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There could be severalThere could be several reasons why a potted dahlia might wilt.   * Does the pot have good drainage? You don't want any water to sit in the pot at the base, which will encourage 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)?   * 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 have gotten too much heat.   * 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. Stem rot causes leaves to yellow and wilt, roots to rot. To avoid disease, disinfect your tools with a dilute solution of bleach before you use them on each plant. Rotate where you plant dahlias each year (because some diseases overwinter in the soil). Provide good air circulation; allow soil to dry a bit before watering. If your soil is heavy, lighten it by adding some peat moss or similar soil amendment. Check tubers before winter storage for any signs of disease or insect problems; cut out any affected parts.   * 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.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?