When to Spray Plum Trees for Leaf Curlers, Home Guides, SF Gate
When to Spray Plum Trees for Leaf Curlers
- 1 When to Spray Plum Trees for Leaf Curlers
- 2 Plum Pockets
- 3 Leaf Curl Plum Aphids
- 4 Controlling Aphids
- 5 Considerations and Precautions
- 6 Comment faire face aux dangereux pucerons de la prune
- 7 ¿Cómo lidiar con plagas peligrosas los pulgones ciruela?
- 8 Integrated Pest Management (I.P.M.) for Aphids
- 9 Scouting & Identification
- 10 Cultural Controls
- 11 Mechanical Control
- 12 Biological Control
- 13 Chemical Control
- 14 Annual Plum Pox Virus Survey
Removing damaged, diseased or dead plant matter will prevent future problems.
When to spray plum trees (Prunus spp.) for leaf curlers varies depending on what is causing the leaves of the fruit tree to curl. These fruit trees — which grow in U.S. Department of Agriculture plant hardiness zones 4 through 10 — are susceptible to insects and diseases that can lead to leaf curling. Once you have identified the culprit, proper treatment can begin and you can implement preventive measures to keep the problem from reoccurring.
The fungus Taphrina communis causes plum pocket disease, which leads to curled, swollen, twisted or otherwise distorted leaves, fruit and shoots. The seeds of infected fruits cannot develop properly causing the inside of the fruit to be hollow. There are no fungicides capable of curing the disease was a plum tree is infected, according to the University of Kentucky Cooperative Extension Service website. However, spraying the plum with Bordeaux mixture before bud swell helps prevent this disease from occurring. One brand of Bordeaux mixture suggests mixing 12 tablespoons of the copper fungicide with 1 gallon of water to treat leaf curl and plum pockets. Since every brand of fungicide is different, check the label of the product you are using for exact instructions. For best results, spray the plum tree during its dormant period between the leaf drop stage in late fall and the bud swell stage in early spring.
Leaf Curl Plum Aphids
Leaf curl plum aphids are tiny soft-bodied insects with a shiny green, yellowish or brown colored body. These annoying pests cause the leaves of infested plums to curl and negatively affect fruit sugar content and tree growth. The aphids are often found hiding inside the curled leaves and will lay their eggs at the base of buds. These overwintering eggs will hatch in early spring and begin increasing their numbers and attacking new foliage. Leaf curl plum aphids create honeydew, which is a sticky material secreted by the insects. Honeydew leads to sooty mold growth and attracts ants, which feed on the sticky secretion.
Attracting natural enemies that prey on these aphids help keep their populations low. Insects such as green or brown lacewings, lady beetles and syrphid flies are a few predators of the leaf curl plum aphids and can be enticed back to the area with annual flowering herbs. Another option is to thoroughly spray the plum tree with insecticides to kill the aphids. The type of insecticide used depends on when you are trying to control the pests. For example, 5 tablespoons of horticultural oil diluted in 1 gallon of water will control overwinter aphids on the plum tree during its dormant stage while 1 ounce of neem oil mixed in 1 gallon of water controls the pests during the tree’s active growing period.
Considerations and Precautions
No matter which pesticide you use to control leaf curlers on the plum tree, make sure to thoroughly cover the plant with the mixture. This includes spraying the top and underside of the leaves as well as the limbs with the solution. Good coverage is essential for proper control of the pest. Furthermore, always follow the mixing and application instructions of the pesticide you are using. Every pesticide has directions designed by the manufacturer for that specific product. Ignoring these instructions could reduce the effectiveness of the product and injury the plum tree.
About the Author
Marylee Gowans has written about gardening for both online and print publications. She attended the University of Akron, graduating with a Master of Fine Arts in creative writing. In 2009, she received master gardener certification from the Master Gardeners of Summit County, Ohio.
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Integrated Pest Management (I.P.M.) for Aphids
Factsheet | HGIC 2009 | Updated: Jul 8, 2019 | Print
Lady Beetle Larvae feeding on green peach aphids (Myzus persicae).
Photo courtesy David Riley, University of Georgia, Bugwood.org
Aphids can be a problem in the home landscape, vegetable garden, or fruit garden. They can vector many viral diseases and can cause significant damage to desired plants if the aphid population is left unchecked. Aphids reproduce quickly and have multiple generations per year. Females of most species can give birth to live young, meaning that within a few days in summer time temperatures aphid populations can grow exponentially. Aphid populations of different species can found at any time from spring to fall in South Carolina. Aphids are most effectively managed when Integrated Pest Management (I.P.M.) is practiced. In short, IPM is the use of multiple control strategies in a comprehensive and preventative approach to reduce pest populations, to maintain plant health, and to minimize the use and impact of pesticides in the environment. These management strategies include mechanical, physical, biological, cultural, and chemical controls.
Scouting & Identification
Oleander aphids (Aphis nerii) feeding on leaves with predatory lady beetle larvae.
Photo courtesy of Anne W. Gideon, Bugwood.org
The first step in solving any pest infestation problem is to determine what exact pest is present. There are many different species of aphids, and the different strategies to manage them can vary with each species. Green peach aphids (Myzus persicae) are the most common aphid species found feeding on many plant species in the garden and the landscape. Another common aphid is the oleander aphid or the milkweed aphid (Aphis nerii). This aphid feeds on oleander, milkweeds, and their relatives.
Aphids can usually be found on young tender growth and on the underside of leaves. Plant symptoms typically are a response to their feeding. Aphids have piercing sucking mouth parts. Some species inject toxins in the plant when they feed; these toxins can cause the tissue to become distorted and deformed. A sign of aphids being present is honeydew, the aphid’s sticky excretion, on plant surfaces. Honeydew looks similar to sugary drink being poured on a plant’s leaves that has dried and has a sticky appearance and feel.
Aphid feeding damage to plum leaves caused by green peach aphid (Myzus persicae).
Photo courtesy of Eugene E. Nelson, Bugwood.org
There are many plant species that can be host to aphids. Vegetable garden hosts include peppers, tomatoes, leafy greens, cabbage, kale, and basil are key host plants. Hosts in landscapes include zinnias, roses, coreopsis, and many others. Fruit trees are also not immune and the Woolly Apple Aphid (Eriosoma lanigerum) can be a severe problem. Not only do they feed on trunks, branches, and twigs, but they may move below ground and feed on apple tree roots.
Another common aphid found in vegetable gardens is the Melon Aphid, also known as the Cotton Aphid (Aphis gossypii), which can be a severe problem for watermelons, musk melons, cucumbers, and squash.
Woolly Apple Aphid (Eriosoma lanigerum) feeding on the stem of an Apple Tree.
Photo courtesy University of Georgia Plant Pathology , University of Georgia, Bugwood.org
There are some basic things home gardeners can do to prevent major outbreaks of aphids. High levels of nitrogen promote succulent, nutritious new growth, which is preferred by aphids and can help boost aphid reproduction. Over fertilizing a plant can enhance aphid population growth and make the problem worse. Using smaller amounts of fertilizer throughout the growing season can help to reduce potential aphid outbreaks.
Another effective preventative method is the use of reflective silver mulch (a Mylar like film placed over the soil surface), especially in vegetable production. A side benefit of reflective mulch is that it can increase crop yields because of the increased amount of solar energy reflected onto the leaves. However, there are some precautions to these types of materials. If purchasing big rolls of the material is not desired, or if the garden space is small, an aluminum pie plate can be cut and placed upside down around the base of the plant.
A good effective method for eliminating aphids is to simply rinse them off the leaves of affected plants. A water hose and nozzle with adequate pressure is enough to knock the aphids from the foliage, but not to damage the plant. CAUTION: A powered pressure washer is much too strong. The jet or shower setting on a dial nozzle is enough to dislodge these pests. Once off the plant, aphids cannot climb back up the plant and will often starve to death. Aphids can also be rubbed off the plants with fingers or a wet cloth. This method effective against small aphid populations and at the very early stages of infestation. Physical removal by rubbing would be ineffective at removing large infestations.
Cotton / Melon Aphid (Aphis gossyppi) feeding on a cotton plant.
Photo courtesy Ronald Smith, Auburn University, Bugwood.org
Aphids have several natural enemies that can be attracted or released to help keep populations in check. The most common one that gardeners are familiar with are lady beetles. Lady beetles and their larvae feed on many different types of aphids, and their presence in the garden should be encouraged by reducing the overall use of broad-spectrum insecticides. Lady beetles are available for purchase, but it is not recommended to buy them. They are typically harvested during their hibernation period in the western U.S., and as a result are often confused when they arrive in a gardener’s home. As a result, the lady beetles fly away in search of a new hibernation spot. For home gardeners, it is best use plants in the garden and landscape that will attract lady beetles, such as sunflowers, clovers, liatris, and coreopsis.
Another effective natural enemy, that can be purchased and realesed is green lacewing larvae (Chrysoperia rufilabris). These larvae are extremely aggressive and will eat numerous aphids a day.
Green lacewing larvae (Chrysoperia rufilabris) feeding on aphids.
Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org
They can be released on specific plants that are infested for direct application or released as adults to establish in the garden. These insects can be attracted by adding plant species such as coreopsis, cosmos, and clover in the garden and landscape. Another way to encourage and protect them is to reduce the use of broad-spectrum insecticides. Lacewings are extremely sensitive to insecticides and even drift from an application can be harmful. Many lacewing adults are often killed in bug zappers used for mosquito control. These helpful predators can be purchased from online sources. It is recommended that only lacewing larvae are purchased and released into the garden and landscape.
Another natural enemy are parasitic wasps (Aphidius species) that sting aphids and impregnate them with an egg. The egg then grows inside the aphid, killing and mummifying it, and a new adult wasp hatches out of the mummified aphid.
If these mummified aphids are seen near active aphid populations, it indicates that the Aphidius wasps are nearby and actively parasitizing the current population.
These wasps are too small to sting humans. In fact, different species of Aphidius are very selective and each species attacks a different species of aphid. Therefore, it is important to have the aphid properly identified before making a purchase in online sources.
A biological control that can be applied similar to a traditional is insecticide is any product containing Beauvaria bassiana. This entomopathogenic fungus is usually applied as a foliar spray and is parasitic to many soft body insects. The organism is available in both liquid or powder form, but the powder is more stable and has a longer shelf life. This can be used for control of aphids or other soft body insect. Apply the product as a preventative every 7-14 days to help keep pest populations low. The downsides to the product is that it can only be found online and needs to be kept refrigerated, but not frozen.
A parasitic wasp stinging and injecting an egg into an aphid.
Photo courtesy David Cappaert, Bugwood.org
When all other control measures have failed to keep the populations under control, a chemical insecticide may be needed. The goal with insecticide use is to choose the one with minimal impact to pollinators and natural enemies, but one that is still effective on the insect causing the problem. It should be noted that although the aphids are killed by insecticides, their dead carcasses can still be on the leaves after the application. Do not panic and make an additional application. Check to see if the aphids are still alive by nudging them with a pencil. Additional applications are only needed if live aphids are still present. The dead aphids can be removed by washing the leaves off with water. Before purchasing and using an insecticide, be sure to read and follow ALL label directions. The label is the law; therefore, the product label is the final authority on what crop or areas the product can be applied and at what rate. When shopping for an insecticide, be sure to look on the package for the active ingredient and choose the product with the proper active ingredient to control the pest. Always spray late in the day for best results and to protect beneficial insects.
A normal aphid (left) versus a parasitized aphid mummy (right).
Photo courtesy of David Cappaert, Bugwood.org.
The first effective choice to spray would be either insecticidal soap or horticultural oil. These insecticidal products coat the aphid’s exoskeleton (body)and cause it to suffocate. These insecticides also can kill beneficial insects upon contact, but they have no residual activity. So only beneficial insects and pollinators that were directly hit by the application will be affected. Pollinators and natural enemies that arrive after the spray solution has dried, will not be impacted by these soaps or oils. Note that these products may be phytotoxic (damaging to the plant) to drought stressed plants, especially at temperatures 90°F or higher. Applications should be made when temperatures are cooler, such as the mid- to late evening to avoid any potential plant damage. For more information on using insecticidal soaps and oils, please see HGIC 2771 Insecticidal Soaps for Garden Pest Control.
An effective step up from the soaps and oils, are insecticides that contain the active ingredient pyrethrin. This botanically derived compound can be very effective in providing a relatively quick knockdown of aphids. These products only affect natural enemies and pollinators that are directly within the application. Additionally, migrating beneficial insects may be repelled by the residue on plant leaves. However, this effect is not long lasting (only hours), so pyrethrins can be an effective choice to help reduce large populations.
Another effective botanically derived chemical is azadirachtin. This compound is a natural insect growth regulator that modifies the way insects grow by inhibiting the shedding of the exoskeleton. It can be mixed with an entomopathogenic fungi or bacteria to allow more contact time between the insect’s exoskeleton and the pathogenic organism. This ensures that the fungi or bacteria have time to grow, penetrate the exoskeleton, and kill the insect.
Systemic insecticides are available for the control of aphids, primarily on ornamentals, although there are formulations for vegetables and fruits. If applying a systemic insecticide to vegetables or fruits, the label will give specific directions as to when the product can be applied prior to harvest. Systemic insecticides contain active ingredients, such as, imidacloprid, or dinotefuran. These products are applied as a foliar spray or as drench application to the roots. The later provides less impact to potential pollinators and natural enemies, as the chemical is applied directly to the roots and taken up by the plant. Applications to the roots can take several days to enter and distribute throughout the plant. Be sure the systemic insecticide product is labelled for use on the specific vegetable or fruit crop.
If this document didn’t answer your questions, please contact HGIC at [email protected] or 1-888-656-9988.
Andrew «Drew» Jeffers, Spartanburg Cooperative Extension, Horticulture and Natural Resource Agent, Clemson University
Joey Williamson, PhD, HGIC Horticulture Extension Agent, Clemson University
This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.
Annual Plum Pox Virus Survey
The foremost control measure of Plum pox virus (PPV) is exclusion. Commercial growers and nursery propagators should only purchase planting stock that is certified virus-tested after it has been assayed for PPV and other viruses. A careful selection and limited exchange of budwood to avoid dissemination of virus-infected materials is critical in avoiding human transmission of the disease. Also, the production of PPV-free trees through the selection of PPV-free budwood and rootstocks is vital to preventing virus spread to new orchards and regions.
The timely elimination of any infected trees is a critically important control method for PPV. There is no treatment for PPV, nor is there any cure in infected orchards; once a tree has been confirmed to have PPV it should be removed as quickly as possible to limit spread of the virus to neighboring trees. If tree stumps are left behind they should be treated with a systemic herbicide to avoid the development of sucker shoots, which can also carry PPV.
Chemical control of aphids is not a feasible management tool of PPV; application of insecticides may reduce the overall population of aphid vectors over a growing season, but a single aphid can transmit PPV to a new host in a matter of seconds. Furthermore, total control of aphid vectors is impossible to achieve.
Plant resistance is the ideal control strategy for PPV. Limited naturally occurring resistance genes are available for use in developing highly resistant stone fruit cultivars through conventional breeding techniques. Most cultivars with host resistance henes are tolerant to the disease in that they express few, if any, symptoms but carry the virus. Such cultivars are of limited value to prevent the spread of PPV. Currently, the most promising prospect for PPV resistance is genetic engineering. The insertion of PPV gene fragments into plum trees confers high resistance to PPV through the anti-viral pathways of RNA silencing, a potent natural defense mechanism against viruses in plants. This resistance is heritable and can be incorporated into other cultivars through standard breeding practices. Although this method is still in early, experimental stages, it offers great promise for the development of commercial stone fruit crops that are resistant to PPV.