Greenhouse and Nursery - Treat for Powdery Mildew

Powdery mildew has really taken off these past two weeks in ornamental plants. I recently received reports of powdery mildew problems on fall asters grown by greenhouse growers with their mum crops. Treatments will be necessary. The following is more information on this disease.

Powdery mildews are one of the most common diseases of ornamental plants; many nursery, flower, and woody plants are susceptible. Greenhouse crops prone to infection include African violet, Begonia , Dahlia , gerbera daisy, Hydrangea , roses, Verbena, Kalanchoe, and poinsettia. Herbaceous perennials particularly susceptible to Powdery mildew include Aster, Centaurea, Coreopsis, Delphinium, Monarda, Phlox, Rudebeckia, and Sedum.

The disease is easily recognizable as a white to gray powdery growth on leaves and sometimes stems and flowers. Powdery mildew may have little or no affect on the plant (other than aesthetic) or it may cause infected leaves to distort, discolor, wither, and defoliate prematurely. Most Powdery mildews have evolved to avoid killing their hosts because they can only survive in living plant tissue. Symptoms and their severity depend upon the cultivar or species of host plant, the powdery mildew species, environmental conditions, and the age of plant tissue when it first became infected. Powdery mildew diseases are caused by species of fungi such as Erisyphe, Leveillula, Microsphaera, Podosphaera, Odium, and Sphaerotheca, although each powdery mildew is specialized to infect only hosts in one genus or one family. Infection does not spread to species of plants in other plant families. Erisyphe has a wide host range and can infect many plants in the Asteraceae family, while Sphaerotheca pannosa var. rosae is confined to roses.

The distinctive whitish powder on leaves is composed of fine threads of fungal vegetative tissue (mycelium) and light colored mats of asexual spores (conidia). Some Powdery mildews produce conidia on short, erect branches that resemble tiny chains, while others form threads so sparse that the mildew cannot be seen without the aid of a microscope. These spores are easily moved by air movement and water splash. Because Powdery mildews are obligate parasites, they do not require plant stress or injury to infect plants. When spores land upon a susceptible host, they germinate and send a specialized feeding structure into the epidermis and obtain their nutrients from the plants. The infection process may take as little as 3 days or as long as 7 days. The pathogen survives in the greenhouse in weed hosts or on crops. Outside, the pathogen can overwinter as mycelium in infected plant parts or in resting structures (cleistothecia) produced by sexual means and visible as small, dark specks on dying leaves. Powdery mildews, unlike most other fungal diseases, do not need free water to germinate and infect. They are favored by high relative humidity (greater than 95%), moderate temperatures (68°-86° F), and low light intensities. The disease is more prevalent in the spring and fall when large differences between day and night temperatures occur. Temperatures above 90° F kill some Powdery mildew fungi and spores and the presence of free water can reduce spore germination.

Monitor crops on a regular basis for Powdery mildew diseases. Epidemics that seem to develop overnight are often the result of undetected low level infections that have spread spores throughout the greenhouse. Rogue infected plants or prune out diseased tissue. Perform this operation when plants are wet or immediately place diseased material into a plastic bag to prevent spores from spreading. The use of resistant cultivars or species is a good management tactic. Although few ornamental crops have been bred for resistance, cultivars of African violet, Begonia, rose, pansy, Zinnia, Monarda, and Phlox with resistance are available. Avoid overcrowding of plants and provide good air movement. Keep relative humidity levels low in the greenhouse by a combination of heating and venting in late afternoon and early morning. Clean greenhouse thoroughly between crops, eliminating all weed hosts and volunteer plants. Unlike most fungi, powdery mildews only colonize the surface of plants making chemical eradication possible. It is not necessary to use fungicides to prevent Powdery mildews. Fungicides with the active ingredients propiconazole (Banner Maxx), myclobutanil (Eagle, Systhane), triadimefon (Bayleton, Strike), fenarimol (Rubigan), thiophanate methyl (Cleary's 3336), potassium bicarbonate (Armicarb, MilStop), or sulfur are registered for Powdery mildew control on ornamentals. Sulfur may cause plant injury if applied when temperatures are high (greater than 85° F). Because the genera and species of fungi causing Powdery mildews are diverse, there may be some variation in fungicide effectiveness across crops. The Powdery mildew fungi can develop resistance to any of the fungicides, except sulfur, listed above so be sure to alternate fungicide applications among chemical classes.

Information from a UMass factsheet on powdery mildew prepared by M. Bess Dicklow, UMass Plant Extension Diagnostic Lab, 108 Holdsworth Hall, University of Massachusetts , Amherst , MA http://www.umass.edu/umext/floriculture/fact_sheets/pest_management/powdery_mild.htm

Greenhouse, Nursery, and Landscape - Powdery Mildew Control Material Trials

The following is a great report on powdery mildew control using fungicides from the Chase Research Gardens. The Chase company is a great resource and has many great products to offer for the horticulture industry. Go to http://www.chasehorticulturalresearch.com/ for more information.

Powdery Mildew Chemical Control Update
A. R. Chase Chase Research Gardens, Inc.

I decided it might be a good time to review some of the research on powdery mildew. We spent a lot of time last year fighting with the disease on various crops. First, we could not make it happen no matter what we tried, even on Gerber daisy. Then when we did have the right weather, we could not seem to get any control even with the best products.

Some of our most recent trials were on roses and they really do seem to be one of the toughest crops to control powdery mildew on. We also tested Gerber daisies, crape myrtle and zinnias in this time frame. Results of national trials for chemical control of powdery mildew are summarized in the table below.

It is interesting to see what all of the researchers are testing as well as what specific results are. Eight researchers from Michigan, New York, Alabama and California performed trials on powdery mildew of rosemary, aster, miniature rose, sedum, poinsettia, phlox, Monarda, crape myrtle, azalea, hydrangea, salvia, Gerber daisy, dogwood, Scabiosa, verbena, zinnia and Ranunculus. While this list does not include all ornamentals that get powdery mildew it certainly does show an effort to work on a wide range of crops.

The fungicides that are colored the same belong to the same chemical class. You can see that a few chemical groups are very good to excellent for control of powdery mildew. The sterol inhibitors (in blue), including Banner MAXX, Bayleton, Immunox, Rubigan, Strike, Systhane (now Eagle) and Terraguard, are a key chemical class for fighting powdery mildew. In addition, the strobilurins (in pink), including Compass O, Cygnus, Heritage and BAS500 (an experimental compound from BASF Corporation) are also very effective and have become standards for powdery mildew control on some important crops at least. These two large chemical groups do not even scratch the surface of products for powdery mildew control. Copper products like Camelot and Phyton 27, oils like Triact and Sunspray and potassium bicarbonates like Kaligreen and Milstop also give good results.

One interesting development is the continued use of thiophanate methyl products (in green) for powdery mildew control. These products were excellent for control for years but rapidly developed resistance if they were overused. The results for Fungo, Spectro, Zyban and 3336 show some of this condition. At times they may give excellent control but if resistance to thiophanate methyl has developed for a particular powdery mildew fungus they will not be effective.

As this illustrates, rotation for resistance management is critical. Make sure you use products from at least two chemical groups in an alternating pattern or in a tank mix.

Conclusions

Many ornamental plants are attacked by powdery mildew fungi we have a very large arsenal of chemicals for their prevention. In many cases, we can even eradicate an infection once it is found instead of spraying preventatively. There are a few plant species that are so badly affected by powdery mildew that eradication is not usually effective. For these crops, like some rose cultivars, you should probably spray preventatively when the conditions are favorable for disease.

Information reprinted from "Powdery Mildew Chemical Control Update" by the A. R. Chase
Chase Research Gardens, Inc. http://www.chasehorticulturalresearch.com/pdfs/pm_2.pdf

Landscape - Powdery Mildew Showing Up on Crape Myrtles This Year

Powdery mildew has been showing up on crape mrytles this year, even in resistant varieties. The following is more information.

Powdery mildew on crape myrtle has not been a problem for many years since the introduction of new cold tolerant varieties (devl. at the National Arboretum and other places) that also happen to have powdery mildew resistance. But, this season has been so favorable for powdery mildew (especially the last several weeks) that we have seen powdery mildew on flower trusses of some of these cultivars. Remember resistance is not immunity. While levels have been low, homeowners have probably never seen powdery mildew on their crape myrtles before. While infection doesn't affect overall plant health, it may be severe enough to prevent flower buds from opening.

Information from Bob Mulrooney, Extension Plant Pathologist, UD

Landscape - Current Pests

The following are some pests currently active in DE landscapes that you should watch for.

First generation fall webworm is being found on apple, crabapple and persimmon. Don't think it's the old tent caterpillar tents! Mechanical disruption of the web now will lessen the second generation.

The “Shothole” damage from Tortoise beetle feeding is prevalent on annuals (Ipomea ,etc.) and some perennials.

Rose midge is active this year again in many locations. Look for the “blighted” shoot tips on the terminal growth only.

Powdery mildew is all over crepe myrtle, lilacs, and other shrubs and flowering trees due to warm moist conditions combined with succulent new growth. Fungicides and sanitation may be required to prevent spread or overwintering.

Cicada Killer wasps are solitary wasps and are very unlikely to sting. Males fly at people, dogs, cats, other insects, etc. but are unable to sting. Females prefer to dig holes and sting cicadas. Treatments are rarely warranted.

Information from the current edition of the Ornamentals Hotline Newsletter from the University of Delaware Cooperative Extension.

Landscape - Powdery Mildew on Dogwood

It is powdery mildew season for dogwoods in the landscape. The following are some control recommendations.

Disease management. Powdery mildew can be confronted by using cultural practices, planting resistant dogwoods, and by using fungicides.

Avoid cultural practices that stimulate succulent growth and encourage powdery mildew. These include applying nitrogen fertilizer, pruning heavily, and irrigating excessively.

Use good cultural practices such as mulching over the root system, pruning out dead branches, and providing good air movement and light penetration by judicious pruning of nearby vegetation.

Plant dogwood species and cultivars resistant to powdery mildew.

Susceptible: All Cornus florida, seedling wild types (but individuals vary in susceptibility) and most C. florida cultivars.

Intermediate susceptibility: C. florida ‘Cherokee Brave’ and cultivars of the C. florida x C. kousa hybrids.

Resistant: Four powdery mildew resistant C. florida cultivars have been developed by the Tennessee Agricultural Experiment Station and are available in the nursery trade. They include ‘Jean’s Appalachian Snow’, ‘Karen’s Appalachian Blush’, ‘Kay’s Appalachian Mist’, and ‘Appalachian Joy’. Also resistant: Cultivars of C. kousa, oriental dogwood.

Immune: Cornelian cherry dogwood, C. mas.

If fungicides are to be used, determine which trees in the landscape are most susceptible so that applications are not made unnecessarily. Those trees most at risk for powdery mildew disease then can be considered for preventive fungicide applications. Most fungicides are capable of stopping the progress of powdery mildew infections fairly quickly, but none will restore already discolored or damaged leaf tissues. Good control can be obtained with as few as four fungicide applications made three weeks apart. Begin applications by early June. Delayed application can still help protect some leaves.

Effective fungicides include:
azoxystrobin (Heritage)
fenarimol (Rubigan)
myclobutanil (Eagle, Immunox, Procoz Hoist)
propiconazol (Banner Maxx, Procon-Z, Procoz Fathom, Propensity)
thiophanate-methyl (Cleary’s 3336)
triadimefon (Bayleton, Strike)

Powdery mildew fungicides requiring more frequent applications to be effective include:
neem oil (Triact)
potassium bicarbonate (Bonide Remedy, FirstStep, Kaligreen, Milstop)
paraffinic oil (Sunspray UF Oil)

When using fungicides for powdery mildew management, be sure that dogwoods are listed on the label and carefully follow all label directions.

Information from an article by John Hartman in the current edition of the Kentucky Pest News http://www.uky.edu/Ag/kpn/kpn_09/pn_090623.html

Landscape - Diseases Currently Present

The following are some diseases now active to be on the watch for in the landscape.

CEDAR-QUINCE RUST is present on Washington hawthorn, Crataegus phaenopyrum. Symptoms include old swollen twigs from last year and swollen deformed fruit (very evident now). Fungicide control is rarely needed. The same fungus infects serviceberry-- seen this past week. The alternate host is juniper, especially ground cover species. Cedar-quince rust rarely causes any serious damage to the host, just loss of fruit display and dead shoots, which can detract from overall aesthetics.

POWDERY MILDEW is present in many perennials. Look for infection on peony, Monarda (Bee-balm), garden phlox and other susceptible plants. It is controlled well with a number of fungicides (apply at the first sign of disease) as well as hort oil, neem oil, and potassium bicarbonate products.

SCLEROTINA SHOOT BLIGHT OF FORSYTHIA was diagnosed twice last week-- from a homeowner sample and one from my yard. Look for dead shoots that often have an orange discoloration. The fungus infects the blossoms and flower stalks, then grows into the twigs and kills them for some distance. The black sclerotia (fungus fruiting bodies) develop inside the infected twigs and are the best diagnostic sign at this point. Prune and destroy dead twigs and stems.

OAK LEAF BLISTER caused by the fungus Taphrina is a common disease during wet seasons of many oaks in the red oak group but mostly pin oak and red oak. The fungus overwinters in infected buds and infects the newly emerging leaf tissue. Symptoms include scattered circular raised areas of varying sizes up to ½ inch over the upper surface of the infected leaves causing a depression on the corresponding lower leaf surface. Lesions are yellow-green when new and brown with age. This disease causes no defoliation and rarely requires chemical control.

Information from Bob Mulrooney, Extension Plant Pathologist, UD

Landscape - Photos of Diseases from Recent Post

The following are photos to go with the recent post of current diseases in the landscape.

Oak leaf blister. Photo by Michael Kangas, NDSU - North Dakota Forest Service, Bugwood.org

Powdery mildew on Phlox leaf. Photo from Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, Bugwood.org

Cedar-Quince rust on Hawthorn. Photo from the Purdue University Plant Diagnostic Lab.

Cedar-Quince rust on Hawthorn twig. Photo from the Purdue University Plant Diagnostic Lab.

Landscape and Nursery - Powdery Mildew on Dogwood

Powdery mildew on flowering dogwood is back again. The following is more information.

Flowering dogwood trees near Townsend Hall on the University of Delaware campus were observed with powdery mildew symptoms on May 13, which is very early. Now is the time to apply a fungicide to valuable susceptible trees. Look for the white talcum-like growth on the leaves in the center of the tree or on water sprouts. When powdery mildew shows up early on dogwood I recommend that infected trees be sprayed to control this potentially debilitating disease. Homeowner labelled fungicides include, Immunox, Infuse, Bayer Advanced Tree, Shrub and Flower Disease Control, and others. I have had very good success with applications of 1% solutions of Sunspray horticultural oil, neem oil (Triact), and potassium carbonate, (Kaligreen, Remedy). Commercial applicators can use Eagle, Cleary's 3336, Spectro, Banner MAXX, Heritage, Compass and others. Apply when first signs appear and reapply every 2-3 weeks until early August. If you use horticultural oil, neem oil or potassium bicarbonate time those sprays to cover new flushes of growth during the season for the best protection.

Powdery mildew on dogwood in a nursery. Photo from the University of Kentucky, http://www.ca.uky.edu/HLA/Dunwell/KHC/NurseryUpdate13_2003.html
Information from Bob Mulrooney, Extension Plant Pathologist, UD.

Landscape - Powdery Mildew on Annuals and Perennials

The following is a good article on powdery mildew on annuals and perennials in the landscape.

Powdery mildew fungi attack many different kinds of herbaceous annual and perennial plants in the landscape. The white, dusty appearance which indicates the presence of powdery mildew on these plants results from fungal hyphae growing on the surface of infected leaves, shoots, and flowers while producing massive amounts of spores, or conidia. In addition to these fungal signs, powdery mildew can cause infected leaves to turn yellow, wither and die, or it may cause leaves and shoots to be abnormally curled and distorted. Powdery mildews generally do not kill their hosts, but infected plants are less productive and not aesthetically pleasing.

Although all powdery mildew fungi growing on plant surfaces tend to look alike, many are fairly host specific i.e., dogwood powdery mildew does not affect zinnia, or phlox powdery mildew does not affect rose. However, some powdery mildew species affect several herbaceous ornamentals and some plants are attacked by more than one species of powdery mildew. Examples of the powdery mildew fungi that attack flowering and foliage plants in the garden include Erysiphe cichoracearum, E. polygoni, Microsphaera begoniae, Oidium begoniae, O. cyclaminis, O. spp., O. verbenae, Sphaerotheca fuliginea, and many other species.

Conditions favoring infection. Powdery mildew fungi are obligate parasites and thus require a living plant host to complete their life cycle. Spores are wind dispersed and when they land on a plant they can germinate and cause infection. Powdery mildew infections are favored by moderate to cool temperatures and high relative humidity. Powdery mildew spores rarely germinate when leaves are wet. As the disease develops on the plant, large numbers of spores can be produced in a matter of a few days, quickly resulting in secondary infections. Continued infections are favored by a humid or almost moist microclimate at the plant surface. Shaded leaves are sometimes more heavily infected because the microclimate favors disease development. Vigorously growing plants tend to be more susceptible to powdery mildew infection than plants growing under stressful conditions. Powdery mildew-infected weeds may be a source of inoculum for garden annuals and perennials.

Powdery mildew diseases may begin early in the season on many flowering plants in the garden. By mid-season many of these plants are practically defoliated by the disease. Reduced photosynthesis by infected or lost leaves reduces flower production. Annuals and perennials that often suffer from powdery mildew in Kentucky gardens include begonia, chrysanthemum, cosmos, crape myrtle, dahlia, delphinium, hollyhock, hydrangea, impatiens, monarda (bee balm), ornamental pepper, primrose, pansy, petunia, phlox (Figure 1), snapdragon, rose (Figures 2, 3, & 4), rudbeckia, verbena, viola and zinnia (Figure 5).

Disease management. Powdery mildew management typically involves an integrated approach to reduce the effects of the disease.

1) Choose powdery mildew resistant or tolerant types. Sometimes seed and nursery catalogs will list plants with powdery mildew resistance or tolerance. There are powdery mildew resistant cultivars of crape myrtle, monarda (bee balm), pansy, viola, and zinnia, for example. Extension publications listing resistant varieties are another good source.

2) Use optimal plant spacing so that plant crowding in the beds doesn’t result in humid conditions favoring powdery mildew.

3) Provide ventilation and sunlight penetration into the foliage by pruning out shading vegetation from nearby plants or from within the mildew-susceptible plants.

4) For some kinds of plants, fungicide applications may be needed. Fungicides for powdery mildew which contain ingredients such as myclobutanil, triforine, thiophanate-methyl, or sulfur would be examples.

Powdery mildew on Gerber Daisy. Photo by Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org.

Article reprinted from "Annuals and Perennials Are Susceptible to Powdery Mildew" by John Hartman in the current edition of the Kentucky Pest News from the University of Kentucky.

Nursery, Greenhouse, and Landscape - New Fungicide Available

The following is information on a new fungicide available for the green industry from Bob Mulrooney, Extension Plant Pathologist, UD.

A new fungicide has been labeled for use on turf and ornamentals in greenhouses, shade houses, container nurseries, retail, conifer and forest nurseries, commercial and residential landscapes, and interiorscapes. It is called Pageant. Pageant from BASF is a combination product of pyraclostrobin (Insignia) and boscalid (Emerald). Those of you that know the vegetable and fruit fungicides will recognize Pristine which is the equivalent combination in Pageant.

I have tested it for powdery mildew and black spot control on hybrid tea rose and found it to be pretty effective at the high rates for black spot but not very effective on powdery mildew.
Trial results are below.

It has a wide label and could be very effective for the diseases on the label. You beed to check the labels for plants with sensitivity such as some grape cultivars, ninebark and Wintercreeper , Euonymus vegetus.

The label can be found at http://www.cdms.net/LDat/ld8JM000.pdf

Click on table for larger version in a new window.

Information from Bob Mulrooney, Extension Plant Pathologist, UD

Nursery, Greenhouse, and Landscape - Powdery Mildew Control Starts With Plant Selection

We had a bad powdery mildew year in 2008. Powdery mildew control starts with plant selection. Choose plants that are not susceptible to powdery mildew or those varieties that have been bred or selected for powdery mildew resistance. The following is a list of plants where powdery mildew is common.

Some of the more common hosts of powdery mildews include: apple and crabapple, azalea and rhododendron, ash, basswood, beech, Berberis, birch, blueberry, buckeye, catalpa, Chinese photinia, chrysanthemum, cotoneaster, crape myrtle, dahlia, delphinium, elm, eucalyptus, euonymus, flowering dogwood, gardenia, hawthorn, holly, honeysuckle, horse chestnut, hydrangea snowball, kalanchoe, Kalmia, leucothoe, ligustrum, lilac, Lonicera, lilac, magnolia, maple, monarda, oak, phlox, Prunus (peach, plum, cherry, apricot), pear, peony, poplar, privet, pyracantha, Reiger begonia, roses, serviceberry, spirea, smoke-tree, snapdragon, sycamore, tulip tree, Vaccinium, viburnum, walnut, wintercreeper, willow, wisteria, and zinnia.

Information from the Plant and Pest Advisory from Rutgers University http://njaes.rutgers.edu/pubs/plantandpestadvisory/2008/ln0918.pdf

Landscape - Pictures of Peony Blotch and Powdery Mildew on Peony

The following are pictures of two diseases on Peonies - powdery mildew and peony leaf blotch.



Gordon Johnson, Extension Horticulture Agent, UD, Kent County

Landscape, Greenhouse, and Nursery - Expanded List of Plants Susceptible to Powdery Mildew

Powdery mildew season is here. The following is a list of some common landscape plants that are susceptible to powdery mildew to expand on the previously provided list.

Some ornamental plants susceptible to powdery mildew

• ash
• azalea, rhododendron
• basswood
• beech
• begonia
• Berberis
• birch
• blueberry
• buckeye
• catalpa
• Chinese photinia
• chrysanthemum
• cotoneaster
• crape myrtle
• dahlia Reiger
• delphinium
• elm
• eucalyptus
• euonymus
• flowering dogwood
• gardenia
• hawthorn
• holly
• honeysuckle
• horse chestnut
• hydrangea snowball
• kalanchoe
• Kalmia
• leucothoe
• ligustrum
• lilac
• Lonicera
• magnolia
• maple
• monarda
• oak
• pear
• peony
• phlox
• poplar
• privet
• Prunus (peach, plum, cherry, apricot)
• pyracantha
• roses
• serviceberry
• smoke-tree
• snapdragon
• spirea
• sycamore
• tulip tree
• Vaccinium
• viburnum
• walnut
• willow
• wintercreeper
• wisteria
• zinnia

Information from Ann B. Gould, Ph.D., Specialist in Plant Pathology in the July 12, 2007 edition of the Plant and Pest Advisory; Landscape, Nursery and Turf Edition; from Rutgers University.

Landscape and Nursery - Plants that are Susceptible to Powdery Mildew

Some Ornamental Plants that are Susceptible to Powdery Mildew are shown below.

Scientific Name/Common Name
Plants listed in boldface are most commonly affected by powdery mildew.

Begonia sp. Begonia
Cotinus coggygria Smoke-tree
Cotoneaster sp. Cotoneaster
Crataegus sp. Hawthorne
Eucalyptus sp. Eucalyptus
Euonymus sp. Euonymus
Hydrangea sp. Hydrangea snowball
Lagerstroemia indica Crape myrtle
Leucothoe sp. Leucothoe
Ligustrum vulgare Privet
Lonicera sp. Honeysuckle
Magnolia sp. Magnolia
Malus sp. Apple, Crabapple
Phlox paniculata Phlox
Photinia serrulata Chinese photinia
Platanus sp. Sycamore
Prunus sp. Peach, Plum, Apricot
Pyracantha sp. Pyracantha
Pyrus sp. Pear
Quercus sp. Oak
Rhododendron sp. Rhododendron, Azalea
Rosa sp. Rose
Salix sp. Willow
Spiraea sp. Spiraea
Syringa vulgaris Lilac
Vaccinium sp. Blueberry
Viburnum sp. Viburnum, Snowball
Wisteria sp. Wisteria
Zinnia elegans Zinnia

Information from North Carolina State University Plant Pathology Extension

Landscape and Nursery - Powdery Mildew Season

Powdery mildew season is upon us. The following is information on this disease group that attacks many of our ornamental plants.

General Information

The fungi which cause diseases known as powdery mildew attack a wide variety of ornamental plants grown in Delaware.

The damage due to infection by the fungi causing powdery mildew can be slight to severe. The disease generally occurs in Delaware during late spring, late summer and fall in cool to warm weather with high relative humidity and in shady areas.

Symptoms

A powdery, fluffy white to light gray-colored fungus growth on succulent stems, leaves, buds and flowers is the characteristic symptom of powdery mildew. Young plants and actively growing shoots are more severely damaged than older plants, leaves or branches. Infected leaves may be dwarfed, curled or deformed. Powdery mildew can destroy the blossoms on crape myrtle. Small dead flecks in the leaves and defoliation may occur on some varieties of azalea and rhododendron that are infected with powdery mildew without the obvious development of white fungus growth.

Disease Cycle

The white mildew on the plant surface is actually composed of the threads (mycelium) and asexually produced spores (conidia) of the powdery mildew fungus. These spores are wind-blown to other parts of the same plant or other plants of the same species. Powdery mildew fungi are quite host-specific, so for example the mildew on zinnia will not spread to dogwood or sycamore, and vice-versa. Also, they are obligate parasites, meaning that they can only grow on living plant tissue.

Some powdery mildew fungi survive the winter as colonies of mycelium, but many switch over to sexual reproduction in the fall, producing minute brown to black specks amid the old mycelium on the dying leaf or other plant part. These survive the winter and in the spring release another type of spore to start the cycle over.

Control

Some highly susceptible plants, such as Chinese photinia or euonymus should be replaced with a similar plant that is not susceptible to powdery mildew. On many trees, the disease causes little or no damage, therefore, control is not necessary. Plants in the landscape which may require fungicide applications to prevent powdery mildew damage include crape myrtle, phlox, rose and zinnia.

If damage from powdery mildew is severe and a susceptible plant(s) must be grown in the landscape, prune out severely diseased portions and use one of the following fungicides according to label instructions:

Propiconazole (Banner Maxx) is registered for control of powdery mildew and numerous other diseases on ornamentals.

Myclobutanil (Systhane, Eagle, and Immunox) is registered for control of powdery mildew and numerous other diseases on ornamentals.

Sulfur as a spray or dust as needed. Sulfur may cause some plant injury if applied when air temperature exceeds 90 F.

Triforine (Funginex) is labeled for use on roses and several other ornamentals. Follow manufacturer's instructions on the label. This product is only available in small packages.

Triadimefon (Bayleton and Strike) is now registered for use on a number of ornamentals.
Fenarimol (Rubigan) is a locally systemic fungicide for the prevention or therapeutic control of powdery mildew of field or landscape grown ornamentals.

In some situations copper fungicides will provide control of powdery mildew.

Note: The powdery mildew fungi can become resistant to any one of the fungicides listed above except sulfur.

Information modified from "Powdery Mildew of Ornamentals and Shade Trees" Ornamental Disease Information Note 4, R.K. Jones, Extension Plant Pathologist and D.M. Benson, Plant Pathologist, North Carolina State University Plant Pathology Extension

Ornamentals Hotline Disease Pictures for Issue 21, August 17, 2007

Drought and heat effects on perennials - Hosta

Photo by Gordon Johnson, University of Delaware

Brown patch produces characteristic symptoms on tall fescue leaves: irregularly shaped lesions that are tan in color with a dark brown border.

From a distance, brown patch appears in brown, tan, or yellow patches ranging from 6 inches to several feet in diameter. The infected leaves typically remain upright, rather than collapsing to the ground.

Brown Patch Pictures From North Carolina State University Plant Pathology Extension
Turfgrass Disease Information Note 15 (TURF-015)

Powdery Mildew on Lilac

Photo from Iowa State University Plant Disease Clinic