- Author: Lauren Fordyce
It may be after Labor Day, but some of your plants may still be wearing white, breaking that long-standing fashion “rule”! While many of us don't adhere to this old rule for our wardrobes these days, you may care about white stuff on your plants this time of year.
There are several white colored pest insects and diseases that you could be noticing on your plants.
Scales
Several types of scale insects are white. Each has their own host preferences, or plants they feed on.
- Cochineal scales feed only on cacti, usually prickly pears. On the outside they are white and waxy but have bright red bodies. If you scrape them off, you'll notice a red stain left behind.
- Cottony cushion scale feeds on a variety of woody ornamental plants. Common hosts in California are citrus, cocculus, nandina, and pittosporum. Adult scales are elongated and fluted.
- Euonymus scale feeds on many different plants but can be a serious pest to Japanese Euonymus (Euonymus japonica). Immature males are white and elongated with a brownish-yellow area at one end.
See the UC IPM Pest Notes: Scales to learn more about these and other scales and how to manage them.
Mealybugs
Mealybugs are whiteish-gray, oval, wax-covered insects. They can be found on many different plants, from fruit trees to houseplants, often hiding in protected places like branch crotches.
Powdery mildew
Powdery mildew often appears as white powdery spots (fungal spores) primarily on the tops of leaves. It can infect fruits and fruit trees, vegetables, and landscape plants in shady conditions when temperatures are between 60 to 80 degrees.
Whiteflies
Whiteflies are tiny insects with white, waxy wings and yellow bodies. They feed in large colonies on the undersides of leaves and have a very wide host range, including trees, vegetables, houseplants, and flowers.
Hackberry wooly aphids
This introduced aphid species infests Chinese hackberry and other hackberry (Celtis) plants. Hackberry wooly aphid adults appear in white, fuzzy masses on shoot terminals and leaves.
Spittlebugs
Adult spittlebugs are often brown, but they produce a white, foamy excrement on plants that looks like spit. They are known to feed on just about any plant.
Mushrooms
Many fungi produce white fruiting bodies that may pop up in your lawn, mulched landscape beds, or on trees and stumps. Puffballs and other fungi in lawns are mostly just a nuisance. However fungi on trees can sometimes cause wood decay and harm trees.
For more help diagnosing a plant problem or identifying a pest, see the UC IPM Plant Problem Diagnostic Tool or contact your local UC Master Gardeners.
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.
Resources:
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:
- 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
- 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
- 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
- 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
- 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
- Author: Alison Collin
I had a good patch of California Poppies (Eschscholzia californica) that had just finished flowering when I noticed the foliage on one of them was somewhat grayer in color than its neighbors. I did not think much of it at the time and put it down to being some sort of natural variant. As a Master Gardener I should be thoroughly ashamed of myself for not taking more notice!
The plant was suffering from powdery mildew, a fungal disease which, unlike most fungal infections, prefers dry weather. I have lived in the Owens Valley for ten years and so far have not seen much of this on my plants in previous years; even my variegated Euonymus which is renowned for its susceptibility has escaped.
Thanks to my inaction a few days later all the rest of my poppies were almost white from the infection, and then I noted the early symptoms on a previously healthy mahonia.
Many plants can be attacked by different species of powdery mildew and it is commonly seen in cucurbits – squashes, cucumbers, melons etc. as well as many fruits such as grapes, apples. Flowers such as zinnias, monarda and lupines, and roses are particularly prone.
It commonly begins on new growth, but if left unchecked can eventually distort the growing tips and buds as well as the leaves.
The fungal spores which overwinter in plant debris are spread by wind, and tend to attack plants in shady areas and also where there is not good air circulation, so avoid planting susceptible plants in those situations, and prune excess foliage to allow air circulation and sunlight to reach more of the plant.
Try to avoid the problem choosing resistant varieties when planting species known to be susceptible .
Some control may be achieved by pruning out affected areas if small, spraying with water to wash spores from the leaves, or spraying with a fungicide.
For more detailed information on how to avoid Powdery Mildew and methods used to contain or control it check out the following:
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7494.html
http://ipm.ucanr.edu/QT/powderymildewcard.html
As you practice social distancing during COVID-19, you may find yourself spending more time in the garden. As we move into nice weather, we are sharing a few gardening tips for pest management.
Powdery Mildew
Check for signs of powdery mildew on fruit trees, landscape ornamentals, and vegetables. Symptoms can vary by plant species but common symptoms include white powdery spots developing on both sides of the leaf, leaves that turn yellow or brown and fall off, twisting and distortion of leaves and shoot. Prune out small infestations and remove infected material to reduce spread.
For more on this disease, see our Pest Notes publications: Powdery Mildew on Fruits and Berries, Powdery Mildew on Ornamentals, and Powdery Mildew on Vegetables.
Check out more April gardening tips on composting and fertilizing; and stay tuned for more!
Subscribe to our Seasonal Landscape IPM Checklist for monthly landscaping reminders delivered straight to your inbox.
/h2>- Author: Anne E Schellman
Over the past few months, gardeners have asked the UCCE Master Gardeners for help with their grapes. They want to know:
- What's this white powdery substance on my grape plants?
- Why are my grapes so small?
- What's causing my grapes to split?
- What can I do to “save” my grape plants?
The culprit is a common grape disease called powdery mildew. This fungus leaves a telltale white powdery coating on plants. It also deforms leaves, shoots, and grapes. Young grapes can be stunted or scarred, and sometimes split open.
We've told gardeners that unfortunately, powdery mildew can't be eradicated. For now, gently hosing down plants weekly with water will help to wash off and kill the spores.
In winter, prune grapes and remove and destroy infected materials. During spring, use fungicides to protect grapevines. Timing is important. Read about how and when to prevent and control this disease in the publication Pest Notes: Powdery Mildew.
Small Grapes
Small grapes are a result of too many clusters of grapes on a vine. The clusters will need to be thinned. Sometimes gardeners have trouble doing this. It may feel like you are throwing away perfectly good fruit! However, thinning out grape clusters is a necessary task that should be done in early spring during the first three to four weeks after fruit has set.
Study up on grapes now! Then you'll know what you need to do next year. For information on pruning, thinning, and growing grapes, visit The California Garden Web page Growing Grapes in Your Backyard.