Viburnum is a genus of approximately 150 species of deciduous and evergreen perennials with colourful blooms and lovely leaves, many of which are endemic to North America.
The majority of species are woody shrubs, though some have a spreading growth habit and others can be pruned into tree-like shapes.
These adaptable, low-maintenance plants thrive in shady or full-sun locations, depending on the species and growing zone.
Fortunately, most diseases do not affect the viburnum. Infected plants have typically been weakened by improper care or are otherwise in poor health.
As a result, you should be able to prevent disease by meeting their needs. In this article, we are going to discuss how we can identify some common viburnum Diseases and how to treat them.
Viburnum Diseases That Are Common
Viburnum shrubs are extremely adaptable. That means they are rarely afflicted with the disease. Fungus-caused viburnum bush diseases are the most common, while other disease issues are uncommon.
Most of the time, proper plant placement, adequate air circulation, and good watering practices can prevent these soil or airborne problems.
Plants that are stressed are more vulnerable to long-term damage from these diseases. Some common viburnum Diseases include:
1. Mildew (Downy Mildew)
Downy mildew on viburnums is caused by the fungus Plasmopara viburni. This foliar disease appears and spreads rapidly during periods of cool to warm weather combined with leaf wetness.
This disease first appears as light green spots on the upper leaf surfaces. The dots grow to create angular patches between the leaf veins. Downy greyish-white fungal growth forms on the undersides of the leaves.
Downy mildew illness differs from powdery mildew in that the fungal growth is seen on the bottom rather than the upper leaf surface.
As the leaf tissue dies, the infected areas redden and turn brown. Infections of the foliage in the spring can be caused by splashing spores released by diseased foliage that has fallen to the ground.
Treatment And Prevention
Downy mildew prevalence and severity can be decreased, like with other foliar diseases, by keeping the leaves as dry as possible.
Use of overhead irrigation is not recommended. Allow for adequate plant spacing when planting viburnum or other nearby plants. Pruning back neighbouring shrubs or overhanging tree limbs is a good idea.
These measures will promote air circulation around the plants and aid in the drying of the foliage. Rake up sick falling leaves and burn or dispose of them, then add another layer of mulch beneath the bushes.
If fungicides are required, sprays should be applied to both the upper and lower leaf surfaces. Mancozeb and chlorothalonil are two fungicides approved for the control of downy mildew.
2. Powdery mildew
Powdery mildew is caused by the fungus Erysiphe sparsa, which appears as white, powdery growth on the tops of young plant leaves and, in some cases, the undersides as well.
It usually does not cause much harm. Severe infections, on the other hand, can cause newly developing leaves to become deformed.
Humidity promotes the spread of this disease, and it is most prevalent when the days are warm and the nights are cool. Rain prevents it from growing.
Treatment And Prevention
Powdery mildew can be avoided by pruning the branches in the plant’s interior to improve airflow. Watering should also be done with caution, and only the ground should be watered, not the foliage.
If you notice this infection, remove the affected leaves as well as any debris beneath the leaves as soon as possible.
Fungicides containing myclobutanil, propiconazole, thiophanate-methyl, or horticultural oil are effective against powdery mildew (see Table 1 for specific products).
To avoid foliar injury, apply 2 percent horticultural oils for powdery mildew treatment only when temperatures are below 85 °F and no rain is expected in the following 24 hours.
Although sulfur is occasionally used to prevent powdery mildew, it is not suggested for sulfur-sensitive plants such as viburnums.
3. Leaf Spots Caused By Fungi
Cercospora species, Phoma species, and Phyllosticta species cause a wide range of leaf spots.
The leaf tissue in these fungal leaf spots on the viburnum is sunken and dry. Spots may start small but enlarge or merge over time, and they can range in colour from reddish to greyish brown.
Fungal leaf spots are most common during the hot, humid summer months and appear first on older foliage.
Anthracnose is a disease caused by the fungus Colletotrichum species that manifests as black, sunken sores. Leaf spots and anthracnose are fairly prevalent on viburnum leaf, but these foliar diseases are rarely fatal.
Treatment And Prevention
Many of these infections can be avoided by keeping the leaves dry, avoiding overhead irrigation, and improving air circulation.
Pruning any overhanging trees surrounding the damaged plants will assist to reduce humidity and allowing the leaves to dry.
Remove the spotted leaves if the infection is minor. Remove and destroy any infected leaves. These steps will help to reduce the likelihood of your shrub becoming infected again the following season.
In the case of severe infections, apply a copper fungicide to the plants as soon as you notice symptoms and repeat at 10- to 14-day intervals as needed.
4. Algae Leaf Spot
As its name indicates, this disease causes an infection that shows symptoms on the leaves. The spots start small and pale green before becoming reddish or light brown.
They often have feathered edges and look like they are slightly raised.
Cool and moist conditions favour the development of algal leaf spots. Wind and splashing rain will spread this type of algae to nearby foliage.
The organism that causes it, a type of algae known as Cephaleuros virescens, overwinters in the leaf spots, ready to strike again in the springtime.
Treatment And Prevention
Camellias, magnolias, azaleas, aucubas, gardenias, and rhododendrons are among the ornamental shrubs and trees that have algal leaf spots.
Monitor the plants for disease problems, practice good sanitation as described in the fungal leaf spot control section, and treat any other landscape plants in the area with algal leaf spots.
Copper fungicide sprays can be used to control algal leaf spots. Apply as soon as symptoms appear and repeat every 10 to 14 days as needed.
5. Armillaria root rot
Shoestring root rot, mushroom root rot, and oak root rot are all names for Armillaria root rot. It is caused by the fungus Armillaria mellea, which is widespread in landscape and garden settings.
This fungus can damage the roots of a wide variety of plants. This disease is most commonly found on trees and shrubs such as oak, pine, rhododendron, and dogwood, although hundreds of plant species, including viburnums, are susceptible.
Typically, the symptoms of this root rot spread throughout the plant. Above-ground parts of the shrub appear stunted and yellowed, and leaves may fall.
Over several years, the unhealthy foliage may become more sparse. However, there may be no signs of a problem and the shrub will die suddenly.
Treatment and Prevention
There is no cure for this disease, so if you find a severely infected viburnum, dig it up and dispose of it in the garbage, not in your compost pile.
Remove as many roots as you can and destroy them as well. This will help to keep the fungus from spreading and infecting other plants.
Furthermore, the soil around the root zone should be removed and disposed of.
Once the soil has become infected, you will be unable to grow viburnum in the same location.
How Do You Treat Viburnum Rust?
Many different types of viburnum rust fungus disease affect specific annual and perennial flowering plants, but gardeners can identify this problem by observing the speckled masses that form on the surfaces of leaves.
Viburnum spots are typically small and dispersed across plant foliage like freckles. Affected plants can have dozens of rust spots on each leaf, and a single leaf can have over a hundred rust spots.
Spray the entire plant with neem oil to prevent rust from attacking the viburnum. Rake up and dispose of any plant debris that has fallen to the ground around the tree.
Rust can overwinter in the perfect hiding place provided by fallen plant matter.
In a quart of lukewarm water, dissolve two 325-milligram uncoated aspirin tablets. As an organic rust treatment, pour the solution into a spray bottle and thoroughly coat both sides of the viburnum plant’s leaves and stems.
7 days later, treat the viburnum again. If the aspirin treatment does not cure the rust, use a chemical fungicide, such as neem oil.
How Often Should You Water Your Viburnum?
Viburnum Watering Instructions. When you first plant viburnums, water once or twice a day, or whenever the top inch of soil is dry.
At each watering, keep the hose close to the root ball and make sure the root zone is thoroughly soaked.
As the plants establish roots, you’ll be able to gradually increase the number of days between waterings.
When Is The Best Time To Fertilize My Viburnum?
Viburnums are not heavy feeders, but they can benefit from spring fertilization once a year. When it is well established, it can survive without being fed.
If you want to increase blooming and foliage colour, feed the viburnum a slow-release tree and shrub formula shortly after it blooms.
In conclusion, Viburnums are generally disease-free and healthy plants. However, when the shrubs are subjected to adverse growing conditions, such as drought stress or excessive moisture, fungal pathogens have an opportunity to establish themselves.
Have your viburnums contracted a disease? If so, please let us know how it went in the comments section below.