Extension Master GardenerSM Volunteers of Buncombe County

Plant Diseases

Mildew on Plants? What to Watch for!

In addition to the mildew that may appear in your house in damp, dark places, you may soon see mildew on your outdoor plants! Both downy and powdery mildews are likely to infest Western North Carolina gardens this summer. Should you be concerned? Is there anything you can do to stop them? 

Powdery mildew_Dr Lina Quesada_NCSU Vegetable Pathology Lab
Cucumber leaf infected with powdery mildew

Identifying mildews
If you see spots on leaves, you may have plant mildew. The spots may appear on the top surface of the leaf, the underside, or both. Powdery mildews tend to start out looking like white powder on the top of leaves, while downy mildews often first appear on the underside of leaves as downy-looking spores that range from white to quite dark, depending on the plant. As the disease progresses, downy mildews may produce yellow lesions on top of leaves that are defined by leaf veins.

Downy mildew_PennState Extension
Downy mildew on underside of leaf

Mildews may also appear on other plant parts, such as flowers. Although mildews affect many plants, the good news is that most mildews on shrubs, trees, and perennials are unsightly, but not fatal to the plant. The bad news is that mildews are much more serious on annuals and cucurbit vegetables, which include cucumbers, squash, pumpkins, gourds, and melons. 

What causes plant mildews?
Many different strains of fungi cause powdery mildews, but downy mildews are caused by different fungus-like organisms. Although you may find both types of mildews on susceptible plants, the strains that cause mildews on one type of plant will not spread to other types. So, no need to worry that powdery mildew on your dogwood tree will spread to your cucumbers!

Prevention is key!
When you buy plants, you may find selections bred for resistance to mildews, or to better tolerate these diseases. If mildew-resistant varieties aren’t available, carefully examine the plants you buy for signs of infection.

If you’ve had problems with mildews in the past, be sure you’ve removed any diseased plants or weeds that may serve as a refuge for the pathogens. Do not save seed from downy mildew-infected plants as they may produce diseased plants.

Give your plants plenty of space for air circulation and avoid overhead watering. Cool-to-moderate humid weather promotes the growth of mildews, so that’s when you’re first likely to spot problems.

Treatment?
Although fungicides are available to help prevent mildews, for home gardens preventive spraying requires vigilance and may not be worth the effort. Instead, remove diseased plants from your garden before the disease spreads.

More information
–  Clemson Cooperative Extension: Charts of plants resistant to powdery mildew and examples of fungicide products.
–  Perdue Extension: List of disease-resistant annuals and perennials.
–  NC State Extension: Curcurbit powdery mildew.

Article written by Debbie Green, Extension Master GardenerSM Volunteer.

Southern Blight: Large Variety of Plants Vulnerable to This Disease

With summer coming, so are plant diseases. One that can affect an astonishing variety of plants—from apple trees to zinnias—is Southern blight, caused by the fungus Sclerotium rolfsii.

Southern Stem Blight_Sclerotium rolfsii_leaf wilt on pepper plant; courtesy of Chatham County NC State Extension
Leaf wilt on pepper plant, a sign of Southern blight

What to watch for
Southern blight thrives in hot, humid conditions. Although the fungus can infect many plant parts—including roots and fruits—it typically shows itself as wilt resulting from rotting stems at the base of the plant. Even before the plant wilts, you’ll often see water-soaked spots on the stems.

Common casualties
Some plants are particularly vulnerable. Hostas may succumb in your perennial garden. Many root crops are affected, including beets, carrots, Jerusalem artichokes, parsnips, radishes, sweet potatoes, and turnips. Other victims may be cantaloupe, eggplant, peppers, snap beans, sweet corn, and tomatoes.

SouthernBlight_Hosta_moccasinlanding_CC BY-NC-ND 2.0_Flickr
Southern blight on hosta

How it survives
One reason Southern blight can persist in soils is that it forms sclerotia: hard, rounded bodies that look like mustard or radish seeds. The disease is sometimes called Sclerotial blight for this reason. It can also remain in plant residue in the soil and spread through surface water.

What to do
As with all fungal diseases, prevention is key. Once the plants are infected, fungicides may help prevent the spread of the disease to other plants, but won’t reverse problems on plants already affected. Depending on the crop, fungicides used as soil drenches prior to planting may prevent infection. Different fungicides are recommended for different crops, however.

Sclerotia or hard nodules on pepper plant stem with Southern blight, Sclerotium rolfsii; courtesy of Chatham County NC State Extension
Sclerotia or hard nodules on pepper plant stem with Southern blight

Immediately remove infected plants along with their roots and the surrounding soil. Do not compost! If container plants are affected, do not reuse the potting soil. If Southern blight takes hold in your garden, solarizing for several weeks using plastic during the summer may kill the fungus.

For more information about recommended fungicides:
https://content.ces.ncsu.edu/north-carolina-agricultural-chemicals-manual/disease-control
(See page 498 for Southern stem blight, Sclerotium rolfsii)

Factsheet from the American Phytopathological Society:
http://www.apsnet.org/edcenter/intropp/lessons/fungi/Basidiomycetes/Pages/SouthernBlight.aspx

Article written by Debbie Green, Extension Master GardenerSM Volunteer.

Time to Maintain: Remove Leaf Gall from Azaleas and Camellias

I am fortunate to have many native flame azaleas growing on my property. But this time of year—April and May—just when they begin to bloom, I see abnormal growths at the tip of some stems. They look like little green apples and are slightly smaller than a ping pong ball. What are they?

Azalea Leaf Gall (Exobasidium vaccinii)
The growths are caused by a fungus that thickens the emerging foliage into these soft-tissue balls. The spores of the ripening fungus are spread by wind and splattering raindrops. They overwinter in the ground and on the plant’s branches and re-infect the azalea again in the spring. While the galls won’t kill the plant, they are unsightly. The best defense is to pick the galls while they are still small and green. Once they’ve turned white and begun to dry up, they release their spores and you’ll contend with even more galls next season. Exobasidium vaccinii primarily affects native azaleas, but it can spread to rhododendrons and hybrid azaleas, too.

Camellia Leaf Gall (Exobasidium camelliae)
I also have a beautiful stand of sasanqua camellias affected by leaf gall caused by a different fungus. The emerging leaves thicken and turn reddish. I check the shrubs every few days and clip off the affected leaves before the galls mature and release spores. Although Exobasidium camelliae mostly affects the sasanquas, other camellia species are also susceptible.

Camellia leaf gall (Exobasidium camelliae). Photo by Joey Williamson, courtesy of Clemson University Extension.
Camellia leaf gall (Exobasidium camelliae). Photo by Joey Williamson, courtesy of Clemson Extension.

Good practices
Be sure to sterilize your pruners between cuts so as not to spread the fungus to other branches. Dispose of the galls in a plastic bag and throw them in your trash—not the compost pile. Manual removal is usually the only treatment needed. Providing good air circulation minimizes the time rainwater stays on the leaves during rainy seasons, reducing the likelihood of fungal infections.

For more information about azalea leaf gall, view this publication from Cornell University: http://plantclinic.cornell.edu/factsheets/azaleagall.pdf

For more information about camellia leaf gall, check out this video from Clemson Cooperative Extension: http://www.clemson.edu/extension/hgic/videos_posters/videos/camellia_leaf_gall.html

Article written by Beth Leonard, Extension Master GardenerSM Volunteer.

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Buncombe County Extension Office
49 Mount Carmel Road
Asheville, NC 28806
Helpline 828-255-5522

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