Five Garden Plant Diseases: What they look like and what they do to your plants
Of the many types of garden plant diseases, Surrounds garden care specialist Margaret Miley says these five are the ones she sees most frequently in our Maryland and Northern Virginia gardens:
- Sooty Mold
- Powdery Mildew
- Needle Cast
Truth is, if you’ve chosen a company to perform scheduled garden care and landscape maintenance programs, you aren’t likely to see these diseases take hold of your plants or trees. But, a basic understanding of what these diseases look like and which plants they attack may perhaps enable you to alert your neighbor to an issue some day. Also worth noting, if your neighbor’s garden has issues, it usually doesn’t take long for them to migrate over to your yard.
1 Sooty Mold
Sooty Mold collects on camellias and other leafy plants. Unlike certain pest insects, it doesn’t feed directly on the plant. Sooty mold is a fungus that feeds on the leavings of sucking insects that eat the plant. For example, scale bugs and aphids excrete sugars called honeydew. The sooty mold spores grow on those excretions. If allowed to propagate freely, a black coating of mold can potentially interfere with the plants ability to breathe and photosynthesize sunlight.
2 Powdery Mildew
Powdery mildew is a fungus that thoroughly enjoys our humid Maryland and Virginia climate—particularly during a dry spell accompanied by moderate (below 90-degrees) temperatures and high relative humidity. That’s because powdery mildew requires a dry leaf surface to form.
There are multiple types of powdery mildew and each one favors a particular species of plant. So, if your lilacs are affected, that mildew will stay put and not spread to your peonies or other species. The fungus won’t kill a plant unless you allow it to progress for several years without treating it. In that case, it would gradually interfere with growth, distort plant shape and cause leaves to drop prematurely. Typical causes of sooty mold are:
- Weather. A moderate summer that combines low precipitation and high humidity
- Crowding. Too many plants too close together—or overgrown plants will trap moisture and impede air circulation.
Anthracnose is a fungus that attacks any tree, but causes serious problems for our native (Cornus Florida) dogwoods. It interferes with the vascular system of the tree. If left untreated for several years, entire branches will start to die off. Bark will peal back. There will be black spots on leaves and reddish colored blotches on fall foliage. And the tree will begin to drop its leaves early.
Canker is caused by fungus or bacterial intrusions. This usually presents as oozing lesions on the trunk of the tree or shrub. This is usually followed by die back of branches. The fungus or bacteria get into the tree when there is a cut or split in the bark, or by the hole left by a boring beetle. Cankar is different from a gall that a tree produces to surround an intruder and pull it away from the vascular system.
5 Needle Cast
Needle cast causes big problems here in Virginia because of the humidity. It is a fungus that comes up through the ground and reproduces when water hits it. When a raindrop hits the fungus, it explodes sending spores spraying upward onto the lowest tier of tree limbs. So the die back of limbs moves from the bottom and upward.
The Blue Spruce is especially susceptible to needle cast because its natural habitat would be a dry climate with rocky, well-drained soil. In Northern Virginia it’s humid and the soil is mostly clay. So this is a situation in which some plant disease problems are tied to our aesthetic choices. We want a particular plant or tree in our garden because of its beauty. But that plant or tree may not be suited to the environment.
We love the blue spruce. Everybody wants one in their yard because they are beautiful and add color to a winter landscape. Margaret Miley, Garden Care Specialist
Worth the Risk?
Miley says any specialty plant or ornamental tree is going require regular care and attention to keep it healthy. And, she isn’t convinced that it is always worth the risk to invest in a vulnerable species like blue spruce because they are so expensive. On the other hand, she says, there are benefits and drawbacks to just about anything you plant. And sometimes you get lucky because pests that might prey on a tree or plant in their native environment may not be present in the non-native setting.
Prevention Is the Best Cure
- Knowledge. Research your plant selections carefully so you understand their needs. Be sure that the soil composition and moisture is right for the plants you choose because plants under stress (too dry, too wet) are susceptible to disease and insect infestation.
- Breathing room. Maintain proper spacing between plants. Sometimes gardeners may try to put too many plants in one place because it gives an instant look of fullness. But it cuts down on air circulation. The plants basically smother each other.
- Source. Purchase plants from a reliable grower. Otherwise, you could get diseased plants from the nursery.
- Scheduled maintenance. Invest in a comprehensive garden care and landscape maintenance program that includes integrated pest management (IPM).
Choosing Disease Resistant Garden Plants
I really love Green Velvet Boxwoods. It’s a really tough variety and rarely has pest problems. Margaret Miley, Garden Care Specialist
Miley recommends several plant species that tend to have few problems.
- Green Velvet Boxwoods are beautiful and rarely have pest problems
- Spirea Japonica is very tough and has almost no problems.
- Viburnum comes in many varieties and most have no pest problems. “Chinese Snowball Bush” is a popular viburnum variety.
- Hydrangeas—especially the “Endless Summer” variety is very adaptable and aside from deer, they tend to be problem-free.
- A variety of Calacarpa called “Beauty Berry Bush” is disease resistant.
A Balanced Garden Plan
In terms of coming up with a balanced planting plan, it is wise to rely on some typically “low maintenance” species to fill in amongst the show stopping beauties that a require (and deserve) high maintenance. If you want advice about adding flowering trees or specialty plants to your landscape or have questions about garden plant diseases, contact our Garden Management Division. One of our certified horticulturists will be happy to answer your questions.