Columnaris, while it may look like a fungus infection, is actually a bacterial-caused illness called Flexibacter. Because it is hard to distinguish, be careful when diagnosing a fish with a fungal infection.
Special things that cause Columaris: along with bad water quality and overfeeding, drastic temperature changes make Columnaris thrive.
Symptoms: White, gray or clear stringy-looking "fungus" hanging off the betta's body or fins (remember, this isn't actually fungus). White or gray patches that look like mold or a slime covering, usually on the body (and most commonly around the dorsal area). White "pimples," generally around the chin and mouth area. White or gray tufts of "fungus" (often looks like cotton candy) anywhere on the body, but especially around the mouth, gills, or edges of scales. Lesions anywhere on the body, generally beginning in the dorsal area. Redness and swelling are not uncommon. In the later stages of infection, the bacteria will begin to eat away at the fish's scales, often leaving behind a red or brown looking bloody area surrounded by fungusy-looking tufts of bacteria. The Betta may also have clear, stringy feces and may be lacking appetite.
Treatment: Columnaris can be difficult to treat. Because high temperatures help bacteria (remember, high temperatures help us treat parasites, not bacteria), keep the temperature around 75 F so that the bacteria don't thrive. Columnaris is a gram-negative rod bacterium, and should be treated with an antibiotic that is effective against gram-negative bacteria or a broad-spectrum antibiotic. Kanacyn (Kanamycin sulfate), Spectrogram (Kanamycin sulfate and Nitrofurazone), Tetracycline, or Furan 2 (Nitrofurazone) work, and you can try medicated foods containing oxytetracycline, but if your fish is having problems with its mouth due to the infection, this may not be an option. Remember to always quarantine sick bettas if they don't already live alone.