Help: RUST

[Claus Fredemann]

Dear Arisaema Friends,

First of all I would like to introduce myself as a (quite) new AEG member.
I am currently growing about 35 different arisaema species (about 2/3 of
them planted out in my garden). I become infected by Ariseams only in
spring last year. In general, I would say that I am a quite experienced
gardener growing (more or less) successfully almost 1000 different species
with a focus on ferns, hostas and bamboos in my garden.

Unfortunately, I am less successful growing Arisaemas and I am wondering
whether anybody can help me. Two arisaemas (A. tortuosum and A. sazensoo)
are full of rust on both surfaces of the leafs. (both grown in pots). The
rust was already on the leafs before unfolding. They grew only have the
size they had last year. Did any of you ever manage to get rid of rust on
Arisaemas successfully? If yes, how?

Thanks for your advice.

Claus
Frankfurt/Germany (Zone 7)