Home orchardists often undervalue the importance of applying a dormant oil spray to their fruit trees.
Dormant oil sprays typically are applied to the point of runoff to the branches and trunks of fruit trees to control overwintering adult insects and insect eggs that were laid last summer and fall as a means of lowering the insect pressure early in the season. Dormant oil sprays do not manage insects that are overwintering off of your fruit trees or in the soil around your trees. These insects will be managed through early multipurpose sprays and cover sprays as the growing season progresses during spring through summer. Dormant oils do not manage fungal diseases that show up in the orchard either.
How they work
Dormant oil sprays work by smothering the adult insect and soaking into the overwintering eggs, effectively controlling any insects and eggs that are found on the fruit tree. This is why the instructions say to thoroughly cover the tree. Dormant oil sprays are temperature-sensitive and will need to be applied when the temperatures are above freezing for a minimum of 24 hours. This is the standard practice, but be sure to read the label of the product you are using for any specific guidelines. Why above freezing? Dormant oils are mixed with water and if temperatures fall below freezing the oil and water separate, the effectiveness is greatly reduced.
Fungus disease timing
Fungal diseases such as Cedar Apple Rust and Apple Scab, and several less-important diseases, are treated with fungicides applied at what is called “bud break.” Bud scales swell and soften to allow either the flowers or foliage to begin emergence.
When to spray
The bud scales soften during spring rains, the same time the fungal spores begin to emerge from their overwintering stages and float through the air infecting the very young flower and leaf tissues. Those first few sprays are critical to prevent early infestations that remain through the entire season and this is what most home orchardists find most challenging. Sprays need to be applied during inclement weather when we would rather be indoors. Another challenge to be recognized is that if it rains within hours of the application, you must go back out and reapply or the treatment is lost and the fungal spores arriving on your leaf tissues can infect anyway.
Apple scab is the one that will cause your leaves to fall off the tree and that impacts the fruit quality since the leaves are the source of the energy that grows the apple. Apple scab also can infect the fruit so the damage to your fruit tree is two-fold. Cedar Apple Rust is damaging, as well, as it impacts the leaves’ ability to produce food; it is just that those leaves remain on the fruit tree.
Matching sprays to tree development
Remember those sprays have to match the trees development and the weather. Use the calendar to count the days between sprays, but you cannot assume that the intervals between sprays remain constant. There will be more sprays more frequently in the beginning of the season and fewer sprays toward the end of the season. Your product label will be helpful in this regard. Lastly, there will be “days before harvest” notation on the label, letting you know when you must stop spraying prior to harvest.
• Richard Hentschel is a Horticulture Extension educator with University of Illinois Extension, serving DuPage, Kane and Kendall counties. This column originates on his blog at go.illinois.edu/overthegardenfence. To get more tips from Hentschel, watch his “This Week in the Garden” videos on Facebook and YouTube.