Garden Pests and Diseases: What could be wrong with my Oxalis articulata ?, 1 by bonitin

bonitin wrote:
On the underside of its leaves it shows these rusty, powdery spots, on the upperside the leaves are yellowing.
Is it a kind of fungus and what could have caused it ? The Oxalis is planted in a spot where it has been growing happily for several years.
There was a very long, more than a month's dry and hot periode in April, then followed by a long very chilly, whimsical, wet, dull weather with little sun.
I would be grateful for a diagnosis and eventual cure (non-chemical though)