots faster than it loses it thru the big leaves. Wilting happens in windy conditions and in almost all summers. If wilted, you just need to check the soil to make sure it is moist and then either water the soil or ignore that wilting episode, unless it is an extreme wilting episode. If it looks waaay wilted compared to other wilting episodes, water it right away (the soil is probably dry). Provided that the soil is moist (you can insert a finger into the soil to check), they should recover from the wilting episode on their own by night time or by the next morning; if they are still wilted in the morning then water the soil. As the shrubs develop a larger root system, they will wilt less in the future but in Year 1, expect a lot of wilting.

Some sun protection in the form of an umbrella or a chair has been used where the plants get afternoon sun. Since all the roots in your plants are still around where the pot walls used to be, water from the center crown outwards.

Reminder: dispose of all plant debris to the trash; and when the plants go dormant, dispose of dried out leaves/blooms to the trash too.

Here are some photos

Since the entire bush is infested with PM should I cut them all back?

what's your zone?

If you sprayed fungicide there is not much more you can do right now other than wait it out.

The good news is fall will be here soon and once the leaves drop carefully remove them and any debris. Be vigilant with spraying next season and perhaps even spray prophylactically starting in spring at regular intervals as provided on the product labeling.

Unfortunately, since you've already sprayed fungicide and it hasn't helped much there is not much more you can do this season to revive them.

I wouldn't cut it back now because with fall arriving the plant will enter dormancy storing energy for the coming winter. That process will take place until the cold of the fall causes the last leaf to fall.

Just do a really good cleanup and get ready for next season.

Thanks gardengal48 and guyground. I will leave them be and keep my fingers crossed that they come back hearty and healthy next season. These bushes are planted under an awning so they get indirect light but never direct light which contributes to much of the mold growth (so the nurseries say). If I run into the same problem next year, I may dig them up and grow them in pots on my deck. They were so beautiful when I planted them.