Unfortunately this fungal disease is rather common on Fever Trees. The fungus has been identified as a rust, similar to Uromyces. The suggestion is to totally spray the trunks with triazole type fungicides. (Defender or Bumper 30 ml per 100 L water) Probably the most popular treatment is a total drench with Trichotec --- Trichoderma spp. A living fungi that is antagonistic to many other pathogenic fungi. After the drench it is essential to cover as much of the treated trunk with newspaper to shield the Trichoderma from ultra violet light for a few weeks. Preventatively - be careful as weed-eater damage can initiate an infection. If this is not successful, call an arborist for their opinion and treatment.