Delayed Leaf Abscission a Trending Issue for Washington Apple Producers
April 24, 2023 What happens when the leaves do not drop? Apple growers in Washington state are in the midst of finding out. Leaf senescence occurs naturally and gradually in the fall in response to key environmental cues, chiefly day length and temperature. But it was not the normal fall last year in much of the Yakima Valley, thanks to a historically cold, wet spring that got the season off to a late start. That growing season then was prolonged due to an unusually warm autumn.
In turn, Washington State University researchers — Bernardita Sallato, Lee Kalcsits, Tory Schmidt, and Matthew Whiting — observed active growth in apple trees in the second half of October.
“This suggests that the normal signals for initiating senescence and leaf abscission had not yet been perceived by trees, and that levels of natural growth hormones (e.g., auxins and gibberellins) were high,” Sallato says. “The relatively warm period of October and early November was followed by a sudden hard freeze in the middle of November. It was this rapid change in weather that froze leaves on the tree before they initiated the abscission processes.”
In frozen ‘WA 38’ and ‘Honeycrisp’ leaves collected by Sallato’s lab, only 13% of the nitrogen had been remobilized from ‘WA 38’ and 21% from ‘Honeycrisp’. Thus, reserves would be slightly depleted.
“However, we anticipate that this may have only a minor impact on nutrient and carbohydrate supply since remobilization would have already started prior to that stage,” Sallato says. “Thus, the warm conditions in October may have reduced nutrient and carbohydrate remobilization; however, large impacts to tree health are not expected.”
Recommendations by the researchers include:
- Maintain your normal delay dormant spray with boron and zinc, which are generally low in Eastern Washington.
- If you had a high crop load in 2022 and are concerned about tree vigor, support nitrogen with ground application during bloom. Nitrogen uptake will start when nitrogen reserves become exhausted. If limited, uptake will likely start earlier than the benchmark of calcium 40 days after full bloom.
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