Answer: A couple possibilities come to mind. Petunias are in the same family as tomatoes, and just like tomatoes, are susceptible to Verticillium wilt. Planting the same species year after year encourages these types of problems, so even though you might really like a particular species, it's always a good idea to rotate, just as you would rotate crops in the garden. Or, you could change the soil and clean the planters thoroughly with a 10% bleach solution before replanting petunias each year. Replace with fresh bagged potting soil that is meant for planters; do not use soil from your garden, which may be a reservoir of fungal spores. For replacements, avoid these susceptible species: snapdragons, cosmos, china aster, bachelor's button, dianthus, and stock.
Alternatively, perhaps the plants were overwatered. In general, annuals and some perennials react to high temperatures (90F and above) by slowing down and sometimes even stopping their respiration and transpiration in an effort to conserve moisture. This results in wilting and can eventually cause cell collapse in both stems and leaves. It's not a lack of water that's causing the wilting but the inability of the water around the roots to be transported up through the plant's vascular system because the stomata (natural openings in the leaves) have shut down in a defense against the heat. Usually heat stressed plants resume transpiration and respiration when the temperatures drop below 90F, and by the next morning they're fully recovered. However, we gardeners see the wilt in the afternoon and apply more water to "fix" the problem. If we do this several days in a row or more, the saturated soil causes root rot (roots need oxygen to thrive as well as water) and the plant collapses. At this point, I'd suggest replacing the soil and sterilizing the containers. Good luck!
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