Ian, Rust! It comes back every year and attacks my citrus. How do I control it organically? I'm just now brewing some compost tea to spray on it tomorrow, but again, wanted to ask you about your solution.
Hi Jan. The first part of the answer is a question. Is your rust on the fruits? If yes, then the cause is a tiny insect and one remedy is dormant oil (also known as horticultural oil).
This is an oil that is highly refined so it's just oil. It's also very light in weight and almost clear. You mix a bit of it with water and spray on the leaves to kill the mites. The oil, being oil, spreads over their bodies and suffocates the mites by covering the pores that bring them air.
Another spray for mites is wettable sulfur.
Both dormant oil and wettable sulfur are available at most garden shops.
I hope this helps.
By the way, your local UC extension agent will know a lot more about rust and remedies in your part of California than I do. Have you called the office?