Common Tree and Plant Diseases

Your ultimate guide to common tree and plant disease comes to you from The Homeowner’s Complete Tree & Shrub Handbook. In this article, you will find practical methods for identifying, preventing, and treating the following: Canker, Leaf Spot, Mildews, Root Rot, Sooty Mold, and Verticillium Wilt.

Knowing the causes for these various diseases can help you prevent tree and plant disease in the future. After all, an ounce of prevention is worth a pound of cure! Our tree service and tree removal experts in Denver are fully qualified to look for signs of disease and can give you guidance on how to avoid or treat the problem!

See Related: Common Insect Pests


Description: Cankers are long, dark, sometimes sunken lesions with defined edges on limbs and trunks of woody plants. The lesions sometimes may ooze tacky, tawny sap. Canker can destroy leaves, twigs, and branches. Trunk cankers can kill your trees. The rim of bark around the canker may turn inside, giving the lesion a rolled edge.

Cause: Canker-causing fungi and bacteria attack weak trees and shrubs, which may have been harmed by environmental stresses such as sunscald, flooding, or frost damage, or human error such as driving a lawn mower into a tree trunk.

Preventive Measures: Mulch around trees and shrubs to avoid damaging them with landscape equipment.

Treatment: Treat your woodies well. Maintain the health of established trees and shrubs by fertilizing when necessary and watering them during prolonged dry spells. Prune off severely damaged limbs.

Our Arvada tree service experts can help you understand how to properly fertilize your tree. We have a team of experts equipped to give you guidance on proper fertilizing techniques and can help you understand when to water your tree.

Leaf Spot

Description: Foliage of numerous trees and shrubs develops spots that vary in shape, size, and color. Sometimes leaf spots grow bigger until they slow the tree’s or shrub’s development. Other kinds of leaf spots may develop holes in
the middle.

Cause: Different fungi and bacteria cause leaf spots. Some infections occur during rainy springs when spattering water carries bacteria from twigs to shoots.

Preventive Measures: After fall leaf drop, rake up and discard diseased leaves and twigs. Weed under evergreens for good air circulation. Plant disease-resistant cultivars.

Treatment: For bacterial leaf spot, which often starts out pale green and becomes brown with clear borders, spray with Bordeaux mix (copper sulfate) as buds begin
to expand in wet weather. If it’s dry when buds expand, although the plant will look bad, it will not be as weakened by the disease.

Photo by Mahima on


Description: Mildews are parasitic fungal diseases that affect live trees and shrubs. Downy mildews show up mostly on the foliage of plants, including redbud (Cercis canadensis), hackberry (Celtis species), Viburnum species, brambles (Rubus species), currants (Ribes species), and roses (Rosa species). If your woodies have downy mildew, you’ll see on leaf tops some spotty discoloration that eventually turns brown. Grayish white fuzz appears on the underside of the patches. The diseased spots can spread so much that early leaf drop occurs. If you see splotchy, pale gray areas on leaf and stem surfaces, this is most likely a layer of white, powdery spores known as powdery mildew. A few of the many, many woodies affected by powdery mildew our common lilac (Syringa vulgaris), apples and crab apples (Malus species), roses (Rosa species), ornamental cherries and other stone fruit trees (Prunus species), redtip (Photinia species), hydrangea (Hydrangea species), crape myrtle (Lagerstroemia indica), and gardenia (Gardenia jasminoides). Both mildews may warp the buds and developing foliage, though powdery mildew rarely causes permanent harm.

Cause: Downy mildew is a current cool to warm humid weather. Water splashing from dead sick leaves on the ground to the plant’s lower leaves replays the cycle of mildew infection year after year. Powdery mildews come about with warm days and cool nights in dry or humid, but not rainy, weather. Where I live, the warm days, cool nights, and dry conditions of the typical summer frequently lead to powdery mildew on susceptible plants.

Preventative Measures: Pick up infected leaves and dispose of them in the trash. Choose mildew– resistant varieties of susceptible plants when available. Prune trees and shrubs to allow maximum air circulation so foliage can dry out quickly from dew, rain, (downy mildew only; rain actually impedes the spread of powdery mildew), or watering. A sunny breeze site not too closely planted also helps air circulate.

Treatment: Improve air circulation. Remove infected leaves, plant parts, or when necessary, the entire plant. Spray plants with 1 tablespoon of baking soda dissolved in 3 quarts of water for powdery mildew. For downy mildew, spray with fungicide derived from copper.

Phytopthora Root Rot

Description: This soil– born organism causes root rot in many trees and shrubs and eventually kills them. You may notice wilting, yellowing, and the preservation of dried leaves due to the roots’ in ability to take up water. You first notice root rot in summer when plans are more water stressed. Some of the fungal spores stay active even after the plant has died.

Cause: Moist warm soils are necessary for this fungus to thrive and spread from plant the plant. The disease disperses downhill.

Treatment: Fungicide in the affected soil mat keep the disease from spreading to other plants. An increase in the amount of organic matter in the soil my further reduce fungal activity.

Contact one of our Denver tree service professionals for a treatment plan to remove the fungicide!

Sooty Mold

Description: If you see black mold growing on twigs and foliage your plant may be infected with sooty molds or dark fungi. These develop on honey dew (a sugary juice) produced by sapsucking insects such as scales, aphids, whiteflies, and mealybugs.

Cause: A fungus that grows on secretions of sapsucking insects. The presence of sooty molds maybe rising due to global warming and the stress of hotter, drier weather, which can increase the number of certain sapsucking insects.

Preventative Measures: Identify the insect by referring to pictures and descriptions. You can also consult your local Cooperative Extension Service for help identifying and dealing with insect problems.

Treatment: Sooty mold rubs easily of leaves and can rinse off in the rain. By decreasing the number of sapsucking insects on your plant, you can control the spread of sooty mold.

Photo by Johannes Havn on

Verticillium Wilt

Description: A common fungal disease among landscape plants, verticillium wilt clogs the vascular system, depriving the plant of water and nutrients and causing leaves to wilt and branches to die back either one by one or on one side of the plant. Verticillium wilt can kill plants quickly or slowly over many years depending upon how far the disease progresses through the root system. Many wood plants are vulnerable to this disease, including maple (Acer species), weigela, magnolias, viburnums, rhododendrons, and tulip-tree (Liriodendron tulipfera).

Cause: Caused by soil-borne fungi, especially in cool areas of North America, the diseases also spread by contaminated garden tools and by the wind.

Preventative Measures: Plant resistant trees such as Hawthorne (Crataegus species), London planetree (Platanus x acerifolia), holly (Ilex species), katsura (Cercidiphyllum japonicum), oak (Quercus species), and thornless honey locust (Gleditsia triacanthos finermis). You can also grow plants with exposed seed, such as conifers and ginko, which are not prone to the disease.

Treatment: None.

If disease has taken hold of your tree, it’s important to reach out to one of our tree care professionals. We offer free estimates on how to treat your tree or if the tree needs to be removed in Denver.

We also provide tree services not only in Denver, but also the surrounding communities in Arvada, Lakewood, Wheat Ridge, Aurora and other areas!

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