rusty leaves on Mandevilla vine
Rust diseases have fairly complicated life cycles. Most require an alternate host (such as cedar-apple rust or quince-hawthorn rust). A few can repeat generations on the same plant, such as hollyhock rust or geranium rust. Because of the alternation of hosts, timing of the disease is often periodic rather than constant.
Some fungicides control the rust fungi, but they must be applied before the rustiness develops.
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