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Plant Diseases

Galls in Your Garden? What to do!

August 26, 2021

Horned Oak Gall

Is there something weird growing in your landscape? I recently spotted this horned oak gall and knew I needed to find out what it was! Galls are something gardeners often notice—usually with alarm. What are they and do you need to do anything about them?

What are galls?
Unusual plant growths are often—but not always—galls.

  • Causes for gall growth include:
    • Critters—from adelgids and aphids to beetles, midges, mites, sawflies, and wasps
    • Bacteria and fungi
  • They affect many plants:
      • Trees
      • Shrubs
      • Perennials
  • And many plant parts:
    • Buds, flowers, and fruit
    • Leaves
    • Stems, twigs and branches
    • And even roots!
  • Galls may appear in different seasons, as well as change across seasons.
    • Azalea leaf gall - three stages of development. Photo by James H. Blake, courtesy of Clemson University Extension.
      Azalea leaf gall – three stages of development. Photo by James H. Blake, courtesy of Clemson University Extension.

      This spring you may have seen signs of azalea/camellia leaf galls (caused by Exobasidiumvaccinii/
      Exobasidium camelliae fungi) or maple eyespot galls (caused by Acericecis ocellaris midges) on your maple tree leaves.

    • Come summer you may have seen signs of crown gall (caused by soil bacterium Agrobacterium tumefaciens) on your roses, or oak apple galls (caused by wasps Amphibolips confluenta or quercusinanis) on your oaks.
    • As summer progresses, you may see evidence of life stages of horned oak galls (caused by Callirhytis cornigera wasps).
Horned Oak Gall leaf gall stage
    • Come fall and winter, goldenrod galls (caused by Eurosta solidagnis flies) are easy to spot.

 Which need attention?
It depends!

Aesthetics. Although galls may be fascinating, if you find them unsightly, you may want to remove them even if they will do no lasting harm to your plants.

Pros:

  • Removal may improve your plant’s appearance
  • Cleanup may help prevent further damage or disease.

Cons:

  • Removal may be impossible—think leaf galls that affect many leaves, such as Witch hazel cone galls.
  • Cleanup may remove the benefits to wildlife of some galls—birds feed on the larvae in goldenrod galls; birds, and some mammals, feast on the wasp larvae in oak apple galls.
    Witch hazel cone galls caused by aphids
    Goldenrod Gall
    Oak Apple Gall

Plant decline. Some galls left untreated will harm plants and may eventually kill them! Do not ignore Azalea/Camellia galls, crown galls, and horned oak galls.

What to do?

Prevention. When bacteria cause galls—crown galls, for example—there must be an opening to cause an infection. Careful handling to avoid wounding your plants as well as removing insect damage can help. Take care to clean and sanitize tools that might spread infection. Sanitation is also important for preventing fungal galls, such as Azalea/Camellia galls—dispose of diseased tissue in the trash, as well as clean any tools used.

Pruning is the answer to many aesthetic as well as more serious problems. Removing affected leaves or other plant parts may be all that is needed. In other cases, it may help, but it isn’t a cure-all. In the case of bacterial disease, for example, the cause of the gall is systemic and may or may not have a cure. Removal of early stages of horned oak gall wasp infestation can save trees.

Article by Buncombe County Extension Master GardenerSM Volunteers

 For more information:

Azalea/Camellia leaf galls: https://www.buncombemastergardener.org/time-maintain-remove-leaf-gall-azaleas-camellias/

Bacterial crown gall: https://extension.umd.edu/resource/bacterial-crown-gall-flowers

Horned oak gall wasp:

https://bygl.osu.edu/node/1019

http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2922&Type=2

Goldenrod gall fly:

https://hort.extension.wisc.edu/articles/goldenrod-gall-fly-eurosta-solidagnis/

Maple eyespot gall: https://content.ces.ncsu.edu/maple-eyespot-gall-midge-acericecis-ocellaris-osten-sacken-diptera-cecidomyiidae

Oak apple gall: https://bygl.osu.edu/node/344

Sampling of gall photos:

https://projects.ncsu.edu/cals/course/ent525/close/gallpix/

https://www.maine.gov/dacf/php/gotpests/diseases/galls.htm

Series of American Nurseryman articles on galls:

https://www.amerinursery.com/plants/plant-galls-myths-misconceptions/

https://www.amerinursery.com/pest-management/insect-and-mite-galls-myths-and-misconceptions/

https://www.amerinursery.com/american-nurseryman/managing-galls/

 

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Categories Plant Diseases, Plant Insects Tags crown gall, diseases, flower gall, gall, insects, leaf gall, Tree gall

Pest Alert: Basil Downy Mildew Found in Western North Carolina – July 22, 2019

July 29, 2019

Basil downy mildew has been confirmed on the variety ‘Genovese’ in the NC State University research plots in Haywood County, North Carolina.

Basil Downy Mildew Spores on Underside of Leaf
Basil Downy Mildew

Basil downy mildew characteristics

  • Pathogen: Basil downy mildew is caused by the fungus-like oomycete pathogen Peronospora belbahrii.
  • Host crop: Basil (Ocimum basilicum)
  • Host parts affected: Leaves are severely affected, which can result in yield reduction or plant death.

 Identification

The disease typically begins in June and lasts throughout the growing season. Symptoms include yellowing or browning of leaves, which may be mistaken for nutritional problems. Infected leaves have many dark spores on the underside of infected leaves that can be seen without a microscope or hand lens.

  • High-humidity and moisture—6 to 12 hours of moisture from morning dew, rain, or overhead irrigation.
  • Cool temperatures (60°F).

Prevention

The disease has not been found on the resistant sweet basil varieties ‘Thunderstruck,’ ‘Devotion,’ ‘Passion,’ and ‘Obsession.’ Planting these resistant varieties or pathogen-free seed of other varieties is essential to prevent basil downy mildew.

Some basil types are less susceptible than sweet basil varieties. Researchers have found red basil (‘Red Leaf’ and ‘Red Rubin’), Thai basil (‘Queenette’), lemon basil (‘Lemon,’ ‘Lemon Mrs. Burns,’ ‘Sweet Dani Lemon Basil’), lime basil (‘Lime’), and spice basil (‘Spice,’ ‘Blue Spice,’ ‘Blue Spice Fil,’ ‘Cinnamon’) to have less severe downy mildew symptoms.

In addition to infected seed. disease also spreads from airborne spores of Peronospora belbahrii. Help control by watering early in the mornings, so foliage dries quickly, and allowing enough space between plants to increase air circulation.

Disease control for home gardeners

Be diligent in checking basil leaves for downy mildew. Fungicides are most effective when applied in a preventive, weekly spray program. Products containing the active ingredients copper or chlorothalonil (‘Daconil’ is the trade name of one product with chlorothalonil) are the only effective products available to home gardeners. Apply to achieve good coverage on both sides of the foliage and apply the products before disease begins.

 For more information:

Pest Alert: https://plantpathology.ces.ncsu.edu/2019/07/basil-downy-mildew-found-in-western-north-carolina-july-22-2019/

Basil Downy Mildew: https://content.ces.ncsu.edu/basil-downy-mildew

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Categories Plant Diseases Tags basil, diseases, downy mildew, pest control, Symptoms

The Garden Detective: Dusty-looking Plant Leaves? Could Be Powdery Mildew.

October 11, 2018

Q: I noticed that my bee balm and phlox looked bad this year. The leaves were covered with a white powdery substance. What is this stuff and is there anything I can do to control or prevent it next year?

Powdery mildew on Monarda didyma_Emma Cooper_CC BY-NC 2.0_Flickr
Powdery mildew on Monarda leaf

A: A fungus (from the genus Erysiphe) attacked your plants! Many Erysiphe species produce powdery mildew—what looks like white dust on plant leaves, stems, and even flowers.

Distinguishing powdery mildew from other plant problems
There are other plant mildews that cause a white, powdery appearance—most notably downy mildew. For more information on the difference, see the blog Mildew on Plants? What to Watch for!

Spider mite damage_AgriLife Extension Service_CC BY-NC-ND 2.0_Flickr
Spider mite webbing is sign of insect problem, not disease.

Also, serious infestations of tiny insect pests, such as spider mites, white flies, woolly aphids, or mealy bugs, may make your plants look white! If you look closely, you can see spider mite webbing, white flies flying, or aphid or mealy bug individuals to determine if you have an insect problem rather than a disease! 

What conditions favor powdery mildew?

  • Low light
  • Wet leaves and high humidity
  • Overcrowded gardens with poor air circulation
  • Excessive nitrogen fertilization

Powdery mildew’s preferred climate—hot dry days, cool nights, and morning fog—means that for gardeners in Western North Carolina, powdery mildew is a fact of life. 

Plant damage
Powdery mildew requires living plant tissue for its vegetative part (the mycelium) to grow. Mats of branching mycelium threads absorb nutrients. As these mats spread, they decrease photosynthesis, causing affected leaves to look yellow (chlorotic). 

Powdery mildew on pumpkin leaves_Jeff Kubina_CC BY-SA 2.0_Flickr
Powdery mildew on pumpkin leaf

Susceptible plants
There isn’t much that powdery mildew doesn’t go after. It affects more than 1,300 plants! The good news is that it is host specific—different fungal strains affect different plants. In addition to the perennials you mentioned, different fungal species cause problems for some ornamental trees, such as our beloved flowering dogwood (Cornus florida), vegetables—cucumbers in particular—and many fruits, including apples, strawberries, peaches, and grapes.

Minimizing powdery mildew infections
Given the growing conditions in Western North Carolina, eliminating powdery mildew is a bit like Don Quixote tilting at windmills! 

Monarda 'Marshall's Delight'_Chadwick Arboretum_CC BY-NC-ND 2.0_Flickr
Monarda didyma ‘Marshall’s Delight’

Selecting disease resistant varieties is a better approach. Ask for disease resistant cultivars when shopping—or go with a list of desirable varieties. One good source is our own NCSU Mountain Horticultural and Crops Research Center in Fletcher, N.C. Testing done by Extension Specialist Dr. Richard Bir gives the following perennial cultivars high marks.

Among bee balm (Monarda didyma) varieties:

  • Early Blooming – ‘Claire Grace,’ ‘Marshall’s Delight,’ and ‘Stone’s Throw Pink.’
  • Late Blooming – ‘Beauty of Cobham,’ ‘Blue Stocking,’ ‘Cambridge Scarlet,’ ‘Elsie’s Lavender,’ ‘Mahogany,’ ‘Marshall’s Delight,’ and ‘Vintage Wine.’
Phlox paniculata 'David'_Karen_Hine_CC BY-SA 2.0_Flickr
Phlox paniculata ‘David’

Among garden phlox (Phlox paniculate) varieties:

  • ‘David,’ ‘Robert Poore,’ ‘Bright Eyes,’ ‘Eva Cullum,’ ‘Fairest One,’ ‘Franz Shubert,’ ‘Natascha,’ ‘Orange Perfection,’ ‘Rosalinde,’ and ‘Starfire.’

Because powdery mildew is such a threat to flowering dogwoods, major agricultural universities, including NCSU, have undertaken decades of research to create disease-resistant varieties. When purchasing look for crosses between Cornus florida and Cornus kousa, such as ‘Stardust,’ ‘Stella,’ and ‘Celestial.’

NOTE: Resistance doesn’t mean immunity, but it does mean that there is a reduction of disease growth in the plant.

Improve your growing conditions. Once you have selected the right variety, give your plants the best chance for success.

  • Grow them in full sun—if your plants can tolerate it. Powdery mildew prefers shade!
  • Give them breathing room. Encourage good air circulation by thinning and pruning.
  • Reduce or eliminate overhead watering.
  • Remove and dispose of infected leaves during the growing season. In the fall, remove and destroy infected leaves and branches.
  • Apply fungicides that target powdery mildew, remembering that most need to be applied before the onset of infection.
  • Use horticultural oil—but only if conditions are right. Some oils may cause damage if applied during warmer summer weather.

Yes, powdery mildew is a formidable opponent. But by selecting the right plants and creating an environment that impedes fungal growth, you and your garden can enjoy success next season!

Article written by Janet Moore, Extension Master GardenerSM Volunteer.

Learn more
Let’s Talk About Powdery Mildew
by Dr. Bill Hanlin, Horticulture Assistant
NC Cooperative Extension, Wilkes County, NC

Monarda and Powdery Mildew Resistance
and
An Evaluation Report of Selected Phlox Species and Hybrids
by Richard G. Hawke, Coordinator Plant Evaluation Programs
Chicago Botanic Garden

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Categories Plant Diseases Tags bee balm, Monarda, Phlox, powdery mildew

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