Brown spots that become especially noticeable in midsummer on leaves and stems of fruiting and ornamental plants may be caused by certain kinds of plant pathogenic fungi known as rusts. Apple trees, asters, daylilies, hawthorns, hollyhocks, pear trees (fruiting and ornamental), ornamental grasses, veronica, and hundreds of other plants are susceptible to rust fungi. The severity of infection and damage is dependent on species and individual plants, as are decisions about management.
Rusts are named for their rust-colored spores. On asters, daylilies, hollyhocks and other plants, spots may appear on the leaf surface with bright orange clusters of rust spores on the undersides of the leaves. In dry weather, spots may look brown on both sides with orange re-appearing after rain or heavy dew. On apple and pear trees, the undersides of the spots may appear orange or even look like they have tiny tentacles coming out of the spots. These are structures produced by the fungus that are unique to certain rust species.
Observation of a rust disease in the garden may be cause for concern or may simply be an aesthetic issue. Gardeners should understand that an immediate cure is impossible and should closely evaluate whether management is warranted based on the plant(s) affected and severity of infection.
For severely infected plants or those whose long-term health seems to be taking a toll, consider removal and replacement with a resistant species or variety. For example, asters that are severely affected by rust year after year lose vigor. Consider replacing rust-susceptible aster varieties with resistant varieties — or plant something else entirely. When replacing plants, research varieties, look for information on the label, and avoid purchasing plants that are already exhibiting signs of infection.
Overhead watering can spread rust spores and create an optimum environment for the disease. Use drip irrigation or take care to avoid wetting leaf surfaces if watering manually.
Learn more about the rust disease specific to affected plants. Some rusts need two different plant species to complete their life cycle. One of the most familiar rust diseases is cedar-apple rust, which needs both cedar (juniper) and apple to survive. The fruiting structures produced on cedars (orange balls that appear in spring and turn into hard brown structures in summer) produce spores that can only affect apple trees and vice versa. In this case, removing nearby cedar trees may eliminate the problem for apple trees. This approach only really works on properties where the gardener can control plants in proximity, however.
Confirm the disease with an expert if necessary. Daylily growers might be especially interested in professional confirmation as daylily rust looks very similar to another fungal leaf spot but can be detrimental to certain species of daylily. Rust that appears on barberry could also be of concern as barberry is an alternate host to a few species of rusts that have an alternate host of wheat. (Current ornamental varieties of barberry are rust-resistant.)
Good sanitation practices can reduce the amount of rust for future infects. For plants like daylilies, clean up and destroy infected foliage as soon as it appears. Under apple, pear, and hawthorn trees, rake up infected leaves that drop in the fall and destroy them.
In crop production, rotation of crops and use of resistant varieties are important to break the cycle of rusts.
Some gardeners might be interested in the use of fungicides for control of rusts. As in any fungal infection, fungicides cannot take away the infect that has already occurred. They work preventatively to prevent fungi from infecting plant tissue. To be effective, determine the specific rust and apply fungicides shortly prior to the expected infection period, which is often on an annual basis.
Gardeners who choose to use fungicides should also be aware that rust fungi (and other fungi) can develop resistance when the same products or similar products are used repeatedly. If spraying is necessary to protect valuable crops, rotate between different classes of fungicides and consider the long-term costs of repeated spraying.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.