I try not to, but sometimes a 'real' chemical is the answer. I've tried baking soda solution, and Neem but neither really has helped with the rust on the plumerias, cannas or my fig tree.
Last year I had the start of a bad bout of black plague on some of my orchids after TS Debbie rained on us for 3 days. Much surgery, re-potting and use of cinnamon (on the cuts and in the medium) later and I had most of it under control, but a little came back. Another orchid nut advised me not to fool around, get some good fungicide. She grows organic veggies for her catering business, so another anti-chemical person, but for this she says it is warranted. Her exact words were 'break out the big guns'.
Anyway, I have a small bottle of Physan 20. This year after all the rain the black spots began to appear again, and I went straight to using the Physan. It is diluted at 1tsp per gallon of water. It's widely used for sanitizing greenhouses and also as an algaecide. Once I got finished spraying the orchids, I had some left, and went out with it to spray the plumeria, cannas and the fig tree. (no figs present) I also sprayed the screen of the pool cage and the pavers around the orchids. It cleaned up the algae growth nicely.
So far, a miracle, I have seen no rust. The black plague has disappeared, too. So now I am asking myself if it is worse for the environment to use gallons of stuff that works marginally if at all, or to use 1tsp. of fungicide that does the trick.
