What is in this article?:
- If leaves are falling prematurely in late summer or early fall, chances are it is due to two diseases, scab and rust.
- Foliar diseases are becoming more problematic in California almond orchards.
- If scab has evolved into a serious problem, a delayed dormant application of copper and oil is now recommended.
“I hate to be a nozzle head and tell growers to spray, but it will do an effective job in cleaning up an orchard to reduce pathogen populations in the long run,” he said.
This delayed spray is effective in reducing and delaying sporulation of scab twig lesions. It should be used in combination with properly timed bloom and petal fall sprays. Doll said orchards with significant scab outbreaks in 2010 that followed the above recommendations had obvious suppression of the disease in 2011, and were able to keep the leaves on the trees until late November.
This delayed dormant period is just when the buds begin to swell, about two weeks before a grower sees bloom, Doll explained.
A spring fungicide spray to control scab should be made between two to five weeks after petal fall.
Orchards with a history of rust should be treated with a fungicide five weeks after petal fall.
With spray timing so close for both scab and rust, many growers try to catch both with one spray.
“If you try to split the difference between the spray timing for scab and the timing for rust, you will usually miss (timing) for both,” Doll told growers and PCAs at the Fresno field meeting. He recommends scab treatments more toward the two week side of the window.
“A spray for rust five weeks after petal fall can be very effective,” he said. Even better he said, if a grower treats for rust with an early May treatment followed by an early June treatment, “I would be shocked if they have a rust problem.”
Another rust control strategy is to apply zinc sulfate (20-40 pounds per acre) in late October/early November. This should be made to help reduce overwintering populations of rust. The zinc will hasten leaf fall, and prevent the rust inoculum from increasing. In orchards of severe infestation, applying a low rate of nitrogen to the surface leaf debris will help speed up the natural degradation of the leaves.