Lowe's here has been seen to display mislabeled trees in the past. You have the wrong cultivar or its resistance was overcome under your conditions. "Resistant" means it puts up a fight at first, not that no infestation is ever seen.

THere are several factors that contribute to powdery mildew.
Powedery mildew likes DRY leaves and high humidity.
Dry leaves on crape myrtle, considering you bought them from Lowes, probably means they weren't watered the way they should have been.
Lowes doesn't really care that much about regular watering of their trees, you have to take that into consideration.
Just make sure that since you just planted it this year, it will get regular waterings.
You have to treat a newly planted tree a little differently than an established tree the first couple of years.
Also, this winter, look at the trees framework carefully.
Pick the number of main branches you want the tree to have, either 3 or mabey 5?
Use a pruner and cut the tree framework to look like that.
CHeck for branches that are growing into one another.
They will mature next year and rub, which is what you don't want.
Cut the branches that you see growing into other branches off at the branch, or stem, or trunk, leaving about 1/2 inch to an inch before the trunk.
Do this every fall when all the leaves fall off, you can see the framework easier without the leaves.
It also helps circulate more air through the tree, which will really help with the powedery mildew.
Regular watering, proper pruning for at least the first 3 to 4 years, (your shaping the tree too), and you should notice a big difference in the tree.
When your tree is established, you will not have to water it unless it is really hot and you havent had any considerable rain in the past couple of weeks.
But it will take a couple years to become established.
Good luck with it.
My crape myrtle from Lowes that I bought only 6 years ago is 30 feet tall, and I still check it every fall for branches that are growing into other branches, for the health of the tree.

We've found resistance often improves as they get established. None of the 8 mature CMs on our current lot have suffered from (or been treated for) powdery mildew in years, but the two or three new shoots the large ones put up each summer still frequently do get PM, so is still active in the immediate area. Not a problem since the shoots are doomed to be cut off anyway, only downside is they go in the trash rather than the compost pile.

No idea if age, growth rate, proximity of foliage to ground, or other factors account for the change, but would be interested to know.

'resistant'... does not mean its impervious to such ...

its simply.. more resistant than other cultivars ... for whatever that is worth ... on some level.. you should be leery of marketing claims .....

there are good years and bad years.... perhaps this was a bad year ... and the fact that the plant was not fully established .... might mean it had some predisposition to such.. meaning.. a stressed plant can attract a multitude of problems.. it may not.. later in life.. as others noted ....

ken

As mentioned new plants being infested more than established specimens of same kind in vicinity would be due to drying out at the retail yard or on the final planting site before establishment.