My immediate response to the silvery patches on the frit leaves was "thrips". This is just a gut response not backed up by any special knowledge, so don't place too much weight on it.
In your shoes, I'd get out my strongest magnifying glass and spend some time meticulously looking over the plant.
Gladiolus growers are old adversaries of thrips, and control is difficult, but infestations of them seem to be associated with overly dry, warm, still-air conditions.