I can't remember ever having grown garlic or leeks that didn't get a bit of rust but never had it on the onions yet. I can't really see the problem. if its really bad- cut and burn the worst leaves.

I am not sure as I fancy breathing/eating anything with just a number -but then again i am too mean to buy the chemicals anyway.

My guinea pig eats rusty leeks and rusty beet!!!

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Paul