Dahlia Flower Diseases: Learn About Dahlia Disease Treatment

Dahlia Flower Diseases: Learn About Dahlia Disease Treatment

By: Mary H. Dyer, Credentialed Garden Writer

Dahlias, available in an incredible range of sizes, colors, and forms, grace your garden from midsummer to the first frost in autumn. Dahlias aren’t as difficult to grow as you may think, but proper care may prevent certain dahlia flower diseases. Read on to learn more about a few of the most common diseases in dahlias.

Common Dahlia Diseases

Below you will find the most common diseases in dahlia plants:

  • Powdery mildew – This fungal disease is easy to spot by a mealy, powdery growth that appears on leaves, usually late in the growing season. Although powdery mildew is rarely fatal, it can definitely affect the appearance of the plant.
  • Botrytis blight – A fungal disease commonly known as gray mold, botrytis blight is initially evidenced by brown, water-soaked spots that enlarge and develop a fuzzy, gray or tan mold as the disease progresses. Botrytis blight is often a problem in humid weather conditions.
  • Wilt – Fusarium wilt and verticillium wilt are fungal diseases that cause wilted, yellowing leaves before the plant turns dark brown or black and eventually dies. Verticillium often appears when weather turns warm after a cool period, while fusarium is most severe when the soil is warm. Never plant new dahlias in affected soil.
  • Stem rot – Dahlias planted in poorly drained, soggy soil are highly susceptible to stem rot. This deadly disease causes stems to become mushy and rotted.
  • Viral diseases – Viruses are often transmitted by thrips, which burrow deep into the stems and buds. The diseases display lines, rings, a mottled appearance, and streaks of dark and light green, as well as wilted, stunted foliage. Infected plants are usually discarded, as thrips are notoriously difficult to control. Insecticidal soaps, neem oil, and botanical, pyrethrin-based products may help. If possible, avoid toxic insecticides that kill bees and other beneficial insects.

Dahlia Disease Control

With the exception of viral diseases, which are transmitted by insects, most common dahlia diseases are the result of damp, humid conditions, overwatering or poorly drained soil. The best way to control disease is to ensure soil is well drained and that plants aren’t crowded.

Don’t water dahlia tubers until sprouts appear above the soil. After that time, a couple of deep waterings per week is usually adequate. Water at the base of the plant and avoid wetting the foliage.

As far as dahlia disease treatment goes, some diseases, including powdery mildew and gray mold, can be treated by fungicides applied when the disease is first noticed. Fungicides can also be used as a preventive measure.

Unfortunately, many diseases are fatal and the best recourse is to start fresh with new, disease-resistant tubers.

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Gardenia Diseases & Other Problems

Factsheet | HGIC 2058 | Updated: Mar 10, 2019 | Print | Download (PDF)

With their wonderfully fragrant blossoms and lustrous, dark green leaves, gardenias (Gardenia jasminoides) are popular shrubs with many southern gardeners. Their positive qualities compensate to a large extent for the fact that gardenias are somewhat high-maintenance plants with fairly specific cultural requirements. To learn more about growing healthy gardenias, see HGIC 1065, Gardenia. In addition to problems resulting from improper growing conditions, gardenias are also susceptible to several diseases, insect pests, and other problems. For information on insects and spider mites

Disease and Insect Pests

Roses are susceptible to a number of disease and insect pests. Roses may survive without a basic pest control program, but they may not be very attractive. A pest control program starts with proper site selection, good soil preparation, good drainage, proper spacing, cultivar selection, and plant maintenance. These all foster healthy roses that are better able to withstand the pressure of disease and insects.

When selecting roses, note the resistance to disease of a particular cultivar or named variety within a class. You want to select by cultivar, not by class. All too often, many gardeners assume that shrub roses (the class) are very tolerant or resistant to disease and make their selection based solely on class. In fact, there are a number of cultivars that are very prone to severe disease injury.

Another interesting fact about diseases is that plants can have two types of resistance: phenotypic or genotypic. Phenotypic resistance is when a cultivar is resistant to a disease in one location or part of the country but not in another. Genotypic resistance is due to the presence of genes that are not affected by climate, location, or horticultural practice. That is why, when the term "resistance" is used as a blanket term and assigned to a variety or class, it may or may not apply depending on where you garden.



This fungal disease can cause almost complete defoliatiation of bushes by early fall, resulting in a weakened bush on which cane die-back and cankers become severe. Blackspot is identified as circular black spots that appear on the upper surface of the leaves, starting at the bottom of the plant and moving upward. Infected leaves turn yellow and fall off prematurely. The fringed margin and black color distinguish this leaf spot from others. Infections on canes are identified as reddish-purple spots. Splashing water spreads blackspot. Infection occurs after leaves are wet for several hours, making it more serious during rainy periods. Some roses are less susceptible than others, so cultivar selection is important. The fungus overwinters in fallen leaves and stem cankers. Raking and removing these leaves as well as pruning out affected canes by spring before the buds swell may help provide some control. Avoid wetting the leaves when watering and locate plants where there is good air circulation. Fungicide spray programs need to be started as soon as new leaves appear in the spring.

Powdery Mildew

Powdery mildew is a fungus disease that affects young leaves, causing them to curl and twist and develop a purple coloration. As the disease progresses, leaves become covered with white powdery fuzz. Whereas blackspot is usually most severe on the lower part of the plant, mildew affects the top part of the plant. Mature leaves are less likely to be affected. Mildew is spread by wind and develops rapidly during periods of warm, dry days followed by cool, humid nights. Infections of mildew are actually discouraged by the presence of water on the leaves. However, keeping plants wet all night to avoid mildew provides an environment that allows other diseases to develop. Infection can be reduced through sanitation and fungicide spray programs. Prune out all dead and diseased canes to reduce initial fungus infection. Because new growth is especially susceptible, thorough coverage of new growth with fungicide is important. Plant roses in areas where they receive good air circulation and where the foliage can dry off rapidly in the early morning to prevent many types of diseases.

Stem Cankers

There are several fungi that cause cankers on roses. The different fungi can cause different-looking cankers, but they usually produce brown, oval-shaped, sunken, or shriveled areas anywhere on the cane. The cane dies, and leaves wilt from that point outward. Sometimes small black specks can be seen on the cane surface within the borders of the canker. These are fungal spore-forming structures. Cankers should be pruned out each year. Make the cut well below the affected tissue. Protect the plant from cold or freeze injury by providing adequate cover over the winter. Do not cover roses too early in the fall. When roses are mulched before the soil freezes, moisture can be trapped around the canes and this can increase the damage caused by canker disease. Keep plants vigorous with proper culture and disease control. Canker is a disease of stress. If plants are kept actively growing, they stand a better chance of avoiding cankers. There are no effective chemical controls for canker disease.

Botrytis Blight

Botrytis blight is a fungal disease that generally attacks dying tissue. It is frequently found on older flowers and other plant parts. Under certain conditions it may also attack healthy tissue. Botrytis favors moist, wet conditions, often causing the disease to attack entire flowers and produce a gray fuzzy mold. This disease is often called gray mold. Good garden sanitation and removing spent flowers often result in good control of this disease. When this is insufficient in providing adequate control, a preventative spray program may be necessary.


Rose mosaic is caused by a virus. Bright yellow patterns made up of wavy lines may appear on the leaves of some varieties. Other varieties may show no yellow lines, but may be stunted and weak due to virus infection. Virus-infected plants cannot be cured. Plant virus-resistant roses if possible. Try to control insects, especially aphids, since they help spread the virus. If you are pruning virus-infected plants, don't prune healthy plants unless you first disinfest your pruners. Dipping the blades in a 10 percent solution of chlorine bleach and water for 60 seconds can do this. A 25 percent concentration reduces the time needed to about 10 seconds. All infected plants should be removed and destroyed to reduce the spread of the virus to other plants.

Crown Gall

Crown gall is a bacterial disease that can survive 15-20 years in the soil. It causes irregularly shaped, rough, dark-colored masses (galls) to appear on stems near the soil line. These galls can appear as small swellings or be several inches in diameter. Severely infected plants become stunted and fail to grow properly. There are no effective controls for crown gall. Severely infected plants should be dug up and discarded and roses should not be planted in that area for at least 5 years. Avoid buying plants with suspicious swellings or gall on lower stems or crowns. However, do not confuse crown gall with normal swellings that you see as a result of the budding process. Protect plants from injury on stems during cultivation. Maintain vigor with fertilization and watering. Crown gall is not specific to roses and can affect apples, raspberries, honeysuckle, euonymus, and many vegetables. For this reason, roses should not be planted where plants susceptible to crown gall have been removed because of the disease. Galltrol-A, a non-pathogenic bacteria, has been used to prevent crown gall. It is often used as a dip on cane root roses prior to planting.

Rose Rosette

Rose rosette is becoming more common and can result in significant damage. This pathogen (not yet positively identified) is spread by an eriophid mite. Symptoms include rapid growth of shoots, development of "witches' broom," development of tufts of small, deformed reddish leaves and excessive thorniness. Plants decline over time. Because affected plants can't be cured, it is best to dig out the affected plant and destroy it. Controlling the mite has been labeled as an option but attempts at controlling it have proven inconclusive. It is very difficult to apply sprays in a timely and satisfactory way.

Insect Pests


Aphids are very common pests. Aphids are soft-bodied insects that can be red, green, yellow, or black. They feed on very young succulent shoots, causing distortion. Aphids are often kept in check by natural predators. Alternative control measures include the use of insecticidal soaps, strong streams of water to knock them off the plant, or insecticides.

Japanese Beetles

These hard-shelled, metallic-green, black, and gold insects can cause extensive damage to roses just by their sheer numbers and voracious appetite. They prefer flowers and flower buds but will also attack foliage. Japanese beetles are difficult to control because they are strong fliers and constantly reinvade the area. Home gardeners still find that Sevin provides the best control, but it is only topical. This means that reapplication needs to be done on a regular basis to protect the foliage and flowers. Beware of Japanese beetle traps. Traps are almost too effective and will draw a great number of beetles into an area, making the problem worse. If they are used, they should be placed in areas away from the rose garden. Hand picking is also a suggested control for small numbers of beetles.

Leaf Cutter Bees

It is unusual to see the insects at work, but they make their presence known by the perfectly round holes cut near the edges of the leaves. These leaf pieces are used to make egg partitions inside their burrows. The damage they cause is strictly cosmetic and warrants no control.

Spider Mites

Mites are very tiny relatives of spiders. They can be red, black, or brown in color. Mites pierce the underside of rose leaves and suck sap, causing the leaf to turn gray or bronze. A fine web is a sign of a heavy infestation. Mites reproduce rapidly, resulting in high populations in a short time. Mites flourish in crowded, stagnant gardens. A high-pressure washing with water from a garden hose directed to the underside of the leaves every 2-3 days can manage mites. This will interrupt their life cycle. Miticides such as dicofol help in heavy infestations. Insecticidal soaps are also effective in controlling mites.


Thrips are extremely small, brown insects usually living and feeding inside of the blooms. A deformed flower with flecked or scratched petals is usually a sign of a thrips problem. The rasping mouth parts of thrips causes this injury when they scratch the petal surface to feed. Thrips are especially attracted to yellow or light-colored roses. Some control can be achieved using materials such as orthene, malathion, or insecticidal soap, but even these often give poor results. They tend to be worse during late June, July and August when temperatures are warm.

Rose Midge

The rose midge is a tiny fly that lays eggs in the buds and shoots of roses. The larvae that develop start feeding and causes bent, mishapen or blasted flower buds and withering of the stem tips. Eventually they turn black. Control consists of pruning out buds and applying insecticide if the problem persists. Midge damage usually shows up in July. Because the larvae fall to the soil to pupate, an effective control is to place weed barrier fabric under the plants to catch the larvae and prevent them from entering the soil to pupate.

Sawfly (Rose Slug)

The common rose slug causes skeletonizing or window pane like damage to rose leaves in spring and early summer. The larvae look like caterpillars but are actually more closely related to bees and wasps. Common rose slugs are green with a light tan head and often have may hairlike bristles. Although they look like caterpillars, products with a BT are not effective because they are not larvae of moths or butterflies. Control can include hand picking and the use of horticultural oils or insecticidal soaps.

Fungicide Spray Programs

Fungicides generally recommended for blackspot control include:

  • Daconil 2787 or fungicides containing Daconil
  • Phyton 27
  • Mancozeb
  • Funginex
  • Orthenex.

Frequently used fungicides for control of powdery mildew include:

  • Captan
  • Daconil 2787
  • Phyton 27
  • Funginex
  • Orthenex

Materials used in limiting botrytis are:

  • Captan
  • Daconil 278
  • Mancozeb

As with all spray materials, follow label directions carefully for mixing and applying.

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Bacterial Diseases

Bacterial Wilt (Ralstonia solanacearum)

This important bacterial disease first manifests as blight in the leaves and flower clusters. However, more severe infestations can cause both wilting and root rot.

Bacterial wilt occurs mainly in hot weather and heavy rains. There are no chemical options to control this disease.

Bacterial Leaf Spot (Xanthomonas campestris)

The bacteria that cause this disease can enter the plant through natural openings like stomata or through wounds.

The first symptoms are water-soaked spots. The spots darken and become angular in shape. These spots become larger lesions and can kill the leaves.

If you have a susceptible plant, you can protect it with copper hydroxide (Kocide).

Powdery Mildew

Powdery mildew, as the name suggests, resembles a white, powdery coating on leaf surfaces. The term is used for several closely related fungal species, each of which affects one or more specific plants. So, the powdery mildew on your cucumbers may not be the same species as the one affecting your phlox.

If severe, in addition to leaf surfaces it also might appear on stems and the flowers themselves. Affected leaves eventually turn yellow, then brown. Dead foliage typically falls off the stem, though it will sometimes remain in place.

Although not fatal to plants, powdery mildew makes the foliage unattractive and repeated bouts of the disease will gradually weaken the plant. Annual flowers that are particularly susceptible to powdery mildew include zinnias, snapdragons, and verbena. Perennials that are commonly infected include delphiniums, lungwort, bee balm, and garden phlox.

Preventing Powdery Mildew

Unlike most fungal diseases, powdery mildew doesn't need water to spread it can spread under conditions of high humidity.

  • Keeping plants well-spaced and removing weeds will help ensure good air circulation and reduce the humidity around plants.
  • Avoid overhead watering, which can increase the humidity in the plant canopy. Instead, water gently at the soil level, near the base of the plant. Or, better yet, use drip irrigation.

Controlling Powdery Mildew

  • Choose varieties that have proven to be resistant.
  • If you see just a few affected leaves, pluck them off and dispose of them.
  • Use a horticultural oil, insecticidal soap or another spray. As with all fungus diseases, it is essential to begin application at the early onset of the disease — often late June or early July — and ensure that all susceptible foliage is treated. Repeated applications are usually necessary right through the duration of the growing season. Read and follow the instructions on the label. Options include:
  • Neem oil, derived from the neem tree.
  • Serenade Garden Disease Control is a spray that contains beneficial bacteria.
  • Garden Dust, an insecticide-pesticide that contains copper.
  • Try a home remedy with baking soda: Mix 1 teaspoon per quart of warm water spray on plants every seven to 10 days.
Black spot disease on rose foliage. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org.

Black Spot

Black spot is another common fungal disease. It is a big problem with roses. (See photo at top.) The disease typically begins as black spots on the foliage. These spots are most prevalent on upper leaf surfaces, and may be up to 1/2" across. Leaves eventually begin to yellow around the spots, then become all yellow and fall off. The spots may also appear on rose canes, first being purple and then turning black.

Controlling and Preventing Black Spot

  • Black spot requires at least seven hours of wet conditions for infection, and it is inhibited at temperatures about 85 degrees F. Although you may not be able to turn up the temperature in your garden, you can minimize the disease by keeping foliage dry through proper watering and good air circulation.
  • Plant susceptible flowers, such as roses, in an open and sunny location and avoid watering during cloudy weather.
  • Black spot fungus overwinters in fallen leaves and infected canes. Prune out infections and rake the fallen leaves at the end of the season.
  • Rose varieties vary greatly in their resistance to black spot, so choose resistant ones. If this information is not indicated on plant labels, you can research the variety online or check with experts at your local nursery. Several of the shrub roses show resistance to black spot — and powdery mildew. Look for cultivars from the Meidiland, David Austin and Explorer series. Many of the other shrub roses and old-fashioned roses are resistant as well.
  • Neem oil can help prevent black spot. Spray every 10 to 15 days during the growing season.

Watch the video: How to Grow Dahlias - flea beetles, rooting dahlia cuttings, how I get longer stems, how to prune