It's caused by a parasite RPF and is transmitted to the rest of the flock via saliva in the communal drinkers and feeders, so the whole flock has it. Oral canker is not initially fungal, just the skin's reaction the the parasite, so an anti-fungal cream will have no effect. It manifests when there is something else wrong and her immune system is depressed. It starts in the mouth but will then migrate to the crop and the airways. The treatment is Flagyl from the vet. The only DIY treatment that works is probiotics, so you could get Avipro Avian. Probiotics take about two weeks to become effective and she may not have that long unfortunately.