Flushing didn't work because that's not burn, burn is when the tips of the leaves are effected not the entire leaf as a whole. This looks like calcium deficiency, what's your soil mix made up of?
p.s. flushing in soil is illogical - one of the biggest misconceptions of the canna community due to the use of petrochemical feeding regimens taking over the industry
p.s. flushing in soil is illogical - one of the biggest misconceptions of the canna community due to the use of petrochemical feeding regimens taking over the industry