"Could it be that the weed killer remained in the soil, or do you think it has dissipated by now?"

Probably not although there are some unusual ones that do. He should know if he used one of them like that. They should basically never be used on residential lots except for extreme applications like putting down pavement, so I doubt it.

"He has since then re-planted the bushes (last fall) with the Encore variety."

Is there only one landscaper in your town? I would have requested a settlement value of your previous plants and moved on to another company.

"However, this spring all of the bushes look dismal - some have blooms but the leaves look brownish and yellow with spots."

Hard to tell w/o pictures but some of the "Encores" were not nearly as winter hardy as initially promoted. This should generally be known now by professionals. If you are in the "upper south" or anywhere north of that, they might simply have winter damage. If that is the case it's another reason to ditch your current plant care provider.

Hi David - thanks for your reply and for letting me know about the Encores! I am in zone 7, VA and PA border, so winter damage is a possibility. Do they recover? All my neighbors' azaleas are thriving but they do not have Encores. I am leaning towards replacing the azaleas myself with a hardier variety. Any recommendations? Thanks again!

Sorry I'm not especially knowledgeable about individual Azalea _cultivars_, maybe someone else will answer. You could go here for advice: http://azaleas.org/