Written by Bernardita Sallato, April 2023
If you wonder what the implications of delayed leaf abscission for spring nutrient management are, a short article has been posted regarding the physiology of leaf abscission, why leaves remained attached, and implications for nutrient remobilization. Based on our leaf tissue analysis during mid-winter, we estimated a reduction in the range of 37% to 29% in nitrogen (N) remobilization for WA 38 and Honeycrisp.
Despite this reduction in remobilization of N, we predicted this would have only a minimal impact on nutrient management requirements. In the following, I attempt to provide further information to help make better decisions and on monitoring nutrient levels in your orchards.
What elements are impacted with reduced remobilization?
Of all the nutrients, nitrogen is the only one that is remobilized during the fall and needed in large quantities for early growth.
Nitrogen, along with carbon (C), is needed to develop new structures, such as fruit, shoots, and leaves. The source of N for new growth comes from stored N (protein and amino acids) in wood, buds, and mostly in the roots that was taken up the previous season and remobilized before leaf abscission in the fall. About 80% of the N utilized in the spring comes from N remobilized from leaves (Gomez et al. 2020); therefore, when N reserves are reduced by about 40%, we could estimate about a 32% reduction in N for spring growth.
Do we need to increase our N applications?
There are several reasons why we don’t need to increase our N applications this spring.
Once the reserves are exhausted, N demand is provided by the current season uptake via new roots. In normal conditions N uptake starts approximately 40 days after full bloom in apples (Hart et al 1997) and around the end of cell division in sweet cherries (Artacho and Bonomelli, 2016). However, if N reserves become exhausted earlier, uptake will commence earlier too, provided the temperatures in the soil are above 59 F. Thus, make sure you start your soil N applications during bloom.
To assimilate N, plants need carbon, which is provided by carbohydrate reserves and photosynthesis (Marschner, 2002). Thus, prior to leaf expansion, C also comes from stored sources. If C reserves were depleted, then N demand will also be lower. If there is excessive N in the system (via sprays) and not enough C to assimilate, N can become toxic.
Most tree fruit orchards have adequate to luxurious levels of N and orchardists need to manage the excessive growth with summer or fall pruning, deficit irrigation, de-leafing, and root pruning, among others. In 2022 we observed increased levels of N (and above adequate range) in sweet cherries and apple orchards. Lower crop loads observed in 2022 in many of these orchards are also reflected in high vigor and higher N levels in the trees. A good example of the minimal impact of reduced remobilization was provided by Sallato and Whiting (2022), where artificial hand defoliation 70, 60, and 50 days before natural leaf drop had no negative impact on fruit quality or yield the following spring.
In what conditions should we support with foliar N sprays?
The following conditions could justify additional N application during spring:
- Orchards where N levels in leaves were low in 2022, with low vigor, smaller leaves or general yellow leaves (chlorosis), where water was not a limiting factor (excessive or deficient).
- Sites where soil temperatures during bloom are still below 59 F.
Based on previous work by several authors, the apple N demand per ton of fruit is approximately 2.1 lb. (adapted by Sallato et al, 2009). Cheng and Raba (2009) indicated that 50% of the season’s demand comes from reserves (1.0 lb. per ton), thus, if stored, N was shortened by 32%, then we could estimate about 17 lb. less N for a 50 ton per acre orchard.
Under the two conditions listed above, a foliar spray of 10 lb. of N per acre could support initial growth. For leaf nutrient sprays, the mix of urea with micronutrients has shown to improve uptake (Fernandez et al 2013). If B was deficient in 2022; leaf levels below 20 mg per kg and below 1.0 mg per kg in soils, adding 0.5 lb. boron (B) per acre to the mix will be beneficial. Do not exceed 1 lb. per acre of actual B as it can cause phytotoxicity (Faust, 1989).
How can we determine if additional N is needed?
While there are few references of nutrient levels for green tip to blossoms in apples or sweet cherry, in our previous work on Honeycrisp, N levels at pink stage vary between 3.8% and 4.0%. Similar values (average 3.2) were observed by E. Smith (personal communication) at pink in several Honeycrisp blocks near Pasco. We will collect blossoms from these same sites to determine differences to previous years.
Monitor nutrient levels by sampling recently mature leaves to determine if your trees are within adequate levels, deficient or excessive. For more information visit leaf tissue analyses. Note that N levels in leaves should always be contrasted with tree vigor.
Why we shouldn’t worry about other nutrients?
Phosphorus (P) is required in low quantities and supply does not depend on remobilization of P from the leaves during the fall. Maximum demand for P coincides with the period of fruit enlargement. Similarly, potassium (K) uptake correlates with photosynthetic activity with higher demand during active growth later in the season. Calcium is a non-mobile element. It is not remobilized from leaves, thus not affected by a lack of remobilization.
The micronutrients B and zinc (Zn) are needed during the spring for pollen tube growth. However, the amount needed is low and commonly supplied effectively through foliar sprays during the fall or spring and not affected by leaf nutrient remobilization.
Artacho P y C. Bonomelli. 2017. Effects of nitrogen availability on root dynamics in ‘Bing’ on Gisela®6 sweet cherry trees. Acta Horticulturae 1161 pp.137-141.
Cheng, L. and R. Raba. 2009. Nutrient Requirement of Gala/M.26 Apple tree for high yield and quality. Cornell University
Faust, 1989. Physiology of temperate zone fruit trees. John Wiley and Sons. New York.
Fernandez, V., T. Sotiropoulos and P. Brown. 2013. Foliar Fertilization; Scientific Principles and Field Practices. First edition, IFA, Paris, France.
Hart. T, T. Riguetti, B. Stevens, B. Stebbins, P. Lombard, D. Burkhart, and P. Van Buskirk. 1997. Pears. Fertilizer guide. Oregon State University Extension Service.
Sallato, B., L. Kalcsits, T. Schmidt and M. Whiting. 2023. Implications of delayed leaf abscission for spring nutrient management. Fruit Matters, February 2023. Online: https://treefruit.wsu.edu/article/implications-of-delayed-leaf-abscission-for-spring-nutrient-management/
Sallato, B. and Whiting, M.D. 2022. Early fall defoliation reduces yield and bud nutrient concentration in ‘Selah’ sweet cherry. Acta Hortic. 1333, 195-202. DOI: 10.17660/ActaHortic.2022.1333.25. Summary report https://treefruit.wsu.edu/article/early-fall-defoliation-in-sweet-cherry/
Gomez, L., Vercambre, G., & Jordan, M. O. 2020. Spatial-temporal management of nitrogen and carbon on the peach tree (Prunus persicae L. Batsch.). Scientia Horticulturae, 273, 109613.