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Plant Answer Line Question

Keywords: Apples--Diseases and pests, Malus domestica

I have a question regarding apple trees and the caterpillars. We have a great apple tree, that I have just noticed has the early nest of these crazy caterpillars that we get around here. Can you help me with the most effective way to get rid of these things before they hatch and start eating our tree???? Is spraying ok for the fruit??


It is possible that your apple tree has an infestation of tent caterpillars, but without seeing the pests, I could not say definitively. If this is what you have, the information below from Washington State University Extension should be of use.

Also, check out Toxic-Free Future's page on managing tent caterpillars. You should be able to prune out the affected part of the tree and dispose of the nest.

Date 2018-06-14
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Plant Answer Line Question

Keywords: Horticultural oil, Apples--Diseases and pests

I have a dwarf Braeburn apple tree that gets spotted apples every year. The leaves drop off and the apples are stunted and not edible. I am spraying with dormant oil spray per the instructions and it looks beautiful right now. I need to know how often to spray it and how long into the season. The instructions aren't clear on this. Also, does the dormant oil spray make the apples unsafe to eat at all?


Here is what Michael Phillips says in his book, The Apple Grower: A Guide for the Organic Orchardist (Chelsea Green, 1998): "Oil sprays smother the overwintering eggs and emerging nymphs of a number of foliar feeders. Use of a highly refined oil is tolerable in an organic orchard, but generally not necessary." He recommends encouraging beneficial insects to control aphids. Aphids may be a sign of a deeper imbalance that needs addressing.

Whether the dormant oil spray makes the fruit unsafe to eat depends greatly on what the oil is made of: many such sprays are petroleum-based and would therefore not be safe. See the following information formerly available online from BeyondPesticides.org:


"Most horticulture oils used today are petroleum based (Grossman 1990), yet a growing number of horticulture oils are being made with vegetable oils, which are considered a least toxic pesticide. Carefully read the label or ask your pest control service provider to determine if the horticulture oil is vegetable or petroleum based."

From Washington State University Extension agent Mary Robson:

"How Do I Use Dormant Sprays?"

"Neither the spray nor the applicator is dormant in a 'dormant spray': the plants to which it's applied are. The term refers to winter-applied sprays for insect pests and diseases, put on before foliage begins to leaf out.

"To use dormant sprays, first identify the reason for the spraying. They are often used on fruit trees to control over-wintering insect pests such as scale and aphids. (The aphids over-winter as eggs, and the spray smothers the eggs, preventing spring hatching.) A dormant spray isn't an all-purpose winter splashing of pesticide around the garden: it's a specific spray chosen for a specific pest. The dormant spray used on fruit trees is often horticultural oil (sold as superior-type oil), and it may be mixed with lime-sulfur depending on the pest to be controlled. It's sprayed thoroughly to give good coverage on the trunk, branches, small limbs and shoots.

"Because dormant sprays are generally applied early in the season, they tend to be less disruptive to beneficial insect predators and parasites which aren't in active life stages in mid-winter. While generally used in fruit tree maintenance, dormant oil sprays are helpful for landscape plants with similar aphid or scale problems. Ornamental plums (purple-leaf plums) often suffer from infestations of aphids or scale; if that's been the case, a dormant oil spray may help reduce the populations."

The following two links are from the Colorado State University Cooperative Extension. Insect Control: Horticultural Oils and Horticultural Oils. Excerpt from a version of this page no longer available online:

"Most horticultural oils contain naphthene and paraffin compounds. Paraffins are valuable to gardeners because they're more toxic to insects and less toxic to plants than other oil compounds. In contrast, oils containing naphthene are less pesticidal and more likely to injure plants than paraffinic types. Oils high in naphthene also contain more impurities such as phytotoxic aromatic and unsaturated hydrocarbons. However, the newest horticultural oils contain only tiny amounts of those compounds."

Have you determined what the cause of the spotting on your apples is? Might it be apple scab? In case that is what you have been seeing, here is what Washington State University Extension says:

"Apple scab is caused by a fungus which also causes scab on crabapple and hawthorn. The first infections occur during wet weather in the spring. Initially, the disease causes tiny, pale, chlorotic, water-soaked spots on the leaves. The spots enlarge and darken to a dark, velvety, olive-green then to black. Leaves may become distorted, puckered, and mottled. Leaves may drop, sometimes resulting in severe defoliation of susceptible trees. Scab can also affect fruit. Fruits infected early in development show olive-green to brown, roughened or corky spots which may develop deep cracks. These apples are often misshapen. Fruits infected at later stages develop small black "pinpoint" scab spots while in storage. The disease is favored by cool, wet conditions and overwinters in infected plant debris.

"Management Options:

"Select Non-chemical Management Options as Your First Choice!!

  • Avoid overhead irrigation.
  • Plant in full sun.
  • Plant scab-resistant varieties such as 'Akane', 'Chehalis', 'Liberty', 'Paulared', 'Prima', or 'Tydeman Red'.
  • Rake and destroy (do not compost) fallen leaves, or cover them with soil.
  • Space plantings and prune to provide good air circulation and light penetration.
  • The application of nitrogen to the leaves in the fall will enhance the decomposition of the fallen leaves."

The following website is for large-scale growers, but may have information of interest to you:
Apples: Organic Production Guide by Tammi Hinman and Guy Ames, 2011

Date 2018-04-05
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Plant Answer Line Question

Keywords: Apple anthracnose, Apples--Diseases and pests

Can dormant oil or lime sulfur sprays help to control or prevent apple anthracnose? Is there any other way to prevent it besides cleaning up old leaves and disinfecting pruning tools between trees?


There are varying opinions on the best approach to controlling apple anthracnose (Cryptosporiopsis curvispora). British Columbia's Agriculture Department suggests that cultural controls (i.e., good garden hygiene) are key, and fungicides have proven ineffective. Here is an excerpt:
"Spores from new cankers are spread by rain or overhead irrigation during the late summer and fall months, and initiate new infections that appear as cankers during April through July of the following year. Cankers that are allowed to overwinter produce airborne spores during the following spring and summer that can initiate new infections at a distance from the source. The airborne spores function mainly to initiate new infections, while the water-borne spores serve to intensify the disease in trees that are already infected.
Cultural Control:
Prune out and remove all cankers during winter pruning. Prune out any new cankers that develop on limbs and trunks as soon as they are discovered, and remove them from the orchard. Developing cankers often girdle 1-year-old wood; remove any shoots that wilt or die suddenly during April through July as soon as they appear.
Nursery trees should be examined carefully for symptoms of the disease at planting and again the following spring. Trees with cankers should be returned to the nursery for replacement or discarded.
The cultivars Elstar, Empire, Gala and Sinta are very susceptible to anthracnose canker.
Chemical Control:
There are no fungicides registered for control of anthracnose and perennial canker in Canada, and fungicides have not proven to be effective."

Although Oregon State University's Online Guide to Plant Disease Control lists several chemical controls, they too indicate that chemical control alone is ineffective.

According to The Apple Grower by Michael Phillips (revised and expanded edition, Chelsea Green, 2005), anthracnose typically follows environmental stresses like cold, drought, or pruning injury. The best control is removing and burning infected parts of the tree. "Bordeaux mixture applied immediately after harvest and again two weeks later can help prevent spore germination in orchards with a severe problem. Any developing cankers the next growing season can be roasted alive using a propane (plumber's) torch."

I'm not sure if you feel comfortable getting out the blowtorch. Bordeaux mixture is one type of lime/sulfur combination, and it has its risks (to plants, the environment, and health), and possibly only limited benefits. Below is more information about this from University of California, Davis Integrated Pest Management.

Date 2018-03-01
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Plant Answer Line Question

Keywords: Codling moth, Apples--Diseases and pests

What can I do to control codling moth on my apple trees? I've tried the nylon footies, but found that the damage was actually greater. It seems as if the rough texture of the stockings actually makes it easier for the pests to get a purchase on the surface of the fruit. Also, the stockings give the fruit a slightly off (petroleum-like) flavor. I don't want to use toxic chemicals.


The web site for City Fruit has useful information about codling moth, including life cycle, traps, and preventive cultural practices (good garden hygiene includes not leaving fallen fruit on the ground, and thinning your apples when they are small--marble or walnut-sized--to one per fruit cluster).

There are several control options described by University of California's Integrated Pest Management page on codling moth.
"Organically acceptable tools for the control of codling moth include cultural control in conjunction with mating disruption and sprays of approved oils, codling moth granulovirus (Cyd-X), the Entrust formulations of spinosad, and kaolin clay (Surround)."

A local home gardener told me that she has had good luck using Surround kaolin clay. You do have to apply it several times (from late June on, every week or two), and it will make your trees look a bit ghostly, but it is worth a try. Although this Washington State University document focuses on apple maggot control, it does describe the use of kaolin clay. A thorough description of Surround kaolin clay spray may be found on the site of the National Sustainable Agricultural Information Service.

Date 2017-05-26
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Plant Answer Line Question

Keywords: Alyssum, Aphids, Apples--Diseases and pests, Beneficial insects

Is there something I can do to prevent my apple trees from getting woolly aphids? I'd rather not have to spray anything.


Encouraging beneficial insects is one step you can take. A 2013 study at Washington State University found that planting Alyssum flowers attracted syrphids which did a good job of reducing woolly aphid populations. Here are highlights of the paper that was published based on the study's findings:

  • Sweet alyssum flowers had the highest attractiveness to syrphids.
  • Faster suppression of woolly apple aphid occurred on trees closer to alyssum flowers.
  • Higher densities of natural enemies were observed near sweet alyssum plantings.
  • Natural enemies were found to move between sweet alyssum and adjacent apple trees.

As Washington State University's HortSense website (search under "tree fruit," "apple," then "aphids") indicates, encouraging beneficial insects is a good practice for the control of all 3 main types of aphids affecting apples, be they woolly, rosy, or green:

  • Control honeydew-feeding ants, which may protect aphid colonies from predators.
  • Encourage natural predators including ladybird beetles, lacewings, syrphid (hover) fly larvae, and parasitic wasps. Avoid use of broad-spectrum insecticides which kill these beneficial insects.
  • Hand-wipe or prune to control small, localized infestations (when practical).
  • Provide proper nutrition. High levels of nitrogen encourage aphid reproduction. Switch to a slow-release or low-nitrogen fertilizer.
  • Wash aphids from tree with a strong stream of water before leaves curl.

Date 2017-04-22
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May 31 2018 13:14:08