Two common diseases in both vegetable gardens and ornamental gardens are powdery mildew and downy mildew. While the two diseases have many similarities the pathogen is different, symptoms are expressed differently, and each flourish in different types of environmental conditions. Learning to prevent and recognize the symptoms of each disease is key to managing these destructive diseases.

Powdery Mildew

Powdery mildew is a fungal disease that can affect over 10,000 species of plants. Rather than just one specific species of fungi there are actually approximately 700 closely related fungi that can cause powdery mildew. Most species are host specific or family specific. Typically, the disease is more cosmetic in nature and does not kill the host, but it can seriously weaken the plant and make it more vulnerable to other stressors.

Symptoms

Powdery mildew is characterized by the formation of a whitish/grey powdery substance on the surface of leaves but also can affect stems and flowers. This powdery growth starts off as small whitish spots that can continue to spread, eventually covering the entire leaf surface, or plant. This white coating reduces photosynthesis and leaf yellowing and premature leaf drop typically follows, beginning towards the base of the plant working up. The powdery growth more commonly occurs on the upper surface of leaves but on some plants, you may also see it on the underside. In addition, or occasionally instead of the powdery growth you will see growth slow, leaves may be discoloured, and may curl or become distorted.

Conditions that Favour The Development of Powdery Mildew

Powdery mildew thrives in warm, dry, humid weather. The ideal temperature for it’s growth is between 21-27C (70-80°F) but once the temperatures go above 32°C (90°F) the fungal spread slows down or stops. Unlike most fungi, powdery mildew does not require wet leaves in order to germinate, just high humidity. In some powdery mildew species, water on the leaves can actually inhibit spore germination. The shadier areas of the garden and areas with poor air circulation are also more prone to develop this disease.

How Powdery Mildew Spreads

The fungal spores spread by wind, insects, splashing water, tools and human activity. The spores can lay dormant in plant buds and over winter in plant debris. Once conditions are right for germination the fungi penetrate the plant tissue and draw nutrients out of the plant. Within 48 hours these newly germinated, parasitic fungi, can produce new spores to spread the infection. While high humidity favours spore formation, low humidity favours spore dispersal.

Powdery Mildew Control

Prevention is worth an ounce of cure. When possible, plant varieties that are resistant to powdery mildew. Ensure there is good air circulation around vulnerable plants. Do not over fertilize, high nitrogen levels favour the disease. Water early in the morning to give moisture a chance to dry before evening. Remove infected plant material to reduce spread. Brown bag, black bag, or burn infected plant material rather than adding to your home composter, which usually does not get hot enough to kill the fungi. City composters get much hotter, about 49 C to 77 C (20 – 170 ℉).

Natural or organic fungicides:

The following treatments are all best used as preventative treatments. Begin spraying before the plant is infected, when environmental conditions are right for the disease. Continue spraying about every 10 days. All sprays should be avoided during very hot weather. When spraying do so in early morning to help avoid burning plants. Do not spray when in bloom. Very young plants may require further dilution. Test an area of the plant first to make sure the plant is not sensitive to the spray.

• Sulfur (either dusted on as a powder or made into a spray).
• Sodium bicarbonate aka. baking soda (1 tbsp. baking soda with to 1 gal. of water add 1 tbsp. of horticultural oil).
• Potassium bicarbonate (1 tbsp. of Potassium bicarbonate to 1 gal. of water, add 1 tbsp. of horticultural oil).
• Copper (follow manufactures instructions to avoid burning plants).
• Neem oil
• Milk and water (3-4 parts milk to 6-7 parts water). The protein in the milk when exposed to sunlight has a mild antiseptic effect.

A list of several plants susceptible to powdery mildew and the specific symptoms typical of each:

• Azalea infected by the powdery mildew fungi Erysiphe azaleae develop a whitish powdery growth on both upper and lower leaf surfaces. Young leaves are more affected. On evergreen Rhododendrons they are more likely to develop subtle leaf spots rather than the white powdery growth.
• Bee balm develop the typical whitish/grey powdery growth on the upper leaf surfaces. Badly infected leaves will drop prematurely. It is the most common disease of Bee Balm. Some resistant varieties include: ‘Marshall’s Delight’, ‘Raspberry Wine’ ‘Gardenview Scarlet’, ‘Violet Queen’, ‘Colrain Red’, ‘Rosy-Purple.’
• Blueberry
• Centaurea
• Coreopsis
• Crabapple infected by Podosphaera leucotricha often develop small, light grey, felt-like patches on the underside of the leaves. Leaves may curl or be distorted and may also appear narrower than normal. New growth is most affected, and the whitish powdery growth can form heavily here, causing distortion and death of these areas.
• Crape myrtle
• Cucumber infected by the powdery mildew fungi Podosphaera xanthii (previously known as Sphaerotheca fuliginea and S. fusca) and Erysiphe cichoracearum, develop whitish powdery growth on both the upper and lower leaf surfaces, as well as the petioles and stems. Yellow spotting on the upper leaf surface may be present. Lower leaves most susceptible. The fruit is usually not affected.
• Current bushes develop the typical whitish powdery growth, more dominantly on the upper leaf surface but also the underside of leaves, stems and fruits. Young leaves are more susceptible. Leaves begin yellowing and may drop prematurely and plants may be stunted.
• Dahlia
• Delphinium
• Dogwood infected by Erysiphe pulchra and/or Phyllactinia guttata, may or may not develop the whitish powdery growth. Leaves may become yellowish or reddish with purplish or brown blotches. Leaves may droop and curl and leaf edges and tips may scorch. Young leaves most susceptible.
• English ivy
• Euonymus
• Honeysuckle
• Lilac can be infected by Microsphaera syringae.
• Maples can be infected by Sawadaea tulasnei.
• Ninebark can be infected by Podosphaera physocarpi. The young succulent growth of the purple varieties is most susceptible. Whitish powdery growth can develop on leaves, stems and flowers. The decaying flowers seem to be particularly susceptible. Premature leaf drop may occur if badly infected. In addition, the shrub can develop ‘Witches’ brooms.
• Oak
• Phlox can be infected by more than one species of powdery mildew fungi Golovinomyces magnicellulatus, Golovinomyces cichoracearum and Podosphaera sp. Symptoms begin small whitish powdery spots on leaf surfaces and stems. As the disease progresses the spots spread to cover whole leaves, stems and flowers and plants will lack vigor, growth may be stunted, and leaves may twist or ripple. Leaves begin to yellow and drop prematurely usually from the bottom up. Phlox paniculata ‘David’ and Phlox paniculata ‘Goliath’ are two varieties that are more resistant.
• Roses can be infected by Sphaerotheca pannosa var. rosae, also known as Podosphaera pannosa. The whitish powdery growth appears on leaf surfaces, both upper and lower may be affected as well as stems and flowers. Young succulent growth most vulnerable. Reddish blister-like areas on the upper leaf surface may appear early on and leaves may distort.
• Rhododendron can be infected by Erysiphe azaleae. Symptoms vary according to cultivar. Common symptoms include pale yellow spots on the upper leaf surface and purplish/brown circular spots on the underside of leaves, sometimes containing fungal growth. In some cultivars purple ringspots or large purplish/brown spots appear on upper leaf surface or the upper surface may have no markings at all, only the undersides. Leaves yellow and drop prematurely.
• Snapdragon
• Squash
• Spirea
• Sunflowers can be infected by Golovinomyces cichoracearum, Poposphaera xanthii, and Leviellula Taurica. Symptoms start off as white powdery growth on leaf surfaces except Leviellula Taurica which starts off as yellow/green spots on upper leaf surface.
• Wisteria
• Zinnias infected by Golovinomyces spadiceus develop white powdery fungal growth on leaves, flowers or stems. The disease is more common late in the season, partially in shady locations. Leaves die from the base of the plant upwards, and plants lose vigor and stop growing.

Downy Mildew

Downy mildew is water mold of which there are more than 300 species. They can affect a wide range of plants but is particularly troublesome to cucurbits (like cantaloupe, cucumber, pumpkin, squash, watermelon, zucchini…) grapes, hops and other vining plants. Unlike powdery mildew, downy mildew can infect a plant systemically and can kill off the entire plant.

Symptoms

Symptoms can vary a bit depending on the host species. Typical symptoms include irregular yellow spots on the upper leaf surfaces between the veins. Lower leaf surfaces develop a greyish fuzzy growth (in some species the growth may appear on the upper leaf surface or may be purplish in colour). Plants may be stunted, leaves may be cupped, and new growth may be distorted, and fruit yield will be reduced or may rot. As the disease progresses the spots spread everywhere but the veins, and leaves eventually turn brown and drop prematurely. Extensive defoliation can kill the plant. Symptoms can be hard to diagnose in the beginning, often being mistaken for a nutritional deficiency or powdery mildew.

Conditions That Favour The Development of Downy Mildew

Rainy weather (or overhead irrigation), temperatures around 10-24C (50-75F) and a relative humidity of 85%. A layer of water on the plant is essential for germination to take place.

How Downy Mildew Spreads

Downy mildew spores can overwinter in plant debris in warmer climates, however in colder climates like Canada, they cannot overwinter. Downy mildew spores can be windblown, splashed up on plants during rain or overhead watering or spread by tools and human activity. If the conditions are favourable for germination, they penetrate the plant and enter the vascular system. If conditions are not favourable the spores will die off without a host to feed on, within 1-16 days. It can take 4-12 days for symptoms to appear.

Downy Mildew Control

Since symptoms can take 4-12 days to appear plants will become infected well before you realize it, thus prevention is the key to controlling this disease.

Prevention

• Plant resistant varieties when possible.
• Use drip irrigation rather than overhead to keep the plant dry, and water in the morning to give the moisture time to dry before evening.
• Provide good air circulation around plants.
• Do not over fertilize with nitrogen.
• Choose sunny planting areas.
• Practice good sanitation as tools and hands can spread the spores.
• Stake the plants or grow them up supports to keep them off the ground.
• Remove diseased plant material and dispose of it.

Natural or organic fungicides:

All sprays should be avoided during very hot weather. When spraying do so in early morning to help avoid burning plants. Do not spray when in bloom. Very young plants may require further dilution. Test an area of the plant first to make sure the plant is not sensitive to the spray.

• Copper spray is best used as a preventative spray applied about every 10 days. Copper spray can make cucurbits photosensitive, follow manufactures instructions to avoid burning plants.
• Potassium bicarbonate (1 tbsp. of Potassium bicarbonate to 1 gal. of water, add 1 tbsp. of horticultural oil).
• Neem oil
• Bacillus subtilis

A list of several plants susceptible to downy mildew

• Basil (can be infected Peronospora belbahrii)
• Blackberry (can be infected by Peronospora sparsa)
• Boysenberry (can be infected Peronospora sparsa)
• Brassicas (can be infected Hyaloperonospora parasitic)
• Centaurea (can be infected by Plasmopara halstedii)
• Coreopsis (can be infected by Plasmopara halstedii)
• Cucurbits (can be infected Pseudoperonospora cubensis)
• Delphinium (can be infected by Peronospora ficariae)
• Dianthus (can be infected by Peronospora dianthicola)
• Evening primrose (can be infected by Peronospora arthurii)
• Grapes (can be infected Plasmopara viticola)
• Hops (can be infected Pseudoperonospora humuli)
• Impatiens (can be infected Plasmopara obducens)
• Phlox (can be infected Peronospora phlogina)  
• Raspberry (can be infected by Peronospora parasitica)
• Roses (can be infected by Peronospora sparsa)
• Rudbeckia (can be infected by Plasmopara halstedii)
• Spinach (can be infected Peronospora effusa)
• Sunflowers (can be infected Plasmopara halstedii)

Summary

References:

Bolda M., (2009). Downy Mildew in Blackberries. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=1053

Burgess C. and rev. by Williamson J., (Feb 26, 2021). Powdery Mildew. Factsheet | HGIC 2049 | Clemson University Cooperative Extension. https://hgic.clemson.edu/factsheet/powdery-mildew/

CABI Invasive Species Compendium, (n.d.). Peronospora dianthicola. https://www.cabi.org/isc/datasheet/39700

Leonberger K. & Ward/Gauthier N. , (n.d.). Plant Health: Dogwood Powdery Mildew. Urban Forest Initiative. University of Kentucky. https://ufi.ca.uky.edu/treetalk/planthealth-dogwood-powdery-mildew

Grabowski M., University of Minnesota, (n.d. rev. 2019). Powdery mildew in the flower garden. https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden

McGrath M. T. (updated 2022). Cucurbit Powdery Mildew. (https://www.vegetables.cornell.edu/pest-management/disease-factsheets/cucurbit-powdery-mildew/

Michigan State University (n.d.). Downy mildew on perennials. Plant & Pest Diagnostics

Moorman G.W. (updated: June 12, 2014). Powdery mildew. Pennsylvania State University. https://extension.psu.edu/powdery-mildew

Ong K. and Brake A., (n.d.). Powdery Mildew of Roses. Texas A and M AgriLife Extension. https://agrilifeextension.tamu.edu/library/landscaping/powdery-mildew-of-roses/

Oregon State University, (n.d.). Crabapple (Malus spp.)-Powdery Mildew. A Pacific Northwest Extension Publication https://pnwhandbooks.org/plantdisease/host-disease/crabapple-malus-spp-powdery-mildew

Oregon State University. (n.d.). Phlox-Downy Mildew. Pacific Northwest Pest Management Handbooks. https://pnwhandbooks.org/plantdisease/host-disease/phlox-downy-mildew

Oregon State University, (n.d.). Azalea (Rhododendron spp.)-Powdery Mildew, A Pacific Northwest Extension Publication. https://pnwhandbooks.org/plantdisease/host-disease/azalea-rhododendron-spp-powdery-mildew

Oregon State University, (n.d.). Ninebark (Physocarpus spp.)-Powdery Mildew. A Pacific Northwest Extension Publication. https://pnwhandbooks.org/plantdisease/host-disease/ninebark-physocarpus-spp-powdery-mildew

Planet Natural Research Center (n.d.). Downy Mildew. https://www.planetnatural.com/pest-problem-solver/plant-disease/downy-mildew/

Rivera Y., Salgado-Salazar C., Gulya T. J. , Crouch J., (2016). Newly Emerged Populations of Plasmopara halstedii Infecting Rudbeckia Exhibit Unique Genotypic Profiles and Are Distinct from Sunflower-Infecting Strains. https://pubmed.ncbi.nlm.nih.gov/27003506/

University of Florida,  (updated July 12, 2017). Powdery Mildew vs Downy Mildew. Gardening Solutions. https://gardeningsolutions.ifas.ufl.edu/care/pests-and-diseases/diseases/powdery-vs-downy.html

University of Massachusetts Amherst, (n.d.). Zinnia – Powdery mildew. https://ag.umass.edu/greenhouse-floriculture/photos/zinnia-powdery-mildew https://www.greenhousecanada.com/downy-mildew-vs-powdery-mildew-1594/

Volesky N., Murray M., Nischwitz C. (2021). Powdery Mildew on Vegetables. https://extension.usu.edu/pests/research/powdery-mildew-vegetables

Photo credits: all photos taken by the author.

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