- How can I kill aphids on crape myrtle?
- Getting Rid of Aphids on your Crepe Myrtles
- WATCH: What’s Wrong With My Crepe Myrtle? 4 Common Problems
- Problem #1 — Sooty Mold
- Problem: Sooty Mold
- Problem #2 — Powdery Mildew
- Problem #3 — Cercospora Leaf Spot
- Problem #4 — No Blooms
- Another Problem You Might Not Think About: Asian ambrosia beetles
- More on Crepe Myrtles
- DON’T COMMIT “CREPE MURDER”
- WHAT CREPE MYRTLES NEED
How can I kill aphids on crape myrtle?
- Aphids feeding on a plant. Aphids feeding on a plant. Photo: Benimoto, Flickr.com
Photo: Benimoto, Flickr.com Image 1 of / 1
Image 1 of 1 Aphids feeding on a plant. Aphids feeding on a plant. Photo: Benimoto, Flickr.com How can I kill aphids on crape myrtle? 1 / 1 Back to Gallery Q. My “Tonto” crape myrtles stay covered with aphids and black mildew. I’ve tried every product available, and none keeps the aphids away for more than two weeks. The trees are close to our house, which blocks the west sun and restricts air circulation.
A. Less sun and restricted air circulation could be factors, but also check soil fertility and drainage.
A healthy tree is less susceptible to pests. Build the soil with compost, and provide adequate water. Avoid overwatering.
When you see the aphids, hit them with jets of water from the garden hose. You will have to repeat. Attract beneficials to help provide control. Spray the tree with a molasses solution (2 ounces of molasses to 1 gallon of water) and then release lady bugs (available at some garden centers). You also could treat with neem or a horticultural oil. Once the aphids are gone, the sooty mold will gradually disappear.
Crape myrtles are commonly attacked by crape myrtle aphids. This often leads to a black fungus, called sooty mold, forming on leaves and branches in mid- to late summer. Control these insects by treating your trees now with the systemic insecticide imidacloprid (various brands, check with the staff at your local nursery).
(NOLA.com|The Times-Picayune archive)
This week’s gardening tips: Control outbreaks of spider mites, common during dry summer weather, with insecticidal soap, a light horticultural oil (Year Round Spray Oil and other brands) or Malathion.
- Crape myrtles are commonly attacked by crape myrtle aphids. This often leads to a black fungus, called sooty mold, forming on leaves and branches in mid- to late summer. Control these insects by treating your trees now with the systemic insecticide imidacloprid (various brands, check with the staff at your local nursery). This is applied as a drench at the base of the tree.
- Hot, dry weather will increase evaporation and lower the water level in aquatic gardens. Add small amounts of water regularly to maintain the level. Adding large amounts of water can hurt the fish by rapidly changing the water temperature and chlorine amounts.
- The New Orleans chapter of the American Hibiscus Society holds its annual show and sale on Sunday at Ursuline Academy, 2635 State St. Everyone is invited to enter flowers for judging between 8 to 11 a.m. The sale and exhibition takes place from 1 to 4:30 p.m. You’ll be amazed by the rare and exotic varieties of hibiscuses on view and for sale. Books and fertilizers also will be sold, and society members are on hand to offer advice. Admission is free.
Getting Rid of Aphids on your Crepe Myrtles
said on October 17th, 2012 filed under: Lake of the Pines, Whimsy
Our friend Anne Frank at Lake of the Pines called in to ask “how do I get rid of Aphids on my Crepe Myrtles?”
After a bit of research, I divided my answer into manual, organic, or synthethic treatments. Synthethic is an euphemism for “chemical,” or course.
The two most highly recommended treatments are below indicated in bold italics.
Spay ‘em off with your hose or pick ‘em off by hand
Neem Oil spray or Safer’s Soap
Ortho Systemic Insect Killer (spray to get rid of them instantly)
Bayer Advanced 12 Month Tree and Shrub Protect and Feed (mix it and pour at the base of the plant)
I might start by spraying the aphids off with water, then putting a bag of ladybugs at the base (sounds entertaining!), then pouring the Bayer Advanced at the base.
If that doesn’t work, nuke ’em with the Ortho.
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This entry was posted on Wednesday, October 17th, 2012 at 1:03 pm and is filed under Lake of the Pines, Whimsy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
- bobjenkins said at October 18th, 2012 at 9:51 am
You are very welcome, Ann (without the “E”).
WATCH: What’s Wrong With My Crepe Myrtle? 4 Common Problems
Crepe myrtles come in all different colors and heights, so find the one that’s right for you. For vibrant summer color, few flowering plants can compete with the crepe myrtle. Just look around your neighborhood. Nearly every street in the South is lined with these blooming trees from mid-June to mid- August. There’s plenty to love about them: They grow almost anywhere, are easy to maintain, and are available in all shapes, sizes, and shades. When picking one for your yard, ask yourself: What color flowers do I want? Choose from red, white, purple, or pink. Is it cold hardy enough? This is key if you live in the Upper South. Is there plenty of sunlight? The more sun, the more flowers you’ll get. How big will it grow? Crepe myrtles have a reputation for growing fast. A larger one can overwhelm your landscape, which could result in the ill-advised pruning practice known as “crepe murder.” At right are four of our favorite selections.
Image zoom emPhoto: Steve Bender/em
Problem #1 — Sooty Mold
Image zoom emYuck! Black mold on crepe myrtle. Photo: a href=
Problem: Sooty Mold
Sooty mold is a fungus that covers the leaves and looks like you just sprayed your crepe myrtle with asphalt. (Note to reader: This is seldom a good idea.) The mold doesn’t feed on the foliage. Instead, it grows on sticky honeydew secreted by sucking insects like aphids, scales, and white flies that do feed on the leaves. Get rid of the bugs and black mold will go with them.
A black spotlike mold covers the leaves. Rubbing removes the mold. Underneath, the leaves are green and healthy.
Sooty molds are also called black molds. These unsightly molds are caused by several species of fungi. More than one sooty mold fungus may appear on the same plant at the same time, feeding on the honeydew of numerous insects. Fungal growth takes place from spring through early fall. Splashing rain or water may spread the fungus to other plants.
Symptoms: Sooty mold usually appears as a dark, brown-black powdery fungus growth covering leaf surfaces and twigs. It can also look like a thin, dark film or black spots. In several cases, the fungus almost completely covers a leaf’s surface. Although the fungus is considered fairly harmless because it does not feed on plants, extremely heavy infestations can block sunlight from reaching the leaves, which may yellow and fall prematurely.
Solution: Look for aphids, scales or leafhoppers higher in the plant. They secrete a sticky substance called honeydew, on which sooty mold grows. Control these insects by blasting them of with water or by spraying with horticultural oil, azadirachtin (Neem), malathion, or acephate (Orthene). Spray your crepe myrtle according to label directions with insecticidal soap, horticultural oil, neem oil, or Natria Multi-Insect Control. All of these are safe, natural products available at home and garden centers. Without the insects and their honeydew, sooty mold will gradually wash away.
Prevention: Control scale, aphids, mealybugs, whiteflies, and other honeydew-excreting insects, as well as ants.
Control: On small plants, wipe or wash off the molds with a small sponge and water. On large trees, use a hose-end sprayer to wash off mold. Spray plants with malathion, horticultural oil, azadirachtin (Neem), or acephate (Orthene) to control insects that secrete honeydew.
STICKY SITUATION: Some sap-sucking insects do not fully digest plant sap. The undigested portion is excreted as a sweet, sticky liquid called honeydew. If copious amounts of honeydew form on trees, sidewalks and other surfaces below may become coated with it and the sooty mold that follows. Ants add to the problem by collecting and tending honeydew-excreting insects, such as aphids, scale, and mealybugs. They milk the insects for the honeydew, which they take back to other ants for food. Ants also transfer honeydew-excreting insects from plant to plant.
Problem #2 — Powdery Mildew
Image zoom emPowdery mildew. Photo: a href=
Problem: Powdery Mildew
The bizarrely cool, rainy summer we’ve experienced in the Southeast this year (sorry, everybody out west) means that if your crepe myrtle is susceptible to mildew, it probably has it. Powdery mildew is a filmy, white fungus that grows on leaves and flower buds. It causes leaves to curl and shrivel. Flower buds drop without opening. Back in the day when we didn’t have resistant selections, powdery mildew was the #1 complaint people had about crepe myrtles.
White powdery spots appear on leaves, stems, and flowers. The spots expand to completely cover leaves in only a few weeks. If the fungus spreads to flower buds, the buds may not open.
There are many different powdery mildew fungi. The fungus spores overwinter on fruit trees. In spring, the fungus begins to grow, and spores are released to travel on the wind to young leaves. Powdery mildew thrives where cool nights follow warm days. Insufficient sunlight and poor air circulation favor its development.
Symptoms: A white or gray powdery fungus appears on foliage and flowers. Round, white spots on upper leaf surfaces expand and merge, covering both sides of leaves. Infected leaves turn yellowish green to brown. New growth may be stunted, curled, and distorted. Infected blossoms may not set fruit; fruit may develop a rough skin or be covered with the powdery fungus. Fruit drops early or is dwarfed.
Solutions: Plant a mildew-resistant crepe myrtle. Almost all of the newer ones are resistant, including those named after tribes of native Americans, such as ‘Acoma,’ ‘Arapaho,’ ‘Catawba,’ ‘Comanche,’ ‘Miami,’ ‘Natchez,’ ‘Sioux,’ ‘Tonto,’ and ‘Zuni.’ ‘Dynamite,’ ‘Early Bird,’ ‘Pink Velour,’ and ‘Red Rocket’ resist it too. If yours isn’t resistant, spray the foliage according to label directions in early summer with neem oil, horticultural oil, Natria Disease Control, Daconil, or Immunox. The first three are natural products. You’ll probably have to spray more than once.
Prevention: Plant resistant selections; see the “Southern Living Garden Book” under the “Practical Gardening Dictionary” or individual plant listings for suggestions. Give plants sufficient light and air circulation. Water plants from underneath rather than above to keep leaves dry.
Control: Reduce nitrogen fertilizer. Pick off and destroy infected leaves and flowers/ Spray ornamentals with horticultural oil triforine (Funginex), thiophanate-methyl (Thiomyl, Domain), azadirachtin (Neem), or triadimefon (Bayleton). Spray fruits and vegetables with wettable sulfur or horticultural oil. Discard infected flowers of annuals and leftover produce in fall.
Problem #3 — Cercospora Leaf Spot
Image zoom emCercospora leaf spot. Photo: Steve Bender/em
Cercospora is a leaf spot fungus that used to be fairly uncommon, but isn’t anymore. Grumpy has a theory that the nearly universal planting of crepe myrtles in the South has made it easy for this fungus to spread. What happens is that in mid- to late summer, angular, brown spots form on the oldest leaves. These leaves then develop fall color prematurely and drop. By fall, the tree may be completely defoliated, except for a few newer leaves at the top. Fortunately, this seems to cause no ill effects the next year.
Solutions: This fungus likes sheltered areas where breezes are blocked and the foliage stays wet for long periods. Grumpy knows this, because the one he planted in front of his house gets eaten up by leaf spot every year, while the one growing in the middle of the lawn is hardly touched. Some websites claim certain selections are resistant, such as ‘Apalachee,’ ‘Catawba,’ ‘Sioux,’ ‘Tonto,’ ‘Tuscarora,’ ‘Tuskegee,’ and ‘Yuma.’ Grumpy has his doubts, because the one that gets devoured every year is ‘Sioux.’ What to do? Plant crepe myrtles in open, sunny spots where air circulates freely. If necessary, spray with Daconil, Immunox, or Natria Disease Control when spots begin to appear.
Problem #4 — No Blooms
No matter where you live in the South, crepe myrtles should have bloomed by now. If yours hasn’t, most likely it’s due to one of three reasons.
1. Your plant is just too small to bloom. Give it time.
2. Your plant isn’t getting enough sun. It likes full sun.
3. Some crepe myrtles bloom better than others. You may have a slacker. If so, replace it with one of the selections named above.
Attention: No ants were harmed in the production of this post.
Another Problem You Might Not Think About: Asian ambrosia beetles
A tiny, foreign invader now threatens peach, plum, pear, pecan, and many other trees in the South — the Asian ambrosia beetle (Xylosandrus crassiusculus). Less than ¼ inch long and cylindrical in shape, it is dark brown, reddish brown, or black and may have pitted wing covers. It entered this country in 1974 near Charleston, South Carolina. Since then, it has spread into North Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, and Texas.
It’s much easier to detect the beetle’s presence than the insect itself. A telltale clue is a white, toothpicklike spike of boring dust that protrudes about 1 ½ inches from the trunk. Female beetles produce theses spikes as they excavate galleries or corridors inside the tree for laying eggs.
Branches wilt and die back. Sawdust protruding from holes in branches looks like tiny toothpicks stuck to the bark.
Solution: These tiny beetles attack both stressed and healthy plants. They lay eggs inside stems and introduce a fungus (ambrosia) with which to feed their young. The fungus clogs the plant’s water transport system and produces toxins, both of which result in wilting. You can reduce stress on plants by making sure they are watered correctly, fertilized annually, and kept free of disease. Once several beetles have invaded the plant, insecticides are not effective. Prune and destroy infested limbs. For prevention, thoroughly spray trunks of susceptible plants nearby with diazinon, endosulfan (Thiodan), or chlorpyrifos (Dursban).
Damage: These aggressive beetles attack both healthy and stressed trees. Attacks on healthy plants usually occur near ground level or at wound sites. As many as 50 beetles may infest a single tree. Beetles excavate a maze of tunnels and cultivate ambrosia fungus (Fusarium solani) on the tunnel walls to feed their developing young. Infested trees may wilt and die.
Life cycle: While adult beetles are present most of the year, they require high humidity to reproduce. Major activity occurs in March. Females bore into stems, twigs, branches or trunks of young trees. They deposit eggs within tunnels and introduce the fungus. Females remain with their young until they mature and exit the tree. Hatching females mate before leaving the tree to infest a new host.
Prevention: Keep trees vigorous with adequate watering and fertilization. Avoid wounding them. Spray trunks with chlorpyrifos (Dursban) or endosulfan (Thiodan) late in the day, so the chemical can dry overnight and avoid breakdown due to sunlight and heat. You can also spray with diazinon. Follow label directions carefully.
Control: Once beetles are in the tree, no chemical will help. Remove and burn the infested tree.
More on Crepe Myrtles
The crepe myrtles are among the most satisfactory of plants for the South: showy summer flowers, attractive bark, and (in many cases) brilliant fall color make them year-round garden performers. Long, cool autumns yield the best leaf display; sudden frosts following warm, humid fall weather often freeze leaves while they’re still green, ruining the show.
Most crepe myrtles in gardens are selections of L. indica or hybrids of that species with L. fauriei. The latter species has attracted much notice for its hardiness and exceptionally showy bark. Queen’s crepe myrtle, L. speciosa, grows only in the Tropical South.
All crepe myrtles bloom on new wood and should be pruned in winter or early spring. On large shrubs and trees, remove basal suckers, twiggy growth, crossing branches, and branches growing toward the center of the plant. Also gradually remove side branches up to a height of 4-5 feet; this exposes the handsome bark of the trunks. During the growing season, clip off spent flowers to promote a second, lighter bloom. Also prune dwarf forms periodically throughout the growing season, removing spent blossoms and thinning out small, twiggy growth.
Crepe myrtles are not usually browsed by deer.
L. fauriei. JAPANESE CREPE MYRTLE. Native to Japan. Tree to 20-30 feet tall and wide, with erect habit and outward-arching branches. Light green leaves to 4 inches long and 2 inches wide turn yellow in fall. Especially handsome bark: The smooth gray outer bark flakes awawy to reveal glossy cinnamon brow nbark beneath. Small white flowers are borne in 2- to 4-inch long clusters in early summer; often blooms again in late summer. Resistant to mildew and best known as a parent of hardy, mildew-resistant hybrids with L. indica, though it is handsome in its own right. ‘Fantasy,’ with even showier bark than the species, has a vase form — narrow below, spreading above. ‘Kiowa’ has outstanding cinnamon-colored bark.
L. indica. CREPE MYRTLE. The premier summer-flowering tree of the South. Tolerates heat, humidity, drought; does well in most soils as long as they are well drained. May be frozen to the ground in severe winters in the Upper South, but will resprout. Gardeners there should plant cold-hardy selections such as ‘Acoma,’ ‘Centennial Spirit,’ and ‘Hopi.’ Variable in size (some forms are dwarf shrubs, others large shrubs or small trees) and habit (spreading or upright). Dark green leaves are 1-2 ½ inches long and somewhat narrower, usually tinted red when new; they often turn brilliant orange or red in fall. Crinkled, crepe-papery, 1- to 1 ½-inch wide flowers in white or shades of pink, red, or purple are carried in dense clusters.
Trained as a tree, it develops an attractive trunk and branch pattern. Smooth gray or light brown bark peels off to reveal smooth, pinkish inner bark; winter trunk and branches seem polished.
Mildew can be a problem. Spray with triforine (Funginex) before plants bloom, or grow mildew-resistant hybrids of L. indica and L. fauriei. Almost all selections with names of Native American tribes, such as ‘Hopi,’ ‘Miami,’ and ‘Zuni,’ are mildew resistant.
L. speciosa. QUEEN’S CREPE MYRTLE. Tree to 25-30 feet tall, 15-25 feet wide. The showiest and most tender of the crepe myrtles, displaying huge clusters of white, pink, lavender or purple flowers in June and July. Individual blossoms reach 3 inches across. Large leaves (8-12 inches long, 4 inches wide) turn red in fall. Smooth, mottled, exfoliating bark. Rank grower; annual pruning in winter is especially important to control size and form.
DON’T COMMIT “CREPE MURDER”
Don’t chop your large crepe myrtles down to ugly stubs each spring just because your neighbors do. This ruins the natural form and encourages the growth of spindly, whiplike branches that are too weak to hold up the flowers. To reduce a crepe myrtle’s height, use hand pruners or loppers to shorten the topmost branches by 2-3 feet in late winter, always cutting back to a side branch or bud. For branches more than 2 inches thick, always cut back to the crotch or trunk. Don’t leave big, ugly stubs.
WHAT CREPE MYRTLES NEED
PLANTING SITE: A sunny location where the air moves freely will help limit powdery mildew and other diseases.
SOIL: Provide moist, moderately fertile, well-drained soil.
PRUNING: Crepe myrtles bloom on new wood, so prune them in late winter or early spring to increase next summer’s flower production. Pruning off old flowers in summer before they set seed may produce a second wave of blooms. Except on dwarf types, remove side branches on trunks up to the 4- to 5-foot level. To reveal attractive trunks, also remove branches growing inward toward center of plant.
Crapemyrtle bark scale often starts up higher on limbs, and sooty mold grows on the honeydew.
As a follow up to last week’s post about crapemyrtles, I want to provide some details about a new pest that has recently been messing with these beautiful plants. It is called Crapemyrtle Bark Scale (CMBS), an exotic scale pest that was discovered in the U.S. only 10 years ago first in the Dallas area. It is very similar to another scale that gets on azaleas and other woody plants, but was recently determined with pretty good confidence that this scale is different and is a newly introduced insect from the Far East.
It was first discovered on crapemyrtles in Richardson in a commercial planting. What drew attention to it was the blackened trunks and limbs of the trees. The black is caused by a saprophytic fungus called black sooty mold that grows on the sugary exudate that is excreted from the scale insects. While the black mold is not harmful to the tree, it sure makes it look unsightly.
The CMBS initially spread slowly in the Dallas/Fort Worth metroplex area for several years, but in the last few years it has now been found in southern Oklahoma, Arkansas, Louisiana and near Memphis, Tennessee. It was found in Tyler last year, and I have seen it on several plants in different locations this year.
The scale feeds on the sugary sap in the phloem that flows just under the bark of the trunk and branches. So far, it doesn’t seem the scales are causing a great deal of harm to the tree, but due to the copious amounts of black sooty mold that grows on the sweet substrate, the trees can look unsightly.
Most of the following information is taken directly from a new Extension publication: Crape Myrtle Bark Scale: A New Exotic Pest (EHT-049) available as a fee pdf file from the AgriLifeBookstore.org web site. Also, check out Dr. Mike Merchant’s Citybugs.tamu.edu website (search for ‘scales’) as he has been involved with this pest since its first report in Texas. In one of his posts, he includes a link to a recording of an informative webinar he gave last April concerning this pest. Note the authors of the publication have chosen the alternative spelling for the common name of this insect.
“Crape myrtle bark scale is relatively easy to identify. It is the only scale known to infest crape myrtles. Adult females are felt-like white or gray encrustations that stick to crape myrtle twigs, stems and trunks. When crushed, the scales exude bright pink “blood”-like liquid.
On new growth and in heavy infestations, the scales may be distributed uniformly on the branch. Up close, CMBS is white to gray and about 2 mm long. Look closely and you may see dozens of pink eggs or the smaller crawler stage.
Most gardeners are first alerted to CMBS by the presence of black sooty mold on the bark. The source of the sooty mold may be mistaken for that produced by the crapemyrtle aphid, a common insect pest the feeds on the leaves of some varieties. However, the presence of the white adult scales on the bark and twigs, and the pink blood exuded when crushed, tells you the sooty mold is from scales, not aphids.
Crape myrtle bark scales may aggregate toward the undersides of young horizontal branches instead of the parts exposed to the sun, and I have often seen them on old pruning cuts.
Control. Based on limited experience with this pest in the U.S., CMBS appears to be difficult to control. Soil-applied neonicotinoids have demonstrated significant suppression. The current best suggestions for controlling this insect include:
- Before buying crape myrtles, inspect plants carefully for signs of CMBS infestations.
- If your plants are heavily infested, wash the trunk and reachable limbs with a soft brush and mild solution of dishwashing soap and water. This will remove many of the female scales and egg masses, making insecticide control more effective. Washing will also remove much of the black mold that builds up on the bark of infested trees.
- Horticultural oil during the growing season has not yet been shown to be effective against this insect. However, it may be beneficial to apply horticultural oil in the winter at dormant season rates to the bark and crotches of the plants where the scales shelter. Use enough oil to reach behind loose bark and into cracks and crevices. Winter is a good time to treat for scales because the higher (dormant season) application rate can be used without damaging the plant. Cover the tree thoroughly with pesticide, especially when using oil.
- Systemic insecticides have shown the most promise in tests to date. Apply them to the root zone as a soil injection or drench. The best control in tests was achieved between May and July by applications of clothianidin, dinotefuran (Safari, Zylam), imidacloprid (Merit, Bayer Advanced Garden Tree and Shrub Insect Control), and thiomethoxam (Meridian). When drenching the soil with a systemic insecticide, allow several weeks for the product reach throughout the plant.
- Although some insect growth regulators are recommended for control of other types of scales in woody ornamentals,they have not yet been evaluated on CMBS.
Some lady beetle species, especially the twice-stabbed lady beetle are effective predators of this scale. However, control by lady beetles is often too late in the season to prevent aesthetic damage from sooty mold.”
See the above referenced publication for more details, including what is currently known about this pest’s biology, more photos and other details.
The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Texas A&M AgriLife Extension Service is implied.
Q: I have a crape myrtle and every year it is covered in a whitish powder. I have been told it is powdery mildew but I don’t know how to control it.
A: Powdery mildew is a plant disease caused by a fungus, Erysiphe lagerstroemiae. It is evidenced by white fuzz on leaves and shoots. Often the leaves are stunted and distorted. Although it usually is not fatal to the plant, it can cause dwarfing of new growth and death of foliage.
Crape myrtles need plenty of sun and air circulation to help fight this disease. The spores and mycelia of powdery mildew are sensitive to extremes of heat and sunlight. Counter-intuitively, rain does not increase the likelihood of powdery mildew, as the spores do not thrive in water.
The most effective way to control powdery mildew is to prevent it. Place crape myrtle trees in full sun, with enough space around them so they are not crowded.
It is wise to choose cultivars of Lagerstroemia that have a resistance to powdery mildew. Many have Native American nation names, such as “Catawba” (purple flowers, 15 by 15 feet), “Cheyenne” (red flowers, 10 by 10 feet) or “Natchez” (white flowers, 20 by 20 feet).
Less toxic fungicides are available including horticultural oils, neem oil, jojoba oil, sulfur, potassium bicarbonate and biological fungicides. Except for the oils, these materials are primarily preventive.
Preventive treatments are applied to susceptible plants before any sign of the disease. Eradicants can help manage an infection after it has started.
Oils work best as eradicants but have some preventive activity. Potassium bicarbonate is available to licensed applicators only.
Never apply oils to water-stressed plants or when the temperature is higher than 90 degrees. Never apply oils within two weeks of an application of sulfur, and never apply sulfur within two weeks of an application of oil.
Sulfur has been used to manage powdery mildew for centuries, but it is effective mainly as a preventive. Of the sulfur products, wettable sulfur formulated with a surfactant is the simplest and most effective for the home gardener.
Biological fungicides are available commercially. These products are usually a beneficial bacterium that can destroy fungal pathogens. They are mainly used as a preventive, although they may kill the mildew they contact.
It is essential to follow the directions on pesticide labels. Follow the fungicide directions exactly, and thoroughly dowse the plant with it. It may be necessary to repeat applications every seven to 10 days because the plant will grow and develop new shoots and leaves that are unprotected.
Many people fight powdery mildew by trimming back all affected branches, but this can reduce a floral display if done too late in the season.
The best practice is to choose a resistant variety of crape myrtle and plant it in the sun. The next best practice is to spray with a preventive fungicide early on. But starting now, a thorough dowsing with a horticultural or plant based oil spray will help reduce a light to moderate case of this fungal disease.
The University of California has additional information on managing powdery mildew on ornamentals and general care and pest management for crape myrtles at www.ipm.ucdavis.edu.
Molly K. Weden is a Contra Costa Master Gardener. The UC Cooperative Extension Master Gardener Help Desk is staffed 9 a.m. to noon Monday-Thursday, 925-646-6586, [email protected]