Gardening Help: Common diseases to watch for in your garden

Like people, plants can become sick due to an infectious pathogen. Many of the most popular landscaping plants in the St. Louis area are susceptible to a number of common diseases.

In some cases these diseases can be treated or managed, but in others the best course of action may be to remove the entire plant. A lot of gardeners are looking for a quick fix to disease problems, such as a chemical spray, but such fixes are rarely available. For example, many fungal diseases cannot be effectively treated using chemicals once symptoms appear. Fungicides are best used as preventatives.

Plants can also be more susceptible to disease when they are weakened from one or more environmental stressors such as drought, temperature extremes, nutrient deficiencies, and air pollution among others.

The Garden’s Kemper Center for Home Gardening is here to help by sharing information on a few of the most common diseases gardeners in the St. Louis area might encounter, providing a description of symptoms, control measures, and what plants are affected.

Leaf Spots

Unidentified fungal leaf spot on a maple leaf. Photo courtesy of the Center for Home Gardening.

Symptoms: Brown, purple, or black spots that may enlarge to form larger areas of necrosis. A lighter yellow ring or “halo” may surround the spots.
Cause: Fungi and bacteria. The most common genera of bacteria that cause leaf spots are Pseudomonas and Xanthomonas. These bacteria can also cause blights, wilts, and cankers. Pathogenic fungi can also cause leaf spots, including species in the genera AlternariaCercospora, and Septoria.
Control: It can be difficult or even impossible to identify the exact pathogen that is causing a leaf spot disease without a diagnostic lab. Luckily, leaf spots rarely kill plants and can be easily controlled by removing infected leaves at the end of the growing season, avoiding overhead watering or watering in the morning, and improving airflow.
Plants affected: Many trees, shrubs, herbaceous perennials, and annuals.

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Rose rosette disease (RRD)

The left side of this rose plant is affected by rose rosette, while the right side remains healthy. Photo courtesy of the Center for Home Gardening.

Symptoms: Deformed stems, leaves and flowers. The stems are usually thickened, with a red coloration and excessive thorniness. The leaves and flowers may be stunted.
Cause: The virus that causes the disease is transferred from plant to plant by a microscopic mite.
Control: Unfortunately, once symptoms appear, there are no viable treatment options and the whole plant (roots and all) must be removed. To prevent the disease from infecting healthy roses, cut back plants by 2/3rds in winter to remove as many overwintering mites as possible. Plant species roses which have shown resistance to RRD including two Missouri natives: Rosa carolina (Carolina rose) and Rosa arkansana (prairie rose).
Plants affected: Roses, particularly hybrid cultivars.

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Cedar Apple Rust

This cedar-apple rust gall erupts into gelanized telia after wet weather in the spring of the second or third spring after infection of leaves on eastern red cedar.

Symptoms: This fungus requires two, unrelated host plants to complete its life cycle: a juniper (usually Juniperus virginiana or eastern red cedar) and an apple or crabapple. On the juniper, the fungus forms a brown gall up to 2″ wide on twigs. In spring during rainy weather, the galls swell and produce bright orange, gelatinous structures called telial horns which release spores. The spores then infect a susceptible apple or crabapple, causing circular, yellow lesions to appear on the upper surface of the leaves. In late summer, small, raised bumps will appear on undersides of the leaves below the yellow spots and release spores that infect junipers, completing the life cycle.Cause: Fungus.
Control: No control is necessary on junipers, although the galls can be pruned off before they form their telial horns to prevent spore formation. Fungicides are generally not recommended to control cedar apple rust on apples or crabapples, and the leaf spots do not significantly impact the health of the trees. Plant resistant apple varieties such as ‘Freedom’, ‘Liberty’, and ‘Redfree’. Resistant crabapple varieties include ‘Purple Prince’, ‘Red Jewel’, ‘Royalty’ and ‘Snowdrift’.
Plants affected: Junipers and apples or crabapples.

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The shepherd’s crook associated with fireblight on crabapple.

Symptoms: Tender, new growth including leaves and twigs take on a shriveled, black or brown appearance and usually form a characteristic “shepherd’s crook” shape.Cause: Bacterium.
Control: The bacterium overwinters in cankers, diseased twigs and fruit. In late summer or winter, prune out all diseased wood including at least 10″ below the infection to make sure all cankers are removed. Disinfect pruners between cuts. Do not over-fertilize or excessively prune trees as this promotes succulent growth that is very susceptible to infection. Young, highly susceptible trees may succumb to the disease in a single year, but it is rarely fatal for otherwise healthy, mature trees if proper pruning and control measures are implemented.
Plants affected: Trees and shrubs in the rose family including apples, crabapples, hawthorns, pears, flowering quince, cotoneaster, and spirea.

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Powdery Mildew

Detail of powdery mildew, plant disease. Photo courtesy of iStock.

Symptoms: White or grey powdery patches appear on leaves, flowers and stems. Leaves and flowers may be stunted, deformed, or discolored.Cause: Fungus. Most species only infect a particular plant, but there are thousands of species.
Control: Cultural control methods are usually sufficient to keep powdery mildew at bay, although some plants are very susceptible and the weather can play a large role in how bad infections are from year to year. Thinning plantings to increase airflow and avoiding overhead watering can help. Remove infected plant debris from the garden in the fall. Look for cultivars that are listed as being resistant to powdery mildew.
Plants affected: Many trees, shrubs, herbaceous perennials, and annuals.

Need more home gardening help?

Check out the Garden’s resources available at including commonly asked questions, monthly tips, and care sheets for specific plants.

Justine Kandra

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