Rust is a fungal disease that attacks the leaves and turns them rusty brown in late summer. Mildew turns the leaves grayish-white. You can help prevent both rust and mildew by spacing plants to allow good air circulation after they are full grown. This might make them look a little sparse when first planted, but they quickly fill-in.
Rust: It usually looks like leaf spotting that's followed by small masses of rust-colored powder on the leaves. Infected leaves die by the end of the season.
Fungal diseases: To avoid fungal diseases, prune plants to encourage good air flow, water at the base of the plant to keep the foliage dry, and mulch the soil around the plants. Dispose of leaves by throwing them into the garbage, do not compose the leaves because the disease will winter over in the compose pile.
If your other plants are not diseased and you continue to have problems you can either throw the diseased plant away or spray it with a fungicide.
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