Its difficult to get and keep, on plants which have been in a large collection for any length of time especially in a greenhouse in a temperate zone - like England. Inevitably there are days in the winter - sometimes spells weeks long , when the temperature never rises above the stat' settings day or night - and someties falls below as today when a gale blows up the English Channel - 60 mph winds, and the mercury rests on 9 degrees at midday. There are heavy showers - one of hail this morning , so humidity is too high . This encourages black spots to develop on th leaves , if not hairy botyritis.
I think a routine spraying say monthly, with a plant disinfectant, may help. We can't get Physan any more, and Phytan has never been sold here.
I read about Citrox - these disinfectants are really designed and marketed for disinfectang say plant tunnels between crops, rather than the plants theselves, but Citrox at 25ml per 100 litre water ( sounds weak ?) has been suggested for seedlings.
I'm thinking of trying it . Any experience anyone,? Any other suggestions ?