White spots on rocket leaves

I don't know what these are but I have been getting them on my rocket in Sydney for the past 6 months. Other friends have reported the same. If you plant new rocket in the same place it comes back, but if you can find somewhere else in a different part of the garden it seems to be OK. The spots look fungal if you scrape a bit off and examine with a microscope; they are definitely not a bacteria.

I keep growing the rocket, but remove affected leaves and destroy (not in the compost!). I haven't been game to eat it (who knows what these plant pathogens may do to us?) but they seem to be able to produce plenty more good leaves and it doesn't seem to shorten the life of the plants.

I would love to find out more about the ID of this bug and will try a plant pathologist. I will keep you posted if I find anything.