APS_April 2023
Volume 77
APRIL 2023
Number 2
AMERICAN POMOLOGICAL SOCIETY F ounded in 1848 I ncorporated in 1887 in M assachusetts
2022-2023
PRESIDENT K. GASIC
FIRST VICE PRESIDENT P. CONNER
SECOND VICE PRESIDENT M. OLMSTEAD
TREASURER A. ATUCHA
EDITOR R. P. MARINI
SECRETARY L. DEVETTER
RESIDENT AGENT MASSACHUSETTS W. R. AUTIO
EXECUTIVE BOARD
N. BASSIL Past President
N. BASSIL President
P. CONNER 1 st Vice President
M. OLMSTEAD 2 nd Vice President
L. DEVETTER Secretary
TOM KON ('19 - '23)
GINA FERNANDEZ ('21 - '24)
DAVID KARP ('22 - '25)
ADVISORY COMMITTEE 2020-2023 B. BYERS M. DOSSETT A. PLOTTO E. VINSON D. WARD 2021-2024 J. SAMTANI D. TRINKA S. MEHLENBACHER M. FARCUH G. BRAR 2022-2025 C. LUBY M. MUEHLBAUER L. REINHOLD A. WALLIS S. YAO
CHAIRS OF STANDING COMMITTEES
Editorial R. PERKINS-VEAZIE Wilder Medal Awards B. BLACK
Shepard Award F. TAKEDA Nominations M. PRITTS
Membership M. PRITTS
U. P. Hedrick Award E. FALLAHI
Website M. OLMSTEAD
Registration of New Fruit and Nut Cultivars J. PREECE, K. GASIC, D. KARP
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April 2023
Volume 77 CONTENTS
Number 2
Published by THE AMERICAN POMOLOGICAL SOCIETY Journal of the American Pomological Society (ISSN 1527-3741) is published by the American Pomological Society as an annual volume of 4 issues, in January, April, July and October. Membership in the Society includes a volume of the Journal. Most back issues are available at various rates. Paid renewals not received in the office of the Business Manager by January 1 will be temporarily suspended until payment is received. For current membership rates, please consult the Business Manager. Editorial Office: Manuscripts and correspondence concerning editorial matters should be addressed to the Editor: Richard Marini, 203 Tyson Building, Department of Plant Science, University Park, PA 16802-4200 USA; Email: richmarini1@gmail.com. Manuscripts submitted for publication in Journal of the American Pomological Society are accepted after recommendation of at least two editorial reviewers. Guidelines for manuscript preparation are the same as those outlined in the style manual published by the American Society for Horticultural Science for HortScience, found at https://cdn.ymaws.com/ashs.org/resource/resmgr/publications/ashspubsstylemanual.pdf Postmaster: Send accepted changes to the Business office. Business Office : Correspondence regarding subscriptions, advertising, back issues, and Society membership should be addressed to the Business Office, C/O Heather Hilko, ASHS, 1018 Duke St., Alexandria, VA 22314; Tel 703-836 4606; Email: ashs@ashs.org Page Charges : A charge of $50.00 per page for members and $65.00 per page ($32.00 per half page) will be made to authors. In addition to the page charge, there will be a charge of $40.00 per page for tables, figures and photographs. Society Affairs : Matters relating to the general operation of the society, awards, committee activities, and meetings should be addressed to Michele Warmund, 1-31 Agriculture Building, Division of Plant Sciences, University of Missouri, Columbia MO 65211; Email:warmundm@missouri.edu. Society Web Site : http://americanpomological.org Fire Blight Susceptibility of 20 Diverse Pear ( Pyrus spp.) Rootstock Breeding Parents – Zara York, Soon Li The, and Kate Evans......................................................................................................... 66 Summer Applications of Plant Growth Regulators, Ethephon And 1-Naphthaleneacetic Acid, Do Not Promote Return Bloom or Reduce Biennial Bearing in Seven High-Tannin Cider Apple Cultivars - David L. Zakalik, Michael G. Brown, Craig J. Kahlke, and Gregory M. Peck. .......... 75 Rio Grande do Sul Feral Olives may Increase the Species Genetic Variability – Juan M. Caballero and Guajara J. Oliveira................................................................................................................... 93 Effects of Vaccinium arboreum Rootstocks on Yield and Fruit Quality of ‘Patrecia’ Southern Highbush Blueberry Grown with Minimum Soil Amendment - Cecilia Rubert Heller, Gerardo H. Nunez, and Jeffrey G. Williamson. .............................................................................................. 103 Sorbus sensu lato: A Complex Genus with Unfulfilled Crop Potential – Ryan King, Nahla Bassil, Todd Rounsaville, and Lauri Reinhold ........................................................................................ 110 About the Cover: ‘Tozumal’ Mamey Sapote.................................................................................. 92 Instructions to Authors. ................................................................................................................ 128
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Journal of the American Pomological Society 77(2): 66-74 2023
Fire Blight Susceptibility of 20 Diverse Pear (Pyrus spp.) Rootstock Breeding Parents Z ara Y ork , S oon L i T eh , and K ate E vans
Abstract Fire blight is a bacterial disease caused by Erwinia amylovora , which can cause devastating losses to pear ( Pyrus spp.) growers. Infections can lead to a reduction in fruit yield, the need to remove some or all scion tissues, and entire tree death. Rootstocks with lower fire blight susceptibility can confer some degree of tolerance to suscep tible scions. Since most U.S. pear cultivars are susceptible to fire blight infection, breeding low-susceptibility rootstock cultivars can help decrease losses for the pear industry. The Washington State University (WSU) Pear Rootstock Breeding Program was established to develop Pyrus rootstocks, with target traits such as dwarfing, pre cocity, cold-hardiness and reduced fire blight susceptibility. This study evaluated fire blight response of 20 diverse accessions, as grafted scion tissue. Two greenhouse experiments were conducted in 2021 on up to 20 individuals per accession, which were arranged in a randomized complete block design with four blocks and five replicates. One actively growing shoot per tree was inoculated with E. amylovora strain 153n. Fire blight response was measured after disease progression stopped and was quantified as percent shoot length blighted (%SLB). Average accession responses ranged from 0.1 to 100 %SLB and were highly correlated between experiments (Pearson’s r = 0.83, P ≤ 0.001). Individuals in Experiment B had significantly higher severity of infection; however, the relative order of accession based on severity was consistent with that of Experiment A. In both experiments, nine accessions consistently exhibited low fire blight susceptibility (0.1 to 10.9 %SLB), while six accessions had high fire blight susceptibility (35.2 to 100 %SLB). Results from this study provide insights for 20 potential breeding parents in the WSU Pear Rootstock Breeding Program.
The Pacific Northwest (PNW) accounts for around 80% of U.S. pear ( Pyrus spp.) production, which was valued at over $290 million in 2021 (USDA-ARS NASS, 2022). Pear orchards in the PNW typically use semi-dwarfing Pyrus rootstocks with only a few hundred trees per acre compared to thousands of trees per acre in high-density plantings (Elkins et al., 2012). Globally, pear producers typically use quince rootstocks to reduce scion vigor and facilitate high-density plantings; however, concerns about lack of cold-hardiness and potential graft incompat ibility with pear scion cultivars have limited adoption of quince rootstocks in major pear producing regions of the United States (Ein horn, 2021). High-density planting systems allow for more uniform canopy structure and disease/
pest management, thereby reducing labor and input costs while increasing production efficiency (Elkins et al., 2012). Transition to high-density pear planting systems has been limited due to the lack of dwarfing, preco cious rootstocks that are suitable for the PNW (Elkins et al., 2012). Breeding for new pear rootstocks also targets traits such as low susceptibility to prevalent diseases and pests (Brewer and Volz, 2019; Guzman and Dh ingra, 2019). Fire blight, a bacterial disease caused by Erwinia amylovora, has a severe impact on rosaceous crops such as pear. Fire blight causes millions of dollars per year in dam age due to loss of fruit production, removal and replacement of hundreds of acres of trees during extreme outbreaks, and labor required for scouting and removal of infected scion
Washington State University Tree Fruit Research and Extension Center, 1100 N. Western Ave, Wenatchee, WA 98801 Corresponding author: kate_evans@wsu.edu
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level of diversity from wild relatives and in terspecific hybrids. In this study, 20 diverse Pyrus accessions were evaluated to identify potential sources of reduced fire blight sus ceptibility. Data from this study can be used to help inform parental selection, which is particularly valuable in a crop that has a long generation time with an extended juvenile phase. Materials and Methods Seventeen accessions from the WSU Py rus parental germplasm collection, along with ‘Beurre d’Anjou’ (referred to as ‘An jou’), ‘Bartlett’, and ‘OH× F 87’ as industry references, were evaluated in this study (Ta ble 1). Dormant budwood of each accession was grafted onto actively growing ‘OH×F 87’ rootstocks, generating up to 20 clones per accession. Trees were grown in half-gallon bags in a greenhouse located at the WSU Tree Fruit Research and Extension Center (47°26’16.5”N 120°20’50”W). Six weeks af ter grafting, each tree was fertilized with 0.85 g of an 18N-7.8P-14.9K blend. Trees were divided into two experiments due to differ ential shoot growth rates, each of which was randomized into a complete block design, consisting of four blocks with five acces sion replicates per block. Secondary shoots from the graft stick were removed, leaving a single actively growing shoot. Adventitious rootstock shoots were also removed if pres ent. The greenhouse was maintained with no supplemental light and maximum cooling for both experiments. Average temperatures re corded were ~21 °C (Experiment A) and ~24 °C (Experiment B), and recorded humidity levels were an average of ~85% (Experiment A) and ~75% (Experiment B). Inoculum suspension was prepared with freeze-dried E. amylovora strain 153n ac cording to the protocol described by Johnson et al. (2009). Inoculum suspension consist ed of 0.01 M dibasic phosphate buffer, pH 7, with an inoculum concentration of 1 ×10 9 CFU/mL. Cut leaf inoculation was per formed once per individual on an actively
tissue (van der Zwet et al., 2012a). Root stocks with low fire blight susceptibility can be re-grafted if an infected scion is removed, reducing losses due to tree replacement and establishment, and are critical for high-den sity planting systems where trees are in close contact (van der Zwet et al., 2012a). Severity of fire blight can vary based on tissue type and maturity, tree vigor, environ mental conditions, and virulence of E. amy lovora strains (Billing, 2011; Norelli et al., 2003a; Norelli et al., 2003b; Schroth et al., 1974). Points of infection include pear blos soms, stomata in young shoots, and wound ing to the scion and/or rootstock suckers (Schroth et al., 1974). Pear typically exhib its high levels of vigor which can facilitate bacterial spread throughout the tree (van der Zwet et al., 2012b). Evaluation of fire blight response can be difficult due to varying symptoms, such as bacterial ooze, shoot cracking, shriveled ne crotic lesions, and/or the characteristic shep herd’s hook at the end of a shoot. Artificial inoculation in a greenhouse allows for stan dardization of bacterial strain(s), inoculum concentration, and inoculation method, such as cut-leaf shoot inoculation, as well as con trolling of greenhouse environmental condi tions (Norelli et al., 1988). While artificial inoculation may not fully replicate natural inoculation in an orchard setting, standard ized inoculation helps minimize external factors when assessing germplasm for use as breeding parents (Peil et al., 2021; Pankova et al., 2023). Low fire blight susceptibility is an impor tant target in pear scion and rootstock breed ing programs (Brewer et al., 2021; Brewer and Palmer, 2011; Musacchi et al., 2005; Peil et al., 2009, 2021). The Washington State University (WSU) Pear Rootstock Breeding Program (PRBP) was established in 2015 to develop pear rootstocks for the U.S. pear industry, and target traits such as conferred dwarfing, induced precocity, low disease susceptibility, and cold hardiness. The WSU PRBP Pyrus germplasm collection has a high
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Table 1. Species and reported susceptibility of 20 diverse Pyrus accessions. Table 1. Species and reported susceptibility of 20 diverse Pyrus accessions. 354 Accession z Pyrus species y
Reported susceptibility x
Anjou (Beurre d’Anjou)
communis L. communis L. communis L. communis L. communis L. communis L. communis L.
Moderate t
Bartlett
High t Low t
Farmingdale Mustafabey OH×F 333
Moderate to high u,v,w
Low t Low s Low t
OH×F 87 Old Home
GE-2004-131
communis L. subsp. caucasica (Fed.) Browicz communis L. subsp. pyraster (L.) Ehrh.
Unknown
P-87 Du Li
Low t
betulifolia Bunge calleryana Decne. calleryana Decne . salicifolia Pall. xerophila T.T.Yu
Unknown
OSU-2 OSU-8
Low t Low t
P. salicifolia (hybrid) - Russia P. xerophila - Lawyer Nursery
Unknown Unknown Unknown Unknown Unknown Unknown Unknown
Hybrid 1 Hybrid 2 Hybrid 3 Hybrid 4 Hybrid 5
Interspecific; dimorphophylla, fauriei
Interspecific; betulifolia, calleryana, communis Interspecific; betulifolia, fauriei, spinosa
Interspecific; elaeagrifolia, spinosa
Interspecific calleryana; salicifolia; ussuriensis
Hybrid 6 ( P. betulifolia -1 × P-79) Interspecific; betulifolia (1), communis (P-79) Low t z Budwood was collected from 17 accessions in the WSU Pyrus parental germplasm collection, along with ‘Bartlett’, ‘Anjou’, and 355 ‘OH×F 87’. 356 y Ten species are represented overall, either as individual accession or within the background of an interspecific hybrid. 357 x Previously documented susceptibility is included when possible. 358 w Aysan et al., 1999 359 v Çitir and Mirik, 1999 360 u Demir and Gündogdu, 1993 361 t USDA-ARS NCGR, 2017 362 s Postman et al., 2013 z Budwood was collected from 17 accessions in the WSU Pyrus parental germplasm collection, along with ‘Bartlett’, ‘Anjou’, and ‘OH ×F 87’. y Ten species are represented overall, either as individual accession or within the background of an interspecific hybrid. x Previous y documented susceptibility is included when possible. w Aysan et al., 1999 v Çitir and Mirik, 1999
u Demir and Gündogdu, 1993 t USDA-ARS NCGR, 2017 s Postman et al., 2013
363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384
growing shoot, preferably ≥ 10 cm in length. Scissors were dipped in inoculum suspen sion prior to bisecting the middle of two unfurling apical leaves (Norelli et al., 1988; Kostick et al., 2019; Zurn et al., 2020). Five replicates (individual trees) of each accession were inoculated per block; however, some blocks had fewer than five replicates due to graft failure. Inoculations were performed on 31 May 2021 (Experiment A) and 2 June 2021 (Experiment B). Experiment B was performed in the previously described man ner, using freshly-prepared inoculum at the same concentration of the same freeze-dried Ea 153n stock. Shoots were assessed for response to fire blight after disease progression had stopped, beginning around six weeks post-inoculation
(Kostick et al., 2019). Length of each necrot ic response was measured and overlapping responses were measured as a continuous length to avoid double counting (Harshman et al., 2017). Total length of the shoot was recorded and percent shoot length blighted (%SLB) was calculated by dividing the sum of necrotic response lengths by total shoot length and multiplying by 100 (Kostick et al., 2019). One-way analysis of variance (ANOVA) was performed to determine significant dif ferences between experiments and among ac cessions. As Experiment A and Experiment B were determined to be significantly different, they were subsequently analyzed separate ly. Pearson’s correlation of average %SLB between Experiment A and Experiment B
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was also calculated. Significant differences among accession %SLB were determined using Tukey’s Honest Significant Difference (HSD). The unbalanced accession replicates due to graft failure necessitated the use of a linear mixed effects (LME) model with a restricted maximum likelihood (REML) fit. Block and replicate were calculated as ran dom effects, while accession as a fixed effect. In addition, statistical differences of each ac cession’s average fire blight incidence (num ber of individuals with any visible response out of the total number of individuals) were calculated using ANOVA and Tukey’s HSD. All statistical analyses were performed us ing R 4.1.2 (R Core Team, 2022) and RStu dio (RStudio Team, 2020), along with soft ware packages ‘agricolae’ for ANOVA and Tukey’s HSD (de Mendiburu, 2019), and ‘lmerTest’ for LME models (Kuznetsova et al., 2017). Results Overall fire blight responses (%SLB) dif fered significantly between Experiment A and B (ANOVA, P ≤ 0.05; LME model, P ≤ 0.01); however, the correlation of acces sions’ average %SLB was 0.83 (Pearson; P ≤ 0.001), indicating overall consistency be tween the experiments (Fig. 1 ). Within each experiment, block and replicate did not have significant effects on %SLB ( P ≥ 0.05). Vari ability at the accession level was determined to be significant for %SLB based on an ANOVA and LME models (Experiment A, P ≤ 0.001; Experiment B, P ≤ 0.001). Accessions were assigned to mean separa tion groups through Tukey HSD tests (sig nificance level < 0.05) for %SLB (Table 2). Five groups were designated within Experi ment A, and eight groups within Experiment B. While significant differences were identi fied for %SLB (i.e., severity), there was no significant difference determined for percent incidence by accession across the two experi ments. Fire blight severity and incidence respons es varied among accessions. For example,
‘Hybrid 6’ had a low severity response with %SLB of 0.1% and 0.8%, maximum %SLB of 1.4% and 15.4%, and a low incidence (5%) in both experiments (Table 2). ‘Hybrid 3’ had low severity for both average %SLB (A: 1.2%, B: 1.7%) and maximum %SLB (A: 3.9%, B: 7.6%); however, the incidence was 55% and 50% for Experiment A and Experi ment B respectively. Accessions with lower %SLB and high incidence include ‘Hybrid 5’ (%SLB A: 4.1%, B: 10.6%; and incidence A: 63.6%, B: 44.4%), and ‘P-87’ (%SLB A: 4.0%, B: 16.3%; and incidence A: 64.7%, B: 89.5%). Moderate incidence with high sever ity was observed for several accessions, in cluding ‘Hybrid 1’ and ‘OH×F 333’ that had Max %SLB of 100% in both experiments, with 40% incidence for ‘Hybrid 1’ in both experiments and respective incidences of 35% and 72% for ‘OH×F 333’. ‘Du Li’ exhibited the highest susceptibility in both experiments. In Experiment A, it had 100% incidence and 100% severity for av erage %SLB, representing total shoot death for all individuals. In Experiment B, it had a 94.7% incidence with an average of 94.7 %SLB, representing 19 out of 20 individuals that had total shoot death. Six accessions consistently showed the highest levels of susceptibility in both ex periments (‘Du Li’, ‘Hybrid 4’, ‘ P. salicifolia (hybrid) - Russia’ , ‘Bartlett’, ‘GE-2004-131’, ‘Mustafabey’; Fig. 1 and Table 2), with high fire blight incidence (85-100%) and average %SLB ranging from 35% to 100%. Each had individuals where the fire blight infection resulted in total shoot death (i.e., maximum %SLB = 100%). Nine accessions were identified with lower susceptibility in both experiments: ‘Hybrid 6’, ‘OSU-2’, ‘OH×F 87’, ‘Hybrid 3’, ‘Old Home’, ‘Anjou’, ‘OSU-8’, ‘Farmingdale’, ‘Hybrid 5’; (Fig. 1 and Table 2). These acces sions had an average %SLB ranging from 0 to 11, and an average fire blight incidence of 5% to 65%. Within all individuals from these nine accessions, a single ‘Anjou’ replicate had total shoot death in Experiment A and
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Fig. 1. Correlation between Experiments A and B for average fire blight response of 20 diverse Pyrus accessions (Pearson’s r = 0.83, P ≤ 0.001). Enlarged inset for accessions with low susceptibility.
none of the individuals had total shoot death in Experiment B. ‘Hybrid 6’ had the lowest average %SLB (Experiment A: 0.1; Experi ment B: 0.8) and lowest incidence (5% for both Experiment A and B). The 5% fire blight incidence represented only 1 out of 20 indi viduals that displayed necrotic response. Fire blight lesions varied among acces sions with some being more prone to crack ing responses (Fig. 2A) or shriveled necrotic tissue (Fig. 2C), while other accessions tend ed to have responses that were necrotic and cracking (Fig. 2B) (data not shown). Discussion Low fire blight susceptibility is an impor tant trait for parental selection in the WSU Pear Rootstock Breeding Program. Up to 20 replicates of 20 diverse Pyrus accessions were evaluated for fire blight susceptibility as scions grafted on ‘OH×F 87’ rootstocks
in two consecutive greenhouse experiments. Results from the second experiment validat ed those of the first. Growth and environmental variability likely contributed to varying disease sever ity between experiments. Slightly warmer and less humid conditions in Experiment B were more conducive to bacterial and shoot growth, resulting in increased severity. De spite these differences, disease incidence was consistent between the two experiments, and the high correlation of disease severity (i.e., %SLB) illustrate repeatability of relative ac cession response (Fig. 1 and Table 2). Our results agreed with previously reported susceptibility levels for ten of the eleven ac cessions; ‘Anjou’ had lower fire blight suscep tibility in this experiment compared with pre vious reports (Table 1) (USDA-ARS NCGR, 2017). This could be due to differences in bac terial strain, orchard/greenhouse conditions
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407
408 Fig. 2. Examples of observed pear fire blight responses: A – cracking; B – necrotic and 409 cracking; C – shriveled necrotic. 410 Fig. 2. Examples of observed pear fire blight responses: A – cracking; B – necrotic and cracking; C – shriveled necrotic.
or rootstock combinations, which have been reported to impact fire blight susceptibility in other accessions (Aleksandrova et al., 2020; Cabrefiga and Montesinos, 2005). Three low susceptibility accessions were consistent with published literature: ‘Old Home’, ‘Farmingdale’, and ‘OH×F 87’ (Postman et al., 2013). ‘OH×F 333’ exhib ited lower susceptibility in Experiment A, with a moderate susceptibility in Experiment B, both of which are consistent with previ ous reports of low and moderate susceptibil ity (Aleksandrova et al., 2020). Accessions ‘P-87’, ‘OSU-2’, ‘OSU-8’, and ‘Hybrid 6’ also displayed low susceptibility according to National Clonal Germplasm Repository information (USDA-ARS NCGR, 2017). Two of the most susceptible accessions were ‘Bartlett’, which is consistent with pre vious reports (USDA-ARS NCGR, 2017), and ‘Mustafabey’, which has previously been
reported as having moderate to high suscep tibility (Aysan et al., 1999; Çitir and Mirik, 1999; Demir and Gündogdu, 1993). ‘Mus tafabey’ was one of the six most susceptible accessions in this study, with Max %SLB of 100 in both experiments and incidences of 100% (Experiment A) and 85% (Experiment B). Average %SLB were more moderate in comparison to the other highly susceptible accessions, with 35.2% and 59.6% respec tively compared to average %SLBs ranging from 59.9-100%. No reports were found for susceptibility levels for the other nine accessions. Of these, four had high susceptibility in this study (‘GE-2004-131’, ‘Du Li’, ‘ P. salicifolia (hy brid) - Russia’, ‘Hybrid 4’), three exhibited moderate susceptibility (‘ P. xerophila - Law yer Nursery’, ‘Hybrid 1’, ‘Hybrid 2’), and two displayed low susceptibility (‘Hybrid 3’, ‘Hybrid 5’).
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Table 2. Severity and incidence of fire blight response in 20 diverse Pyrus accessions. Table 2. Severity and incidence of fire blight response in 20 diverse Pyrus accessions. 385 Experiment A Experiment B
MS groups y
Ave. %SLB x
Max %SLB v
% Incid. u
Num. indiv. t
MS groups
Ave. %SLB
Max %SLB
% Incid.
Num. indiv.
Accession z
Du Li
a
100.0 100.0 100.0
19 20 20 20 19 20 20 20 20 17 20 11 16 17 20 20 20 20 20 20
a
94.7 100.0 94.7
19
Hybrid 4
ab
76.7 100.0 67.0 100.0
95.0 90.0
ab ab
84.7 100.0 100.0 18
P. salicifolia ( hybrid ) - Russia
b b
85.5 100.0 95.0
20
Bartlett
62.0 100.0 100.0 35.2 100.0 100.0 59.9 100.0 100.0
abc 85.4 100.0 100.0 20
Mustafabey GE-2004-131 OH×F 333 B
bc bc
bcd 59.6 100.0 85.0
20
bcde 65.8 100.0 100.0 20
cdef 44.6 100.0 72.2 defg 33.5 100.0 40.0 defg 25.0 100.0 68.4
18 20 19
Hybrid 1 Hybrid 2
cd cd de de de de de de de de de de de
26.8 100.0 22.9 100.0 14.6 100.0
40.0 60.0 35.0 64.7 45.0 63.6 31.3 29.4 30.0 20.0 55.0 25.0 20.0
OH×F 333 A
P-87
4.0 8.0 4.1 3.1 7.5 8.7 1.2 0.9 1.1 0.1
12.0 71.5 30.7 19.4 86.9
defg 16.3 76.7 89.5
19 20
P. xerophila – Lawyer Nursery
efg fgh
27.4 100.0 60.0 10.6 31.7 44.4 7.0 63.8 33.3 2.9 21.2 37.5
Hybrid 5
9
Farmingdale
gh gh gh gh gh gh gh
15 16 19 20 20 20 17
OSU-8
Anjou (Beurre d’Anjou)
10.9 100.0
1.4
9.5
26.3
Old Home Hybrid 3 OH×F 87
87.7
2.2 40.6 20.0
3.9 6.7
1.7
7.6
50.0
2.6 16.7 50.0 1.5 12.5 23.5
OSU-2
10.6
z Accessions were ordered within their respective mean separation groups for p ercent shoot length blighted (%SLB) for easy comparison between experiments. Accession ‘OH×F 333’ was unable to be aligned across experiments and is designated with an ‘A’ or ‘B’ superscript to indicate the respective experiment. y Mean separation groups within experiments were determined using an analysis of variance and Tukey’s Honest Significant Dif ference test for %SLB. x Average of individuals’ %SLB within accession, calculated by dividing shoot length blighted by total shoot length, multiplied by 100. v Maximum %SLB represents the most severe response for an individual within each accession u %Incidence is calculated using the number of trees with fire blight response divided by total number of individuals per accession , multiplied by 100. t Number of individuals inoculated per accession. 20 z Accessions were ordered within their respective mean separation groups for percent shoot length blighted (%SLB) for easy 386 comparison between experiments. Accession ‘OH×F 333’ was unable to be aligned across experiments and is designated 387 with an ‘A’ or ‘B’ superscript to indicate the respective experiment. 388 y Mean separation groups within experiments were determined using an analysis of variance and Tukey’s Honest Significant 389 Diff renc test f r %SLB. 390 x Averag of individuals’ %SLB within accession, calculated by dividing shoot length blighted by total shoot length, multiplied 391 by 100. 392 v Maximum %SLB represents the most severe response for an individual within each accession 393 u %Incidence is calculated using the number of trees with fire blight response divided by total number of individuals per 394 accession, multiplied by 100. 395 t Number of individuals inoculated per accession. Hybrid 6 ( P . betulifolia -1 × P-79) e 1.4 5.0 h 0.8 15.4 5.0
396 397 398
Ideally, a rootstock would have both low disease severity and incidence, such as ‘Hy brid 6’ (Table 2). ‘Hybrid 3’ maintained low severity (%SLB and Max %SLB), but had moderate incidence. A rootstock that sus tains mild infections has a greater chance of surviving and can be re-grafted if neces sary. However, high incidence of rootstock infection can potentially lead to an increased number of susceptible scions at risk for bac terial transmission (Santander et al., 2020). ‘Hybrid 5’ and ‘P-87’ had lower %SLB with a moderate Max %SLB and moderate inci dence of infection. When an accession has
moderate incidence with high severity, fewer trees may be infected, although the infection is more likely to lead to tree loss. In summary, this fire blight study pro vides comparative data for potential breed ing parents evaluated with the same strain and similar greenhouse growing conditions. Six accessions were identified as highly sus ceptible, and new information is reported for nine accessions with previously unknown fire blight responses. Accessions were iden tified in this study that had comparable or lower susceptibility than the industry stan dard ‘OH×F 87’. The nine accessions that ex -
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hibited consistent lower susceptibility could be candidates for parents in future rootstock breeding crosses. Acknowledgements This work was supported by the USDA National Institute of Food and Agriculture Hatch project 1014919 and the Fresh and Processed Pear Committee research project PR-19-108. We would like to thank Bonnie Schonberg and Sarah Kostick for help with experimental set up and design, as well as Caitlin Madden and Nancy Buchanan for their help with data collection. We also thank Frank Zhao and Sarah Kostick for their feed back on the manuscript. Literature Cited Aleksandrova, D., P. Chavdarov and M. Nesheva. 2020. Fire blight susceptibility of pear cultivars grafted on OHF 333 rootstock. Scientific Papers. Series B, Horticulture. Vol. LXIV, No. 2, Print ISSN 2285-5653, CD-ROM ISSN 2285-5661, Online ISSN 2286-1580, ISSN-L 2285-5653. Aysan, Y., Ö. Çinar, S. Tokgönül and A. Küden. 1999. Biological, chemical, cultural control methods and determination resistant cultivars to fire blight in pear orchards in the Eastern Mediterranean region of Turkey. Acta Hort. 489:549–552. Billing, E. 2011. Fire blight. Why do views on host invasion by Erwinia amylovora differ? Plant Pathol. 60:178–189. Brewer, L., M. Aldsworth, V.G.M. Bus, M. Horner, L.K. Jesson and R.K. Volz. 2021. Breeding for fire blight resistance in an interspecific pear breeding programme. Acta Hort. 1303:49–54. Brewer, L. and J. Palmer. 2011. Global pear breeding programmes: Goals, trends and progress for new cultivars and new rootstocks. Acta Hort. 909:105– 119. Brewer, L. and R. Volz. 2019. Genetics and breeding of pear, pp. 63–101. In S.S. Korban (ed.). The pear genome. Springer International Publishing, Cham, Switzerland. Cabrefiga, J. and E. Montesinos. 2005. Analysis of ag gressiveness of Erwinia amylovora using disease dose and time relationships. Phytopathol. 95:1430– 1437. Çitir, A. and M. Mirik. 1999. Fire blight of pome fruits and search for resistant or tolerant cultivars in Amasya and Tokat regions in Turkey. Acta Hort. 489:215–220.
de Mendiburu, F. 2019. agricolae: Statistical proce dures for agricultural research. (R Package version 1.3-1) [Computer software].
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fire blight across different conditions. Plant Protect. Sci. 59:48–58. Peil, A., V.G.M. Bus, K. Geider, K. Richter, H. Fla chowsky and M-V. Hanke. 2009. Improvement of fire blight resistance in apple and pear. Int. J. Plant Breeding. 3:1–27. Peil, A., K. Richter, A. Wensing, M. Höfer, O.F. Em eriewen and T. Wöhner. 2021. Fire blight resistance breeding. Acta Hort . 1327:65–72. Postman, J., D. Kim and N. Bassil. 2013. OH ×F pa ternity perplexes pear producers. J. Amer. Pomol. Soc. 67:157-167. R Core Team. 2022. R: A language and environment for statistical computing. R Foundation for Statisti cal Computing.
USDA-ARS National Clonal Germplasm Repository. 2017. USDA Pyrus germplasm collection (Septem ber 2017). 4 August 2022.
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Journal of the American Pomological Society 77(2): 75-92 2023 Summer Applications of Plant Growth Regulators, Ethephon And 1-Naphthaleneacetic Acid, Do Not Promote Return Bloom or Reduce Biennial Bearing in Seven High-Tannin Cider Apple Cultivars D avid L. Z akalik 1 , M ichael G. B rown 1 , C raig J. K ahlke 2 , and G regory M. P eck 1 Abstract Biennial bearing in high-tannin cider apple cultivars ( Malus ×domestica Borkh. ) exacerbates supply chain issues for cidermakers in North America. Two experiments investigated the efficacy of using the plant growth regulators (PGRs) ethephon and 1-naphthaleneacetic acid (NAA) in midsummer (i.e., not for fruitlet thinning) to promote return bloom in cider apple trees, and effects of these PGRs on yield. One experiment was con ducted over three years at a commercial orchard in Lyndonville, NY, and the other over two years at a research orchard in Lansing, NY. The Lyndonville experiment compared hand-thinning against various combinations of ethephon and NAA, while the Lansing experiment compared hand-thinning and midsummer PGR applications alone against a combination of both treatments. The Lansing experiment also assessed effects on fruit maturity and juice quality. At Lyndonville, bloom and yields followed a highly “biennial” pattern for the PGR treatments and unsprayed control, while hand thinning reduced biennial bearing index (BBI) but also reduced three-year cu mulative yield (kg/tree) compared to PGR treatments and control. Cumulative yield, BBI, and return bloom were not significantly different among PGR treatments and the control for any cultivar in any year. Return bloom did not differ significantly for any treatment compared to the control following the “off” year (2017). In the two-year experiment in Lansing, neither hand thinning, PGR sprays, nor a combination of the two increased return bloom relative to the control for ‘Brown Snout’, while for ‘Chisel Jersey’, hand-thinning did significantly increase return bloom in the first year, and PGRs did promote return fruit set in the second year. The inefficacy of hand thinning and PGR sprays over a single season may be attributable to extreme long-term biennial tendencies at the Lansing orchard, which had little to no crop load management in the years preceding the experiment. Further study is needed to identify ideal crop load and application rates for bloom-promoting PGRs for these and other cider cultivars. Additional index words: crop load, hard cider, Malus ×domestica Borkh. , polyphenol, pre-harvest fruit drop, thinning, alternate bearing, alternate flowering
Introduction Biennial bearing is a phenomenon in many perennial tree crops, comprising extreme year-to-year variations in bloom and yield. Inconsistent supply of apples due to biennial bearing is a major horticultural and supply chain challenge to growers and cidermakers, particularly those using high-tannin cider cultivars which are particularly prone to bi ennial bearing (Pashow 2018; Zakalik 2021).
Horticultural strategies are needed to make yields of these specialty high-tannin culti vars more annually consistent (Bradshaw et al. 2020; Hoad 1978; Merwin 2015; Miles et al. 2017; Wood 1979). The inhibitory in fluence of seed-derived phytohormones, primarily gibberellic acids (GAs) on return bloom in apples is well-established (Dennis and Neilsen 1999; Green 1987; Hoad 1978; Wood 1979). Biennial bearing is mediated in
1 Horticulture Section, School of Integrative Plant Science, Cornell University, Ithaca, NY 2 Lake Ontario Fruit Program, Cornell Cooperative Extension, Lockport, NY Corresponding author: Gregory Peck, email address: gmp32@cornell.edu
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large part by seed-derived GAs, though also by other bloom-suppressing and -promoting phytohormones and the putative ‘florigen’ (FT) protein (Elsysy and Hirst 2019; Haber man et al. 2016; Mimida et al. 2011). In par ticular, GA 1 , GA 3 , and GA 7 are considered the main inhibitors of return bloom in apples. Besides blossom and early fruitlet thinning, growers can also counteract return bloom inhibition using plant growth regulators (PGRs) such as synthetic auxins or ethylene analogs in a non-thinning capacity in mid summer (McArtney et al. 2007; Robinson et al. 2010; Schmidt et al. 2009; Wood 1979). The interaction between crop load and summertime PGR applications is complex and depends on many factors, including ge netics, long-term bearing history of a given tree or orchard (Schmidt et al. 2009), current year crop load, and timing and rate of appli cation. Excessive crop load can reduce or ne gate the efficacy of bloom-promoting PGRs, while the absence of a crop does not neces sarily make bloom-promoting PGRs more effective (McArtney et al . 2007, Schmidt et al. 2009). Crop load is often measured as “crop den sity”, the number of fruits borne per cm 2 trunk cross-sectional area (TCSA). Crop load is widely understood to correlate negatively with various apple juice quality measures, such as soluble solids concentration (Alegre et al. 2012; Awad et al. 2001; Guillermin et al. 2015; Musacchi and Serra 2018; Stopar et al. 2002) and titratable acidity (Henriod et al. 2011; Peck et al. 2016). The effect of crop load on phenolic concentration, an im portant quality attribute for cider apples, is less well-studied, particularly in high-tan nin cider cultivars, though Guillermin et al. (2015) and Karl et al. (2020) found increased crop loads reduced phenolic concentrations in cider cultivars by as much as 25%. The negative effect of crop load on fruit size is well-established in the literature (Guiller min et al. 2015; Henriod et al. 2011; Robin son and Watkins 2003; Wood 1979; Zakalik 2021). Though crop load exerts these effects
throughout the growing season, it is often measured at-harvest, despite being treated as a predictor variable. The ripeness-advancing effects of both exogenous ethephon (Eth) and 1-naphthale neacetic acid (NAA) are well-known (Cline 2019; Singh et al. 2008; Stover et al. 2003; Wendt et al. 2020). Ethephon promotes pre harvest fruit drop (Singh et al. 2008; Stover et al. 2003), while the opposite is the case for NAA (Cline 2019; Dal Cin et al. 2008; Sto ver et al. 2003). Like crop load, the effect of PGR sprays on phenolic concentration in ci der apples has been under-explored. Because phenolic synthesis in apples is thought to oc cur within the first 40 days after full bloom (DAFB) (Renard et al. 2007), it is currently unclear how, or whether, PGR applications in midsummer (i.e., 35–80 DAFB) affect phe nolic concentration at harvest. Crop load often has a negative effect on average fruit size, resulting in a non-linear relationship between crop density (fruit count per TCSA) and yield efficiency (yield weight per TCSA) in the same season. This often results in a “diminishing returns” or “plateau” effect; conversely, even when thin ning is quite drastic, increased fruit size can compensate for potential yield weight losses due to thinning (Zakalik 2021; Wood 1979). The objectives of our experiments were to compare the effects of hand-thinning and mid-summer PGR sprays on return bloom, and to identify PGR application programs that promote return bloom in highly biennial bearing cider apple cultivars. Our hypotheses were: (1) hand-thinning would have a sig nificant positive effect on return bloom; (2) PGR applications would have a significant positive effect on return bloom; and (3) hand thinning combined with PGR sprays would be more effective at promoting return bloom than either treatment alone. Materials and Methods Lyndonville, NY Experiment Experimental design. In June 2016, an ex periment was initiated to investigate the ef-
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fectiveness of different NAA and ethephon (Eth) spray combinations at promoting return bloom in seven high-tannin European cider cultivars. This experiment was carried out at a commercial orchard in Lyndonville, NY located near the southern shores of Lake On tario (43.324, –78.373) on a Galen very fine sandy loam soil (Soil Survey Staff 2014). The cultivars were: ‘Binet Rouge’, ‘Brown Snout’, ‘Chisel Jersey’, ‘Dabinett’, ‘Harry Masters Jersey’, ‘Michelin’, and ‘Geneva Tremlett’s Bitter’. (‘Geneva Tremlett’s Bit ter’ is a bittersharp cultivar of unknown prov enance, mistakenly propagated as the English bittersweet cultivar ‘Tremlett’s Bitter’.) All trees were grafted on ‘Budagovsky 9’ (‘B.9’) rootstock and planted in 2014, at 1.2 m × 3.7 m (~2,220 trees/ha) spacing in a tall-spindle training system with a single high trellis wire and a conduit on each tree. Conventional disease, insect, and weed control were used throughout the orchard (Agnello et al. 2018). There was no irrigation applied. The experiment was set up in a random ized complete block design with blocking based upon location within the orchard. Treatments were randomly assigned to a tree within a block, for a total of 5 treatments × 5 blocks (25 trees per cultivar). The same treatments were applied to the same trees for three consecutive years: two high-crop “on” years (2016 and 2018) and one low-crop “off” year (2017). Rates were not adjusted in the “off” year. All trees within a cultivar were visually as sessed to have similar fruit set before treat ments were implemented. Each treatment tree had a buffer tree on either side, with buf fer trees not overlapping, so two buffer trees separated each experimental tree from the next. Treatments were as follows: (1) non thin control, (2) hand thinning to 6 fruit/cm 2 trunk cross-sectional area (TCSA) in mid- to late June, (3) four applications of 5 mg·L -1 NAA (Fruitone-L ® , AMVAC, Los Angeles, CA, USA), (4) one application of 150 mg·L -1 ethephon (Ethephon 2 ® , Arysta LifeScience North America, Cary, NC, USA) followed by
three applications of 5 mg·L -1 NAA, and (5) two applications of 150 mg·L -1 ethephon fol lowed by two applications of 5 mg·L -1 NAA. Applications started on 29 June 2016, 22 June 2017, and 21 June 2018, and were made on approximately two-week intervals. These rates and timings were based on recommen dations for ‘Honeycrisp’ and ‘Fuji’ (Agnello et al. 2018). Full bloom dates, and thus days after full bloom for hand-thinning and PGR applica tion dates differed slightly among cultivars and years due to weather and time constraints (data not shown, Zakalik 2021). PGR sprays were applied using a Solo ® MistBlower backpack sprayer (Newport News, VA). Bloom assessment. All blossom clusters on trees were counted using a tally counter at the “pink” stage in May 2017 and 2019 (Chapman and Catlin 1976). In Spring 2018 fruitlet clusters were counted in late June on the day of hand-thinning (Zakalik 2021), due to scheduling and labor constraints. Trunk measurement. Tree trunk diameter was measured 40 cm above the graft union in autumn or winter of 2016, 2017, and 2018, after growth had ceased for the season. TCSA was calculated using the formula for the area of a circle. Harvest. Pre-harvest maturity was as sessed for each cultivar using fruit from non-experimental trees via the starch pat tern index (SPI) assay (Blanpied and Silsby 1992) to determine appropriate harvest dates (Zakalik 2021). For cultivars with strong ten dencies to pre-harvest drop, such as ‘Harry Masters Jersey’, fruit was harvested before horticultural maturity for cider production (i.e., at <6 SPI). Pre-harvest drops were counted and removed immediately before the fruit remaining on trees was picked. Drops were counted within the midpoints between an experimental tree trunk and its neighbors on either side. All fruits harvested from trees were counted and weighed in the field on a platform balance (Adam CPW 75, Oxford, CT). Calculation of biennial bearing index. Bi-
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PGR applications combined with hand thin ning to one fruit per cluster throughout the tree. In 2016, the PGR treatment consisted of one application of 150 mg∙L -1 ethephon (Ethephon 2 ® , Arysta LifeScience North America, Cary, NC, USA), followed by two applications of 5 mg∙L -1 NAA (Fruitone L ® , AMVAC Chemical Corp., Los Angeles, CA, USA). In 2017, a third NAA applica tion was added. PGR sprays were applied using a Solo ® MistBlower backpack sprayer (Newport News, VA). Hand thinning and the first applications occurred approximately 5 weeks after full bloom in both years, with subsequent spray applications made on ap proximately two-week intervals (Zakalik 2021). These rates and timings were based on recommendations for ‘Honeycrisp’ and ‘Fuji’ (Agnello et al. 2018). Tree selection. Trees were visually as sessed for bloom. Only trees deemed to have sufficient bloom were selected for this experiment. Subsequently, several branches from each selected tree were quantitatively assessed for initial fruit set, and branch fruit set was used as a proxy for whole-tree fruit set. Treatments were randomly assigned, with blocking by fruit set. Harvest . Harvest dates were chosen based on previously observed harvest dates for the Lansing Orchard (Zakalik 2021). Pre-harvest drops were counted and removed from under experimental trees on multiple dates before harvest, due to a prolonged drop period and large crop size. Pre-harvest drops were also counted and removed on the day of harvest before picking fruit remaining on trees. Drop counts from all dates were summed and re ported as one number per tree. In 2016, all fruit harvested from the tree were counted and weighed; average fruit weight was cal culated by dividing harvest weight by har vest count. In 2017, the crop was so great that counting on-tree fruit was not feasible; instead, the first 100 fruit harvested were counted and weighed, and then the remaining on-tree fruit were weighed without counting. Total on-tree fruit count was estimated by 8
ennial bearing indices (BBI) were calculated using Equation 1 below, adapted from Hob lyn et al. (1937). BBI is a unitless measure of variation in yield among consecutive year pairs. A value of 0 indicates completely con sistent yields from year to year; a value of 1 indicates complete absence of fruit borne on the tree in one year. BBI was calculated on a yield mass (kg) basis. yield among consecutive year pairs. A value of 0 indicates completely consistent yields from 9 year to year; a value of 1 indicates complete absence of fruit borne on the tree in one year. BBI 0 was calculated on a yield mass (kg) basis. 1 Equation 1. = ∑ " !"#$%& #'( )"#$%& # ! "#$%& #'( '"#$%& # # * #+( $%& 2 …where n is the total number of consecutive years observed. 3 6 orchard in Lansing, NY to investigate the efficacy of hand-thinning and PGR sprays at 7 promoting return bloom in two high-tannin cider cultivars, ‘Binet Rouge’ and ‘Chisel Jersey’. 8 The orchard, located at 42.57004°, –76.59507°, was established in 2003 and is located on a 9 hillside of 12–20 percent slope, facing southwest, leading down to Cayuga Lake, on a Hudson 0 Cayuga silt loam (Soil Survey Staff 2014). Experimental trees were grafted onto ‘Geneva 16’ 1 (‘G.16’), ‘G.30’, and ‘Malling 9’ (‘M.9’) rootstocks. In 2016, sixteen ‘Chisel Jersey’/‘M.9’ and 2 eight ‘Chisel Jersey’/‘G.30’ trees were used, with six trees assigned to each treatment. In 2017, 3 due to insufficient bloom and fruit set, a different set of twenty-four ‘Chisel Jersey’ trees (sixteen 4 on ‘M.9’ and ight on ‘G.30’) and sixt en ‘Brown Snout’ trees (twelve on ‘M.9’ and four on 5 ‘G.16’) were selected, with six ‘Chisel Jersey’ and four ‘Brown Snout’ trees assigned to each 6 treatment group. All trees within a replicated block had the same rootstock (one of the three 7 aforementioned). 8 Trees were randomly assigned to one of four treatments, as follows: (1) control, (2) PGR 9 applications, (3) hand-thinned to one fruit per cluster throughout the tree, or (4) PGR 0 Equation 1. …where n is the total number of consecutive years observed. Lansing, NY Experiment Experimental design. This experiment was carried out at a Cornell University research orchard in Lansing, NY to investigate the ef ficacy of hand-thinning and PGR sprays at promoting return bloom in two high-tannin cider cultivars, ‘Binet Rouge’ and ‘Chisel Jersey’. The orchard, located at 42.57004°, –76.59507°, was established in 2003 and is located on a hillside of 12–20 percent slope, facing southwest, leading down to Cayuga Lake, on a Hudson Cayuga silt loam (Soil Survey Staff 2014). Experimental trees were grafted onto ‘Geneva 16’ (‘G.16’), ‘G.30’, and ‘Malling 9’ (‘M.9’) rootstocks. In 2016, sixteen ‘Chisel Jersey’/‘M.9’ and eight ‘Chisel Jersey’/‘G.30’ trees were used, with six trees assigned to each treatment. In 2017, due to insufficient bloom and fruit set, a dif ferent set of twenty-four ‘Chisel Jersey’ trees (sixteen on ‘M.9’ and eight on ‘G.30’) and sixteen ‘Brown Snout’ trees (twelve on ‘M.9’ and four on ‘G.16’) were selected, with six ‘Chisel Jersey’ and four ‘Brown Snout’ trees assigned to each treatment group. All trees within a replicated block had the same root stock (one of the three aforementioned). Trees were randomly assigned to one of four treatments, as follows: (1) control, (2) PGR applications, (3) hand-thinned to one fruit per cluster throughout the tree, or (4) 4 Lansing, NY Experiment 5 Experimental design. This experim nt was carried out at a Cor ell University research
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