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Key questions regarding nitrogen fertilization strategies for sweet cherries include:
  —When is nitrogen needed for the various demands of the cherry tree growth cycle?
  —Does the nitrogen needed for these demands come from soil uptake or remobilization from storage tissues?

Our research has focused on when nitrogen is needed for the earliest stages of fruit and spur leaf growth, especially on precocious and dwarfing rootstocks like Gisela. Since cherries bloom, are pollinated, and set fruit concomitantly with spur leaf bud break, this critical phase of fruit formation occurs prior to the normal spring uptake of nitrogen from the soil, which Dr. Constanza Zavalloni has reported does not begin until four to five weeks after bloom. Dr. Marlene Ayala has shown that spur leaves supply the majority of carbohydrates for fruit development, rather than shoot leaves, and that the formation of spur leaf size reaches completion within three to four weeks of bloom. Hence, both of these important aspects of the cherry growth cycle depend on nitrogen either stored in the spurs and flower buds, or on nitrogen remobilized from storage tissues elsewhere in the tree. Once nitrogen supply shifts from storage sources to new uptake from the soil, bloom, fruit set, and spur leaf development have already occurred (see chart "Cherry growth and cropping timeline").

What orchard management strategies can promote optimal levels of storage nitrogen prior to bud break? In 2004, we began studying how to boost the stored nitrogen levels in spurs, thinking this would promote larger spur leaves in spring that could generate greater subsequent carbohydrate availability to developing fruit. However, we did not want this approach to stimulate late-season vegetative growth or delay cold acclimation, which could render the tree susceptible to damage from early fall freezes.

Our first study simply quantified the amount of nitrogen that is remobilized naturally in the fall from leaves to storage before leaf drop. In the leaves of Rainier and Sandra Rose on Gisela 5 rootstocks, nitrogen levels ranged from 2.1 to 2.4% during most of October, declining rapidly into early November as leaf senescence occurred. Leaf nitrogen content was 1.2% at leaf drop, so about 50% of the leaf nitrogen content was remobilized back into storage tissues in the tree, and 50% fell to the orchard floor to decompose and be recyled into the soil nitrogen pool. Concomitantly, the nitrogen levels in fruiting spurs rose from 1.3 to 1.9% by leaf drop, a 50% increase. Thus, during senescence, the recycling of nitrogen from leaves is an important means for the tree to "fertilize" its fruiting spurs in preparation for initial spring growth when root uptake of nitrogen will be nil.

The spur nitrogen levels consequently did not change appreciably during winter, but increased rapidly to 3.6% during bud swell in spring, as a result of additional nitrogen remobilization from other storage tissues throughout the tree. When we defoliated trees in early fall prior to natural leaf senescence, thereby eliminating the natural direct recycling of nitrogen from leaves, the spur nitrogen levels were 30% lower going into winter, and this deficiency remained throughout the winter.

So, how might we supplement the nitrogen reserves that are building up in cherry spurs and other storage tissues? In apple production, a late-season foliar urea application often is used with positive results. However, studies in the 1950s with foliar urea on peach found little to no uptake, leading to the "common knowledge" that stone fruits in general don’t take up foliar applications of urea. In the late 1990s, Dr. Rich Rosecrance and colleagues refuted this, showing clear uptake of urea by peach leaves in the fall. Within 48 hours of application, 50% of the urea applied was found within the leaf. Only 20% of the applied urea could be washed off the leaf surface, and it was concluded that up to 30% had already been remobilized out of the leaf to storage tissues. So much for 50 years of orchard dogma!

Consequently, we began testing single and multiple foliar applications, ranging from 3 to 5% urea solutions (6 to 10 lb urea in 25 gallons water, applied to run-off), to Hedelfinger trees on Gisela 5 in early to mid-October. Indeed, sweet cherry leaves took up urea like peach, resulting in spur nitrogen levels at leaf drop that were 30 to 40% higher than the untreated controls. These higher levels persisted in the dormant spurs throughout winter, and in the spring we measured an increase in spur leaf area of up to 20%. However, the crop loads on these older trees were highly variable, and we did not detect any ultimate effect on fruit size. Thus, while we can infer that the fall foliar urea treatments were likely to have improved the carbohydrate supply to fruiting spurs, experiments to detect any effect on fruit size will require much more highly controlled experimental trees.

Cold acclimation

Next, we focused on whether this strategy might have negative impacts on fall cold acclimation. This experiment examined cold acclimation following not only early fall treatments, but also late summer applications, when warmer temperatures might be more conducive to a potentially dangerous late flush of growth. For each treatment, two 3.5% foliar urea sprays (7 lb urea in 25 gallons of water) were applied about eight days apart to eight-year-old Ulster trees on Gisela 6. The four urea treatments were initiated on August 31, September 14, September 28, and October 12, and the trial included untreated control trees.

At four days and about three weeks after the repeat application for each treatment, as well as after leaf drop, sample shoots from current-season growth were tested for cold acclimation by stepwise exposure to low temperatures in a programmable freezer. Samples were removed at progressively colder temperatures and evaluated for tissue damage after an incubation period above freezing.

Surprisingly, the results indicated that the foliar urea treatments not only had no negative effect on cold acclimation, but actually enhanced it. At the fully dormant shoot sampling on December 12, all of the urea-treated trees were 5 to 6°F more cold hardy than the untreated control trees. Those shoots treated in early to mid-September were slightly more hardy in December than those treated later. Moreover, those early treatments also showed greater cold acclimation than the controls during the fall progression of acclimation. For example, the earliest treatment (August 31 + September 8) was 6°F more hardy on September 12 and 8°F more hardy on October 2, and the second-earliest treatment (September 14 + September 21) was 8°F more hardy than the control on October 17. The later treatments were consistently about 2 to 3°F more cold hardy than the controls, when tested from early October through mid-November.

We recommend that growers test applications of foliar urea (low biuret) at rates of about 39 to 52 pounds of urea per acre (18 to 24 lb. actual nitrogen per acre) in early to mid-September, making two applications seven to ten days apart. 

Acknowledgements

This work also involved graduate student Theoharis Ouzounis and has been funded by the International Fruit Tree Association and Michigan Agricultural Experiment Station.