Reporting
There could be several
There could be several reasons. If the pot is well-drained, is the dahlia is getting plenty of water, but not too much to cause rot? Do you think that the store where you purchased it gave it reliable care, not overwatering it to start root rot (that is just now showing up in symptoms)? Have you fertilized the plant recently? Dahlias are big feeders (but use a low-nitrogen fertilizer; too much nitrogen will decrease flowering). Was there a hot spell recently? Depending on where you live, where the pot is (on hot pavement or cool grass), and what the weather has been like, the plant may be getting too much heat; if you think this is a possibility, try placing the pot in part shade for a while. There are several wilt diseases (fungal, viral, or bacterial) that appear first as wilting of lower leaves. Do the stems have any black or mushy areas? Are there any yellow streaks/veins on leaves or stems? If this is fusarium or verticillium wilt, or a virus, you should destroy the plant, and get rid of the soil (the pathogens live in the soil). Botrytis blight can cause browning of flowers. Check for insects; entrance holes at the base of the stem; or tiny spots along the petals, base of the flower, or along the leaves. Sometimes thrips, aphids, or other sap-sucking insects can cause damage, such as ragged or brown areas on the flower, or wilting leaves. Borers can tunnel into the stems.