Pests of the Season - Powdery Mildews

About: Powdery mildew is a group of fungal diseases that afflict a wide variety of plants. There are many species of fungi (e.g., Erysiphe spp., Sphaerotheca spp. Golovinomyces cichoracearum) that infect plants and cause the suite of symptoms we commonly call powdery mildew. Each species of fungus commonly attacks a limited group of closely related plants. A wide variety of plants are susceptible, including vegetables, fruits, ornamentals and even turfgrass.

The fungi collectively known as powdery mildew are unusual in that they don't require moisture to establish an infection and grow. Similarly, unlike most fungi, powdery mildew fungi do well in warm weather, and so are common at this time of year when other fungi are less active.

Like all diseases, powdery mildew fungal infection occurs when the fungal pathogen contacts a suitable host and environmental conditions are favorable. Most prevention involves decoupling this disease triangle by changing environmental conditions to make them unfavorable for fungal growth and survival, by planting non-susceptible or resistant host plants or by controlling or removing the pathogen.

Signs and Symptoms: Powdery mildew fungi first appear as white powdery, cottony patches that form on the plant. White spots spread over a large area of the plant in the most susceptible species. Vegetables and other annuals are more often severely damaged, but woody plants like roses and sycamores can also suffer significant damaged. In vegetables, leaves infected with powdery mildew may turn yellow, die, and fall off, twist, buckle and become distorted. On woody plants,  new growth may become distorted, dwarfed, and die back.

Lifecycle: Most species of powdery mildew fungi grow as thin layers of mycelia (fungal tissue) on the surface of the plant. Spores, which act as the dispersal unit for the fungi, create what we see as a white powdery growth that gives these fungi their common name.  Powdery mildew spores are transported by wind (and humans) to other locations on the plant or nearby plants

Powdery mildew fungi require living tissue to grow. On perennial plants, powdery mildew survives in buds or on the bark of branches and stems. On vegetables and annuals, the fungi survive attached to plant parts and plant debris such as fallen leaves. Some species of powdery mildew fungi produce special resting spores; other reservoirs of infectious material include weed hosts that persist when the cultivated hosts die back. For example, the form of powdery mildew that infects barley (Hordeum vulgare) also infects related weeds in the genus, Hordeum.

Where: Powdery mildew fungi and the diseases they cause are prevalent in both inland and coastal areas of Del Norte and Humboldt Counties. The white patches associated with powdery mildew can be foundboth upper and lower surfaces of the leaves, on shoots and, in some cases, on flowers and fruit. Vegetables susceptible to powdery mildew fungi include artichoke, beans, beets, carrot, cucumber, eggplant, lettuce, melons, parsnips, peas, peppers, pumpkins, radicchio, radishes, squash, tomatillo, tomatoes, and turnips. On ornamentals and perennials, the fungi may infect old or new leaves. In some species, like roses, sycamores and crape myrtles, the fungi attack attacks new growth including buds, shoots, flowers, and leaves.

When:  Although humidity requirements for germination vary among powdery mildew fungi, all species can germinate and infect their hosts in the absence of free water—an unusual trait among fungi. Indeed, standing water on plant surfaces inhibits germination and kills some species of powdery mildew fungi. Temperatures between 60-80° F and shady conditions favor disease development, extreme heat (>90°F) and direct sunlight, damage mycelia and spores.

Prevention: Wherever possible, plant resistant cultivars. Seed packets and catalogues will often indicate resistant varieties by listing PM following the description of the plant. Follow good sanitation practices. Plant in sunny locations and provide good air circulation. Make sure the leafy canopies of your vegetables, fruits, and ornamentals don't shade themselves and adjacent plants. Avoid applying excess fertilizer, especially high nitrogen fertilizers that promote leaf growth. For ornamentals, overhead sprinkling can reduce the dispersal and viability of spores. The best time to spray off your ornamentals is mid-morning, so that the plants dry rapidly. Sprinkling is contraindicated for some ornamentals and most vegetables because it makes them susceptible to other infections.

 

 

 

 

 

 

 

 

 

Resources:

Powdery Mildew Biology               

Powdery Mildew on Vegetables               

Powdery Mildew on Ornamentals

Powdery Mildew on Cucurbits          

Powdery Mildew on Grass

Powdery Mildew on Artichokes

Powdery Mildew on Apples

Powdery Mildew on Apricot

Powdery Mildew on Caneberries

Powdery Mildew on Carrots

Powdery Mildew on Cherry

Powdery Mildew on Grapes

Powdery Mildew on Lettuce

Powdery Mildew on Nectarines

Powdery Mildew on Peaches

Powdery Mildew on Peppers

Powdery Mildew on Pistachio

Powdery Mildew on Plum

Powdery Mildew on Strawberries

Powdery Mildew on Field-Grown Tomatoes

Powdery Mildew on Greenhouse and Coastal Tomatoes

 

Links—not hyperlinked

Powdery Mildew Biology https://pnwhandbooks.org/plantdisease/pathogen-articles/common/fungi/powdery-mildew-diseases

Powdery Mildew on Vegetables: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html

Powdery Mildew on Ornamentals: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7493.html

Powdery Mildew on Cucurbits: http://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/cupowerymildew.html

Powdery Mildew on Grass: http://ipm.ucanr.edu/TOOLS/TURF/PESTS/dispowder.html

Powdery Mildew on Artichokes: https://www2.ipm.ucanr.edu/agriculture/artichoke/powdery-mildew/

Powdery Mildew on Apples: https://www2.ipm.ucanr.edu/agriculture/apple/Powdery-mildew/

Powdery Mildew on Apricot: https://www2.ipm.ucanr.edu/agriculture/apricot/Powdery-mildew/

Powdery Mildew on Caneberries: https://www2.ipm.ucanr.edu/agriculture/caneberries/powdery-mildew/

Powdery Mildew on Carrots: http://ipm.ucanr.edu/PMG/r102100111.html

Powdery Mildew on Cherry https://www2.ipm.ucanr.edu/agriculture/cherry/Powdery-Mildew/

Powdery Mildew on Grapes https://www2.ipm.ucanr.edu/agriculture/grape/Powdery-Mildew/

Powdery Mildew on Lettuce https://www2.ipm.ucanr.edu/agriculture/lettuce/Powdery-Mildew/

Powdery Mildew on Nectarines https://www2.ipm.ucanr.edu/agriculture/nectarine/powdery-mildew/

Powdery Mildew on Peaches https://www2.ipm.ucanr.edu/agriculture/peach/Powdery-mildew/

Powdery Mildew on Peppers http://ipm.ucanr.edu/PMG/r604100111.html

Powdery Mildew on Pistachio https://www2.ipm.ucanr.edu/agriculture/pistachio/Powdery-Mildew/

Powdery Mildew on Plum http://ipm.ucanr.edu/PMG/r611100711.html

Powdery Mildew on Strawberries https://www2.ipm.ucanr.edu/agriculture/strawberry/Powdery-mildew/

Powdery Mildew on Field-Grown Tomatoes https://www2.ipm.ucanr.edu/agriculture/tomato/Powdery-Mildew-on-Field-Grown-Tomatoes/

Powdery Mildew on Greenhouse and Coastal Tomatoes https://www2.ipm.ucanr.edu/agriculture/tomato/Powdery-Mildew-on-Greenhouse-and-Coastal-Tomatoes/

 

Images: 

1. D-SP-EPOL-FO.001-Caption Sugar pea foliage damaged by powdery mildew Erysiphe polygoni. A Charles Crabb UC IPM image Link: http://ipm.ucanr.edu/PMG/E/D-SP-EPOL-FO.001.html
2. D-SP-EPOL-FR.001- Caption Brownish spots on pea pod from powdery mildew infection-A Charles Crabb UC IPM image Link http://ipm.ucanr.edu/PMG/E/D-SP-EPOL-FR.001.html
3. D-TO-LTAU-FO.001- Caption Powdery mildew causes irregular yellow blotches on tomato leaves-Jack Kelly Clark UC IPM image Link http://ipm.ucanr.edu/PMG/L/D-TO-LTAU-FO.001.html
4. D-WO-MEUO-FU.007- Caption White patches of powdery mildew on euonymus leaf-Jack Kelly Clark UC IPM image Link http://ipm.ucanr.edu/PMG/M/D-WO-MEUO-FU.007.html
5. D-WO-SPAN-FO.001- Caption Powdery mildew, Sphaerotheca pannosa, on rose-Jack Kelly Clark UC IPM image Link http://ipm.ucanr.edu/PMG/S/D-WO-SPAN-FO.001.html