There have been several questions over the past two weeks regarding applications of dormant oil onto almonds. Concerns of phyto-toxicity or oil burn have been raised over the dry weather, lack of rain, and lack of moisture within trees. Although the exact conditions that will favor oil burn in almonds is unknown, the following are a few considerations when making this decision.
Oil type. Petroleum-based horticultural oils are typically refined to make them safer. There are standard specifications that describe the oil. These include:
- Unsulfonated residue (UR) is a measure of oil refinement. It is expressed as a percentage. The higher the percentage, the less likely to see plant injury. Traditionally, dormant oils have a UR between 50 and 90 percent, summer oils between 92-96%, and stylet oils are refined to 99% or greater.
- Viscosity is the oils “thickness.” The lower the viscosity, the lower the chances for plant injury.
- Distillation Temperature is the temperature in which the oil boils. Oils vary with boiling temperatures between 412°F and 468°F, and oils with boiling points over 455°F may damage plant foliage and should only be used for dormant applications (i.e.455 and 470 oil).
- Distillation Range is a measure of purity and describes the range of boiling point temperatures for oil components at the 10 and 90 percent point of distillation. Narrow range oils have distillation ranges of 80°F or less, and are more predictable when mixing and applying than other oils.
If an application of oil is needed, selecting an oil that has specifications with lower risk of oil burn should be considered (i.e. a narrow range, 440 or 415 summer oil).
Pest Considerations and Oil Rate. Most dormant pest management applications make use of dormant oil at high rates – around 5-8 gallons per acre. Applications at higher rates do provide better control of the insect pests, but also increase the risk of plant injury. If dropping below 5-6 gallons/acre, the insect management benefits of oil applications will be limited and may not be worth the risk of injury.
Dormant treatments that include oil are used to successfully manage scale, peach twig borer, brown almond mites, and scab. If an oil application is planned check to see if the treatment is needed. If a treatment is warranted, see if another treatment timing could be used to manage the targeted pests to avoid making a dormant spray. For scab, treat only problematic blocks. Only 4 gallons/acre of oil is needed for the copper and oil dormant scab treatment and lower rates may be as effective.
Weather conditions. Do not apply oils when it is too hot or too cold. Oil applications should be avoided when temperatures are below 35°F. Spraying at temperatures near freezing may lead to an uneven deposition of oil on the plant surface, leading to plant injury. If overnight freezing temperatures are forecasted, oil applications should be completed by early afternoon.
Moisture status. Plants that are not water stressed are more resistant to oil burn. If the almond trees are excessively dry, it may be best to withhold the oil application as the oil may be absorbed into the wood, damaging the tissue. Late post-harvest or winter irrigations will reduce the risk of oil burn.
Oil Application. Oil can separate from solution. Rigs should be checked to make sure that the agitator is functioning and that the spray pattern is evenly distributed. Be careful at the end of rows to prevent over-application of oil as the rig slows down.
If anyone has experienced oil burn on almonds outside of application issues and is willing to share the circumstances, please feel free to comment. I am sure that we all would learn from what we can and can not do in the current conditions.