(101) Effects of Fall Applications of Urea and Zinc Sulfate to `Bing' Sweet Cherry Spring Budbreak

Summary: A single treatment of nutrient/defoliant chemicals was found to advance bloom significantly in the first half of the bloom period; this treatment was applied on 25 October. This treatment advanced bloom from 4% to 73% in 5 days (March 3 to March 7), compared to the control (0-40%) and the hand defoliation on the same date (0-11%). Hand defoliation tended to result in a bloom progression that was equivalent to, or delayed, compared to the untreated control. Chemical nutrient/defoliant treatments did not remove leaves very far in advance of normal leaf fall, although leaves were ‘burned’. The treatments did result in bloom advance that was equivalent to, or slightly ahead of, the control. By March 9, bloom had progressed so far that treatments generally did not differ in the percentage of truss buds open. All truss buds opened and no vegetative or reproductive bud death was found. The significance of the results is discussed as they relate to previous defoliation work and the use of the Dynamic model for calculating chill accumulation. Problem and its Significance: While rest-breaking agents have become commonly used in California cherry production, application timing continues to involve a certain amount of uncertainty from year-to-year. In order to use any chilling model adequately and thereby schedule rest-breaking treatments, both the beginning point of dormancy and the beginning point of chill accumulation must be understood. One method of testing dormancy onset is tree defoliation, which may be used to alter the pattern of budbreak and regrowth in spring (Couvillon and Lloyd, 1978). Originally, we believed that California sweet cherry growers might benefit from fall defoliation with regard to helping trees adapt to less than adequate chilling conditions. With that in mind, we conducted a preliminary experiment in Fall, 2002, investigating the effects of fall defoliation on budbreak and fruit set in the subsequent growing season. We found that chemical defoliants (20 lb per acre each of zinc sulfate and urea) tended to advance bloom and increase the percentage of reproductive buds that opened, compared to hand defoliation and the control, despite the fact that leaves were removed slowly.. Three timings were applied in October, 2002, and the last timing (October 30) resulted in the greatest number of open buds and the greatest advance in bloom. Hand defoliation tended to reduce the percentage of open-flower truss buds, tended to delay bloom and increase both vegetative and reproductive bud death. Defoliant/nutrient treatments of zinc + urea (10 and 15 lb per acre, respectively) in 1996 and 1997 had improved pollen viability in these ‘low chill’ years (with chill portions of 58 and 52, respectively, and chill hours of 695 and 424, respectively), which appears to be reduced when chill accumulation is inadequate (Weis et al., 1996). We repeated our trial in 2004-2005 season and evaluated budbreak as a result of treatment. Objectives: Test chemical defoliants and hand-defoliation in the fall and evaluate the effects on budbreak and fruit set in >Bing= sweet cherry.