Early Fall Defoliation in Sweet Cherry

Written by B. Sallato and M. Whiting, Revised 2024.

Original publication: 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

In perennial species, natural leaf senescence in the fall is part of a remobilization of mobile nutrients and carbohydrates to wood, buds and roots. These resources are utilized as the main source of energy and nutrient for bud development in the following spring (Weinbaum et al. 1984). Early defoliation in fruit trees refers to the practice of inducing leaf drop prior to natural drop in the fall. Early defoliation has been utilized in different species for various purposes, including homogenizing bloom time (Cruz-Castillo et al., 2010), advancing budbreak (Glozer and Grant, 2006) and management of certain diseases (Burchill et al., 1968, Beresford et al., 2015). However, the potential benefits and risks from early defoliation will depend on the timing of the defoliation and vigor level of the orchard. If nutrients are well managed throughout the season the risks of premature defoliation are minimal, since trees are under sink limiting conditions in late summer and whole-canopy net CO₂ exchange rates are low (Whiting and Lang, 2004, Sallato and Whiting, 2021). In this article we review potential benefits and drawbacks to early defoliation and highlight some of our recent findings in sweet cherry.

Possible benefits

1. Reduce the risk of LCD transmission by insects.

The “Little Cherry Disease” (LCD) is caused by two viruses: LChV1 and LChV2, and a phytoplasma (Candidatus phytoplasma pruni). In WA, the disease has been associated mostly to the phytoplasma, previously described as X-Disease (Gold and Sylvester, 1981). The phytoplasma is transmitted by several species of leafhoppers from their feeding on infected leaves (Jensen, 1969; Gold and Sylvester, 1981), with higher density and transmission risk during the fall (Nielson, 1968). Several measures, including cultural practices, and pesticide sprays are required throughout the season to reduce the risk of the disease (for more information on LCD, visit WSU tree fruit extension crop protection). Pesticide applications for whole season control are limited, thus an early fall leaf removal provides an alternative to reduce the risk of infection and additional pesticide sprays in the orchard and sweet cherry nurseries.

2. Reduce the risk of bacterial infection

Bacterial infections requires a wound (natural or induced) to penetrate plant tissue. When the leaves drop in the fall and detaches from the tree, it creates a natural opening, increasing the risk of bacterial infections if conditions are favorable. Bacterial canker in sweet cherries is caused by the bacterium Pseudomonas syringae pv. syringae (PSS) van Hall and is commonly associated to wet and cold weather conditions (Sallato et al., 2021).  Inducing early defoliation prior to cold conditions in the fall, can reduce the risk of bacterial canker infection in sweet cherries.

3. Reduce inoculum of fungal diseases

The control of Powdery mildew (Podosphaera clandestina) has been accomplished historically by fungicidal treatment, with repeated pesticide applications leading to fungicidal resistance (Swamy and Grove, 2021).  The disease progresses rapidly during the season, with highest levels of pathogen during late summer. Later, the pathogen overwinters in the sexual stage as chasmothecia on senescent leaves, which serve as the primary source of inoculum in the following spring (Grove and Boal, 1991). While early fall defoliation has not been evaluated for its effects on P. clandestina, this technique has been utilized to reduce initial inoculum in other fungal diseases in tree fruit (Kelly and Bai, 1997, Vincent et al., 2004, Beresford et al., 2015). It is hypothesized that inducing early leaf drop could reduce inoculum pressure, while reducing dependence on pesticide applications during the fall.

4. Reduce the risk of frost damage

Milder winters, extreme heat and drought can modify susceptibility to frost and plant developmental processes (Stöckle et al., 2010). In our study, early defoliation increased hardiness levels in ‘Skeena’, when applied 70 and 60 days before natural defoliation (Sallato and Whiting, unpublish data).

5. Reduce vigor

During the fall, nitrogen accumulated in leaves is remobilized and stored in the root system in the form of amino acids (arginine). This nitrogen becomes the main source of nitrogen for early growth the following spring. Thus, early defoliation with products other than N sources, may be useful if one wants to manage excessive vigor in sweet cherry orchards.

Precautions 

1. Reduce vigor and yield

As indicated above, during the fall, nitrogen accumulated is transported and stored in the root system, becoming the main source of nitrogen for early growth and fruit development the following spring. Thus, early defoliation in orchards with low vigor can reduce yield and fruit quality (Sallato and Whiting, 2021, Ouzounis and Lang, 2011).

2. Induce regrowth

If the defoliation is done too early, before the end of active growth, it can induce regrowth and increase the risk to frost injury.

Research of early defoliation in Washington

Below we describe the methods used in Sallato and Whiting, (2021) study, as well as other highlights from previous research. 

Chemical defoliation  

• Urea at 2% to 5% plus 2% zinc sulfate (Sallato and Whiting, 2021, Burchill 1968). In our study, chemical defoliation with urea + zinc didn’t induce complete leaf drop, but leaves became yellow between 4 to 7 days after the spray application.
• 2 – 3 % CuEDTA alone and in combination with 3% urea (Dong et al., 2004)
• Plant growth regulators such as abscisic acid (ABA) have been successfully used as a defoliant in apple and pear nursery stocks. Larsen and Higgins (1998) used two applications of ABA at 1000 ppm or one application at 2000 ppm. Currently ABA (ProTone ®, Valent Bioscience) is registered for defoliation in fruit trees when applied in combination with other defoliants. For more information, review ProTone SG ® Label

In 2020, we evaluated the application of ABA at 1500 ppm + 200 ppm ACC (1-Aminocyclopropane-1 Carboxylic Acid). Note that in 2021 Valent Bioscience registered ACC under Accede ™, for thinning in apples, for more information review Accede ® label). Both treatments were applied under experimental-use pesticide policies and procedures. Treatments and rates were applied on October 1^st 2020, approximately 50 days before natural leaf drop. Both treatments induced leaf drop after 7 to 10 days after the application, accelerating natural defoliation compared with the untreated control (Figure 2).

 

Table 1. Examples of possible defoliants and fall nutrient applications utilized in sweet cherries.

Recommended concentration of chemical	Formulation or Salt	Rate (Pounds of product in 100 gallons of water per acre)
2 – 5% Nitrogen	Urea 46% N (CO [NH2]2)	17 – 42 lb of Urea. Make sure it has less than 0.25% biuret.
2% Zinc	Zinc sulfate (ZnSO4)	17 lb of ZnSO4
2 – 3% Cu	CuEDTA 7.5%	17 – 25 lb of Chelated Cu
ABA	Abscisic acid + organosilicone surfactant	4 – 8 lbs ProTone SG ® (Valent Bioscience) + surfactant.

Note: ABA (ProTone ® and surfactants are pesticides. Please read disclaimer below.

Physical defoliation

The use of pneumatic technology for leaf removal has become a nonchemical alternative to induce leaf drop. There are several alternatives in the market and the following articles provide further information.

• Good Fruit Grower
• Review by Emily Lavely, Michigan State University Extension Specialist

Timing of defoliation

Previous work has induced premature defoliation between late September and early November (Ouzounis and Lang, 2011, Sallato and Whiting, 2021). The earlier the defoliation, the greater impact on reserve accumulation, which can be a positive or negative outcome depending on the vigor of the orchard.  No defoliation treatment in 2017 had an effect on yield in 2018 of ‘Selah’ sweet cherry (Fig. 3). However, after two consecutive years of total defoliation by hand (70 and 60 days before natural leaf drop), yield was lower in the second year; though this orchard received no additional N fertilization during the two years of study. The latest timing of manual defoliation, 50 days before natural leaf drop, did not negatively impact yield.  Chemical defoliation with Urea + Zinc Sulfate had no negative impact on fruit quality nor yield, regardless of timing (Figure 3).

CONCLUSIONS  

Inducing early leaf drop can be accomplished effectively in sweet cherry orchards.  Our recent work suggests that there may be little negative consequences on fruit yield or fruit quality in the following season.  Adopting early defoliation in any block would require a thorough assessment of orchard vigor, especially nitrogen levels in leaves, to prevent deficiencies. For more information, visit nutrient management in tree fruit, or review nutrient management in sweet cherry to determine demand and nutrient adequate levels.

IMPORTANT: Some of the pesticides discussed in this presentation were tested under an experimental use permit granted by WSDA. Application of a pesticide to a crop or site that is not on the label is a violation of pesticide law and may subject the applicator to civil penalties up to $7,500. In addition, such an application may also result in illegal residues that could subject the crop to seizure or embargo action by WSDA and/or the U.S. Food and Drug Administration. It is your responsibility to check the label before using the product to ensure lawful use and obtain all necessary permits in advance.

Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied. 

Contact

Bernardita Sallato
Tree Fruit Extensio Specialist
mail: b.sallato@wsu.edu

Phone: 509 4398542

Literature cited

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