{"id":2661,"date":"2020-01-31T00:03:09","date_gmt":"2020-01-31T08:03:09","guid":{"rendered":"https:\/\/depts.washington.edu\/hortlib\/pal\/pal-question-457\/"},"modified":"2024-04-18T10:42:35","modified_gmt":"2024-04-18T17:42:35","slug":"use-of-horticultural-oils","status":"publish","type":"pal","link":"https:\/\/depts.washington.edu\/hortlib\/pal\/use-of-horticultural-oils\/","title":{"rendered":"Use of horticultural oils"},"content":{"rendered":"

I have a dwarf Braeburn apple tree that gets spotted apples
\nevery year. The leaves drop off and the apples are stunted and not
\nedible. I am spraying with dormant oil spray per the instructions and it
\nlooks beautiful right now. I need to know how often to spray it and how
\nlong into the season. The instructions aren’t clear on this. Also, does
\nthe dormant oil spray make the apples unsafe to eat at all?<\/strong><\/p>\n

Here is what Michael Phillips says in his book, The Apple Grower: A Guide
\nfor the Organic Orchardist<\/em> (Chelsea Green, 1998):
\n“Oil sprays smother the overwintering eggs and emerging nymphs of a
\nnumber of foliar feeders. Use of a highly refined oil is tolerable in an
\norganic orchard, but generally not necessary.” He recommends encouraging
\nbeneficial insects to control aphids. Aphids may be a sign of a deeper
\nimbalance that needs addressing.<\/p>\n

Whether the dormant oil spray makes the fruit unsafe to eat depends
\ngreatly on what the oil is made of: many such sprays are petroleum-based
\nand would therefore not be safe. See the following information formerly available online from BeyondPesticides.org:<\/p>\n

Excerpt:<\/p>\n

“Most horticulture oils used today are petroleum based (Grossman 1990),
\nyet a growing number of horticulture oils are being made with vegetable
\noils, which are considered a least toxic pesticide. Carefully read the
\nlabel or ask your pest control service provider to determine if the
\nhorticulture oil is vegetable or petroleum based.”<\/p>\n

From Washington State University Extension agent Mary Robson:<\/p>\n

“How Do I Use Dormant Sprays?”<\/p>\n

“Neither the spray nor the applicator is dormant in a ‘dormant spray’: the
\nplants to which it’s applied are. The term refers to winter-applied
\nsprays for insect pests and diseases, put on before foliage begins to
\nleaf out.<\/p>\n

“To use dormant sprays, first identify the reason for the spraying. They
\nare often used on fruit trees to control over-wintering insect pests such
\nas scale and aphids. (The aphids over-winter as eggs, and the spray
\nsmothers the eggs, preventing spring hatching.) A dormant spray isn’t an
\nall-purpose winter splashing of pesticide around the garden: it’s a
\nspecific spray chosen for a specific pest. The dormant spray used on
\nfruit trees is often horticultural oil (sold as superior-type oil), and
\nit may be mixed with lime-sulfur depending on the pest to be controlled.
\nIt’s sprayed thoroughly to give good coverage on the trunk, branches,
\nsmall limbs and shoots.<\/p>\n

“Because dormant sprays are generally applied early in the season, they
\ntend to be less disruptive to beneficial insect predators and parasites
\nwhich aren’t in active life stages in mid-winter. While generally used in
\nfruit tree maintenance, dormant oil sprays are helpful for landscape
\nplants with similar aphid or scale problems. Ornamental plums
\n(purple-leaf plums) often suffer from infestations of aphids or scale; if
\nthat’s been the case, a dormant oil spray may help reduce the
\npopulations.”<\/p>\n

The following link is from the Colorado State University Cooperative Extension: Insect Control: Horticultural Oils<\/a>. Excerpt from a version of this page no longer available online:<\/p>\n

“Most horticultural oils contain naphthene and paraffin compounds.
\nParaffins are valuable to gardeners because they’re more toxic to insects
\nand less toxic to plants than other oil compounds. In contrast, oils
\ncontaining naphthene are less pesticidal and more likely to injure plants
\nthan paraffinic types. Oils high in naphthene also contain more
\nimpurities such as phytotoxic aromatic and unsaturated hydrocarbons.
\nHowever, the newest horticultural oils contain only tiny amounts of those
\ncompounds.”<\/p>\n

Have you determined what the cause of the spotting on your apples is?
\nMight it be apple scab? In case that is what you have been seeing, here
\nis what Washington State University Extension says:<\/p>\n

“Apple scab is caused by a fungus which also causes scab on crabapple and
\nhawthorn. The first infections occur during wet weather in the spring.
\nInitially, the disease causes tiny, pale, chlorotic, water-soaked spots
\non the leaves. The spots enlarge and darken to a dark, velvety,
\nolive-green then to black. Leaves may become distorted, puckered, and
\nmottled. Leaves may drop, sometimes resulting in severe defoliation of
\nsusceptible trees. Scab can also affect fruit. Fruits infected early in
\ndevelopment show olive-green to brown, roughened or corky spots which may
\ndevelop deep cracks. These apples are often misshapen. Fruits infected at
\nlater stages develop small black “pinpoint” scab spots while in storage.
\nThe disease is favored by cool, wet conditions and overwinters in
\ninfected plant debris.<\/p>\n

“Management Options:<\/p>\n

“Select Non-chemical Management Options as Your First Choice!!<\/p>\n