While it is a bit early yet for home orchardists to begin a spray program, Extension offices have begun to get phone and email inquiries on timing for dormant oil sprays on fruit trees in home orchards and calls on managing Cedar Apple Rust and Apple Scab fungal diseases.
Dormant oil sprays are typically applied to the point of run off to the branches and trunks of fruit trees to control over wintering 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 your fruit trees or in the soil around your trees. These insects will be managed through your early multipurpose sprays and cover sprays as the growing season progresses during the summer months.
Dormant oil sprays work by smothering the adult insect and soaking in to the overwintering eggs, effectively controlling them as well. 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 24 hours. This is the standard used, yet be sure to read the label of the product you are going to use for more specific guidelines. The dormant oils are mixed with water and if temperatures fall below freezing the oil and water separate, the effectiveness is greatly reduced.
The sprays used to control Cedar Apple Rust and Apple Scab diseases are most often applied at what is called "bud break" This is when the bud scales start to swell and soften to allow either the flowers or foliage to begin to emerge. The bud scales soften during our spring rains, the same time that 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 disease 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 can also infect the fruit so the damage to your fruit trees 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. The important thing to remember is those sprays have to match the trees development and the weather. You can use the calendar to measure the days between sprays, yet 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 towards the end of the season. Your product label will also 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.