If you’re lucky, your only experience with rust in the garden is on an old shovel. But rust is also the name of a fungal parasite. There are many different kinds of rust, and they are highly specialized, meaning they affect only certain types of plants. One of the most common rust diseases is cedar apple rust, which alternates between two hosts: the Eastern red cedar (Juniperus virginiana) and apple or crabapple trees. (Related rusts include cedar hawthorn rust and cedar quince rust.)
Cedar apple rust typically causes minor, but very visible, damage. Cedar apple rust galls on junipers start out tan or brown, but in the spring develop masses of bright orange spores several inches across. Likewise, when the disease spreads to an apple or crabapple tree, it shows up first as orange-yellow leaf spots; later, leaves may drop. Fruit can be infected, too. Other kinds of rust can cause more serious damage to fruit, leaves, and twigs.
If you are planting apples or crabapples, look for cultivars that are resistant to cedar apple rust. If you’ve had trouble with rust on your geraniums, try resistant types, including scented, regal, and ivy geraniums. If you have problems with any kind of rust, remove and destroy infected plant material as soon as you see it. Don’t leave any diseased stalks or leaves in the garden through winter. Clean tools with a solution of one part bleach to nine parts water. If the problem persists, use a fungicide formulated specifically for rust diseases.
—photo courtesy of the Kemper Center for Home Gardening at the Missouri Botanical Garden