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Plant Diseases

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

Mildew on Plants? What to Watch for!

June 14, 2017

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.

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Categories Plant Diseases Tags downy mildew, powdery mildew

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