Hello, anita55. This could be powdery mildew or some other type of fungal disease. The plant seems fine but surely looks like the nursery was watering the leaves. They should be watering the soil in the pot instead. You should do the same; water EARLY IN THE MORNING ONLY and water the soil so you do not let more fungal spores develop on the leaves. Do not overwater in order to dry out the supply of moisture which helped the fungal spores develop.

Because the infestation is severe, I would apply some fungicides although you are going to loose the leaves soon when the plant goes dormant. Normally by Fall, some leaves can suffer from PM but are not worth the expense of buying and messing with fungicides.

Fungicides whose active ingredient is either azoxystrobin (Heritage), chlorothalonil (Daconil Ultrex), fenarimol (Rubigan), parafinnic oil (Sun-Spray Ultrafine Oil), potassium bicarbonate or thiophanate-methyl (Cleary's 3336) can be used to control PM.

Because all those products have variants, verify that the product label says that it can be used against powdery mildew. You will have to re-apply multiple times: every so many days (7 to 14+, depends on the product). See the fungicide label for appropriate rates and application intervals. To avoid fungicide resistance, I suggest you apply two types with different active ingredients; apply the first one twice; then apply the second one twice (repeat if necessary). Rotating products in this manner also helps avoid insecticide resistance by garden pests.

If you are into organic solutions, consider baking soda (1 tbsp of baking soda + 1/2 tsp of liquid soap + 1 gallon of water) or cornmeal (2 pounds per 100 square feet; work it into the soil and water well). You can also prepare cornmeal overnight: add 1 cup of cornmeal to 1 gallon of water, let it sit overnight, strain and spray on the plants.

Mulch with 3-4" of any type of cheapo acidic mulch. This mulch will serve to collect all the spores that fall off into the ground from the leaves. It will protect the plant through winter as well. However, when spring arrives and the plant leafs out, I would replace the mulch with new mulch (throw away the old mulch; should be full of spores).

Remove and destroy next. When the shrub goes dormant, you will have lots of dry leaves (no blooms from what I can see in the picture). Dispose of all leaves (and any blooms) in the trash instead of the compost bin; you do not want to spread the spores elsewhere either as some may survive in the compost bin.

Do not fertilize the plant. One application (1/2 or 1 cup) of manure or cottonseed meal in June should normally suffice (start in 2009) but consider that (1) the potting soil in your plant already contains fertilizer pellets, (2) we are in September and you should stop fertilizing by August-September to make sure your plant goes dormant at the proper time and besides (3) powdery mildew likes to develop on tender new leaves so do not allow it to develop in any new growth.

Separation between plants is important as this allows good air flow. Good air flow keeps moisture down and prevent the fungal spores from developing. From your picture, this looks fine now but your hydrangea could get large and in a few years, it may be much closer to your other plants (spirea?). Keep that in mind.

Do not worry if you keep the plant. PM is not going to kill this plant and the shrub will recover but you will have some hassles for the first few years. The problem will return again after your rainy seasons until the number of spores has gone down enough to no longer be an issue. Other plants nearby could catch PM so be aware of that too.

Does this help you?
Luis

PS - For a brand new plant from a landscaper, I would have declined that specimen and told the landscaper to go shop at a better nursery. Do not pay regular top dollar for a plant that is not in top shape; see if they can give you back some of what you paid.