If I did not live in rust country, I would isolate the plant also . I only sprayed my plants one time this year, and some of them reacted negatively to the soap mixture I used. It caused many of the leaves to turn brown, none of the plants died. I will say that the rust is now only a small percentage of what it was in my garden. Whether that is due to the spray or the weather conditions changing, I don't know for sure. Maybe a combination of the two. I used four oz. to the gal. of a store brand of detergent, I did not bother with the math. But there are 128 oz. to the gal. so that would be just over an ounce of detergent to the gal. But with only one plant to spray that would be way to much to use. 4 quarts to the gal., so a 1/4 of an ounce, 2 pints to the quart so 1/8 of an ounce?
Just mix a gal. maybe and wash dishes with what is left over.
