Fruit Tree Powdery Mildew Control – Treating Fruit Tree Powdery Mildew
By: Mary Ellen Ellis
Powdery mildew is a fungal infection that can affect a lot of different types of fruit trees and berry brambles. It can be damaging to yield because it tends to infect new growth, buds, and flowers, causing fruit to either not develop or to be stunted and damaged. Be aware of this disease and know how to prevent and treat it before it ruins your fruit harvest.
Recognizing Fruit Trees with Powdery Mildew
Knowing how to treat powdery mildew requires being able to recognize it. The clearest sign of powdery mildew is the characteristic spores and mycelium. They are white or gray in color and appear like a powdered substance on both sides of leaves. Less commonly you will see the powder on new shoots and flowers. When flowers are infected, the fruit will then either not set or be stunted, russeted, or will develop rough spots.
How to Treat Powdery Mildew
Treating fruit tree powdery mildew once you see the signs of disease can be attempted with fungicides and cultural practices. If there are only a few areas of infection, trim off those shoots and destroy them. Look for early signs as new shoots form. The new leaves will be puckered. If you can trim them off early enough, you may be able to prevent widespread infection.
Powdery mildew on fruit trees can be treated with the right fungicides at the right time if trimming off affected shoots does not prevent the spread of spores. A present infection requires an eradicant fungicide, so be sure you get the right kind. A protectant fungicide will only help prevent the infection in healthy trees.
Some fungicides that may eradicate a powdery mildew infection are horticultural oils, sulfur, and biological fungicides. Sulfur has to be applied before symptoms show up in a tree, and biological products use bacteria that consume the mildew. The latter is not as effective as oils or sulfur.
Good fruit tree powdery mildew control should also include preventative measures. Start by choosing less susceptible varieties. These are available for strawberries, apples, raspberries, cherries, plums, and peaches. Blackberries are always immune.
Plant fruit trees with enough space to provide good air flow and keep them pruned for good flow between branches on each tree. Avoid too much shade and excessive amounts of fertilizer. Overhead watering can actually help in the case of powdery mildew, as it washes spores off leaves.
With good prevention, cultural practices, and fungicides when needed, you can avoid big losses from powdery mildew.
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George Sundin and Amy Irish-Brown, Michigan State University Extension, Department of Plant Pathology - June 8, 2010
Apple powdery mildew is caused by the fungus Podosphaera leucotricha. This fungus grows as a white mass on new terminal growth of trees, eventually enveloping shoot tips. These symptoms can result in loss of vigor and potential effects on return bloom and yield of bearing trees and stunted growth of nonbearing trees. In addition, powdery mildew infection of fruit can cause russeting if the disease is not controlled before fruit are present. The powdery mildew fungus is unique in that it can infect trees in the absence of wetting from rain or dew.
Apple powdery mildew is effectively controlled by sterol-inhibitor (SI) fungicides or strobilurin fungicides. However, because of resistance in the apple scab fungus to these classes of fungicides, there has been a sharp cutback on use of these materials in Michigan in 2010. The result – powdery mildew infection is fairly common this year, with infection as much as 30-50 percent in some orchards.
The peak risk for mildew infection is the period right after petal fall, which coincides with rapid leaf growth of trees. This timing is likely when the initial mildew infections occurred this year in Michigan. Since only new unfolding leaves are susceptible to infection, and since infection risk ends when the trees set terminal buds, the questions of the day are (1) “What should be done, if anything, in orchards with current heavy mildew infection?” (2) “What should be done on trees with no fruit?”
Besides the impact of mildew on the current season, buildup of mildew infection will typically ensure the presence of plenty of inoculum for next season. Thus, control is probably warranted if trees are still actively growing. This is particularly important on young trees, where active growth continues, and the consequences of mildew infection are more severe.
Here are some suggestions for mildew control in orchards exhibiting infection currently. We consulted with Dr. David Rosenberger from Cornell University as well to determine possible fungicides to utilize at this time. On nonbearing trees or trees where the fruit crop has been lost to frost, further mildew spread can be effectively slowed with sulfur applications at 10 lbs per acre applied every two weeks. A reduced amount of sulfur should be used on bearing trees to avoid scorching the fruit. This would be the cheapest option to arrest further mildew infection.
On young trees, it is likely worth the investment in an SI, such as Rally, to help limit the inoculum carryover to next year. The new SI fungicide Topguard that was recently registered is an excellent mildewcide and could be used, if it is available. There is some concern with possible development of resistance to SI’s in the mildew fungus in New York. We don’t have any evidence currently to suspect a resistance problem in Michigan. Flint and Sovran, Bayleton, Topsin M, and Pristine are also effective fungicides for powdery mildew control. EBDC’s and Captan are not effective, and may kill other fungi that can inhibit powdery mildew somewhat, making the problem worse.
The powdery mildew fungus overwinters in infected buds. Winter temperatures below -5°F can eliminate some of this inoculum as infected buds can be killed following exposure to these low temperatures (killing the fungus with it). However, according to Dave Rosenberger, the only locations that can really count on cold temperatures taking care of powdery mildew inoculum are locations such as the Champlain Valley in Vermont, which can get temperatures as low as -40°F. Thus, the more mildew can be controlled now the less chance there is for extensive inoculum to be present next year.
Photo 1. Here's a single leaf showing anadvanced level of powdery mildew.
Photo 2. Powdery mildew affecting agrowing apple shoot.
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In New England, spray applications specifically targetting powdery mildew usually are not generally necessary, though warm, dry springs can lead to increased infection of susceptible varieties. For those varieties, in blocks with a history of powdery mildew, scab managment fungicides used from pink through second cover should also have activity against powdery mildew. During extended dry, warm weather during this period, specific applications for mildew may be needed, even when scab fungicide sprays are not.
The most common multi-site fungicides, captan and mancozeb, are ineffective against powdery mildew. DMI fungicides vary in effectiveness. Unfortunately, the DMIs most effective against powdery mildew, Rhyme, Rally, Rubigan and Procure, are least effective against scab, and vice-versa. QoI fungicides, Flint, Flint Extra and Sovran (and pre-mixes with Group 11 ingredients) have good efficacy against powdery mildew. SDHI fungicides, Aprovia, Fontelis and Sercadis (and pre-mixes with Group 7 ingredients) are somewhat less effective, but still provide good control.
Low rates of sulfur are effective in low disease pressure environments, such as New England, but the risk of sulfur injury increases as temperatures go over 85°F. In organic production systems, sulfur applied at weekly intervals, and bicarbonate and peroxide-based fungicides applied on 3-5 days intervals are the best options.
How to Manage Pests
Powdery Mildew on Fruits and Berries
White, flattened, pointed apple buds are overwintering sources of powdery mildew, Podosphaera leucotricha.
Powdery mildew is a common disease on many types of plants. Several powdery mildew fungi cause similar diseases on different plants (such as Podosphaera species on apple and stone fruits Sphaerotheca species on berries and stone fruits Erysiphe necator on grapevines, see Table 1). Powdery mildew fungi generally require moist conditions to release overwintering spores and for those spores to germinate and infect a plant. However, no moisture is needed for the fungus to establish itself and grow after infecting the plant. Powdery mildews normally do well in warm, Mediterranean-type climates. Thus powdery mildews are more prevalent than many other diseases in California’s dry summer and fall seasons.
IDENTIFICATION AND DAMAGE
Powdery mildew can be recognized easily on most plants by the white to gray powdery mycelium and spore growth that forms on both sides of leaves, sometimes on flowers and fruit, and on shoots.
The disease can be serious on woody plants such as grapevines, caneberries, and fruit trees where it attacks new growth including buds, shoots, and flowers as well as leaves. New growth is dwarfed, distorted, and covered with a white, powdery growth. On apple and grape and to a lesser extent apricot, nectarine, and peach, infected young fruits develop weblike, russetted scars. On tree fruits a rough corky spot on the skin will develop where infection occurred. Grapes with a severe infection may also crack or split and fail to grow and expand.
On strawberry, affected leaf edges curl upward. Infected leaves later develop dry, brownish patches along with nondescript patches of white powdery fungus on the lower surface and reddish discoloration on the upper surface. When foliage infections are severe, flowers and fruit may also be infected.
All powdery mildew fungi require living plant tissue to grow. On deciduous perennial hosts such as grapevine, raspberry, and fruit trees, powdery mildew survives from one season to the next in infected buds or as fruiting bodies called chasmothecia, which reside on the bark of cordons, branches, and stems. On strawberry the fungus can survive on leaves that remain on the plants through winter.
Most powdery mildew fungi grow as thin layers of mycelium on the surface of the affected plant part. Spores, which are the primary means of dispersal, make up the bulk of the powdery growth and are produced in chains that can be seen with a hand lens. In contrast, spores of downy mildew grow on branched stalks that look like tiny trees. Also downy mildew colonies are gray instead of white and occur mostly on the lower leaf surface.
Powdery mildew spores are carried by wind to host plants. Although humidity requirements for germination vary, many powdery mildew species can germinate and infect in the absence of water. In fact, spores of some powdery mildew fungi are killed and germination and mycelial growth are inhibited by water on plant surfaces. Moderate temperatures and shade are generally the most favorable conditions for powdery mildew development, since spores and mycelium are sensitive to extreme heat and direct sunlight.
The best method of powdery mildew control is prevention. Avoiding the most susceptible varieties and following good cultural practices will adequately control powdery mildew in many situations. However, where conditions are favorable, susceptible fruit trees and berries may require protection with fungicide sprays. Fungicide applications are most often needed on susceptible varieties of apple and on almost all grape and strawberry varieties.
Where possible, choose resistant varieties that meet your growing requirements and personal preferences. Be aware that control actions will probably be necessary when planting more susceptible varieties.
Apple. The most resistant varieties are Red Delicious and Stayman Winesap. Moderately susceptible varieties include Braeburn, Golden Delicious, Granny Smith, Jonagold, and McIntosh. The most susceptible varieties include Gravenstein, Jonathan, Rome Beauty, and Yellow Newtown.
Caneberries. Blackberry is not affected by powdery mildew. Resistant raspberry varieties include Chief, Marcy, Malling Orion the variety Logan is immune. Highly susceptible raspberry varieties include Glen Clova, Latham, Ottawa, and Viking.
Cherry. The most susceptible varieties are Bing, Black Tartarian, and Rainier.
Grapevines. Most varieties are susceptible.
Nectarine. Most varieties are susceptible.
Peach. Freestone varieties such as Crest, Flame Crest, Flavor Crest, and O’Henry are less susceptible than varieties such as Elegant Lady, Fairtime, Fay Elberta, and Summerset.
Plum. Some highly susceptible varieties of plum that may need protection are Black Beaut, Gaviota, Kelsey, and Wickson.
Strawberry. Day-neutral (everbearing) varieties such as Fern, Seascape, Sequoia, and Yolo are more susceptible than short-day varieties (those that fruit in May and June only) such as Chandler.
|apple, nectarine, peach, quince||Podosphaera leucotricha||tolerant varieties prune out infections in apple trees during dormant season fungicides if necessary|
|cherry||Podosphaera clandestina||fungicides if necessary|
|apricot, plum, prune||Podosphaera tridactyla||tolerant varieties fungicides if necessary|
|strawberry (a different strain infects caneberries)||Sphaerotheca macularis||resistant varieties removing infected tissue fungicides if necessary|
|apricot, nectarine, peach, plum, roses||Sphaerotheca pannosa||fungicides if necessary remove or treat roses|
|grape||Erysiphe necator||tolerant varieties water sprays prune during dormancy fungicides|
Shade and moderate temperatures favor most powdery mildews. Plant in sunny areas as much as possible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow-release fertilizer. Long duration overhead sprinkling may actually reduce active powdery mildew infections because spores are washed off the plant. However, spores can be disseminated in water to new noninfected leaves if watered only briefly.
As new shoots begin to develop on perennial plants, watch closely for the appearance of powdery mildew. Where infection is limited, prune out and bury or discard diseased tissue as soon as it appears. Infected tissue can be recognized by the young emerging leaves being deformed or showing a puckered condition. Soon after emergence infected leaves begin to exhibit white mycelial growth on the leaf surface. This combination of symptoms is characteristic of early season mildew onset. If powdery mildew has been present during the season on woody species, prune out infected tissue during the dormant season.
Prune grapevines during dormancy and position shoots during the growing season to allow exposure of fruit to sunlight and good air flow through the canopy. Pruning and training are also helpful in controlling Botrytis bunch rot.
Because one common powdery mildew fungus, Sphaerotheca pannosa, often spreads disease from roses to stone fruits, try to avoid planting apricot or plum trees near highly susceptible rose bushes. If roses are nearby and can’t be removed, control powdery mildew infections on them.
On apple trees, look carefully for infected shoots and buds in the dormant season and remove them. Infected buds are flattened or shriveled compared to normal buds. The buds and infected shoots have a thin layer of fuzzy white fungus on their surfaces that usually is easy to see. Where practical, remove and dispose of overwintering leaves on strawberry plants that are infected. If raspberry canes develop powdery mildew, remove the canes down to the roots during the dormant season. Infected canes of berries and grapevines have distinctive weblike russetting. Remove infected prunings from the garden area and destroy them.
Where powdery mildew has been a problem in the past, fungicides may be needed. Fungicides function as protectants, eradicants, or both. A protectant fungicide can only prevent a new infection from occurring, but an eradicant will kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Eradicants should be used at the earliest appearance of the disease. Once mildew growth is extensive, control with fungicides becomes more difficult.
Fungicides. Several least-toxic fungicides are available for backyard trees and vines, including horticultural oils, neem oil, jojoba oil, sulfur, and the biological fungicide Serenade. With the exception of the oils, these materials are primarily preventive. Oils work best as eradicants but also work as good protectants. The fungicides listed here are registered for home use. Commercial growers should consult the UC IPM Pest Management Guidelines for fungicides for commercial use.
Oils. To eradicate powdery mildew infections, use a horticultural oil such as Saf-T-Side Spray Oil, Sunspray Ultra-Fine Spray Oil or one of the plant-based oils such as neem oil (such as Green Light Neem Concentrate) or jojoba oil (such as E-rase). Be careful, however, never to apply an oil spray within 2 weeks of a sulfur spray or plants may be injured. Some plants may be more sensitive than others, however, and the interval required between sulfur and oil sprays may be even longer always consult the fungicide label for any special precautions. Also, oils should never be applied when temperatures are above 90°F or to drought-stressed plants. Horticultural oils and neem and jojoba oils are registered on a wide variety of crops.
Sulfur. Sulfur products have been used to manage powdery mildew for centuries but are only effective when applied before disease symptoms appear. The best sulfur products to use for powdery mildew control in gardens are wettable sulfurs that are specially formulated with surfactants similar to those in dishwashing detergent (such as Safer Garden Fungicide). To avoid injury to the plant or tree, sulfurs should not be applied within 2 weeks of an oil spray, used on any plant when the temperature is near or over 90°F (80°F for caneberries and strawberry), and never applied at any temperature to apricot trees.
Biological Fungicides. Biological fungicides (Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on the plant, inhibit or destroy fungal pathogens. The active ingredient in Serenade is a bacterium, Bacillus subtilis, that helps prevent the powdery mildew from infecting the plant. While this product functions to kill the powdery mildew organism and is nontoxic to people, pets, and beneficial insects, it has not proven to be as effective as the oils or sulfur in controlling this disease.
How to Use. Apply protectant fungicides to susceptible plants before disease develops. Once mildew growth is mild to moderate, it is generally too late for protective fungicides to effectively control powdery mildew except for on new plant growth. The protectant fungicides are only effective on contact, so applications must provide thorough coverage of all susceptible plant parts. As plants grow and produce new tissue, additional applications may be necessary at 7- to 10-day intervals as long as conditions are conducive to disease growth. On highly susceptible plants, sulfur can be applied early in the season when temperatures are below 90°F and then to switch to other materials as the season progresses. However, applying oil, which is both a protectant and an eradicant, for the early sprays provides the best control.
If mild to moderate powdery mildew symptoms are present, the horticultural oils and plant-based oils such as neem oil and jojoba oil can be used.
Caneberries and Grapevines. Dormant or delayed dormant sulfur sprays can be used as a preventive measure before canes begin to grow in spring. Fungicides registered for use on caneberries include wettable sulfur and oils including neem oil. Don’t apply sulfur when temperatures exceed 90°F.
Strawberry. Treat as soon as symptoms appear. Be sure to spray both upper and lower leaf surfaces. It may help to remove and destroy affected leaves before treating the rest of the planting. Materials registered to control powdery mildew include sulfur and oils. The sulfur treatments also reduce mite populations, but don’t apply sulfur when temperatures exceed 80°F because it damages foliage and fruit.
Apple and Stone Fruit. Sprays are not necessary in many backyard situations. However, if you have had serious powdery mildew damage in past years, treat at 2-week intervals, beginning when buds just start to open (green tip stage), until small, green fruit are present. (Caution: Do not use sulfur on apricot trees.) Sulfur, horticultural oils, neem oil, and Serenade are all registered for powdery mildew on backyard trees.
Grapevines. Powdery mildew is a perennial problem in grapevines. Begin applying treatments when all buds have pushed. Thereafter, repeat at 10-day intervals if disease pressure is high otherwise, extend intervals when temperatures are above 90°F until the sugar content of the grapes is 12 to 15%, which is when they begin to soften and approach ripeness and are no longer susceptible to infection. You can measure the sugar content with a refractometer, if you have access to one, or you can see if sample berries sink in a 15% sucrose solution. (Prepare the sucrose solution by dissolving 8-1/2 teaspoons of table sugar in a half cup of warm water, then mixing in enough cold water to make the total volume 1 cup.) Sulfur, horticultural oils, neem oil, jojoba oil, and Serenade are registered for controlling powdery mildew in home vineyards.
Gubler, W. D., and D. J. Hirschfelt. 1992. Powdery Mildew. In Grape Pest Management. Oakland: Univ. Calif. Agric. Nat. Resources Publ. 3343.
McCain, A. H. 1994. Powdery Mildew. HortScript #3. Univ. Calif. Coop. Ext. Marin County.
Pest Notes: Powdery Mildew on Fruits and Berries
UC ANR Publication 7494
Authors: W. D. Gubler, Plant Pathology, UC Davis S. T. Koike, UC Cooperative Extension, Monterey County
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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