Fire Blight Sensitivity of Cider Apple Cultivars

Introduction

There is increasing interest in growing apple cultivars (Malus domestica Borkh.) of European origin for the production of hard cider in Canada; however, little is known about their susceptibility to fire blight (FB). Specialty cider cultivars have been selected for their fruit and juice characteristics, and in some cases, for ease of management but not necessarily for yield or disease resistance. Traditional European cider cultivars were selected from areas with climates and pathogens vastly different from those found in southern Ontario’s humid continental climate growing region of Canada. Consequently, several cultivars are challenging to grow due to higher disease susceptibility, biennial bearing, and pre-harvest drop. Fire blight can spread rapidly through apple (and pear) orchards causing extensive tree mortality and economic loss. The aim of this article is to report the FB susceptibility of twenty-seven cider apple cultivars in their seventh year (2021) of production when they were severely infected by a FB (E. amylovora) outbreak.

Materials and Methods

The orchard was established in 2015 at the Ontario Crop Research Centre – Simcoe on M.9 rootstock and trained in a vertical axis orchard system. Trees were spaced 1 m within and 4.5 m between rows (1667 trees/ha). Trees were planted in a randomized complete block, with four replications of five trees for each of the 27 cultivars, for a total of 560 trees. Bloom and harvest data were collected from the middle three trees of each five-tree block, with the two outside trees in each block acting as guard trees. Each replicate occupied two rows, resulting in a total of eight rows. The cultivars evaluated consisted of: Ashmead’s Kernel, Breakwell, Brown’s Apple, Bulmers Norman, Binet Rouge, Bramley’s Seedling, Brown Snout, Calville Blanc d’Hiver, Crimson Crisp®, Cox Orange Pippin, Dabinett, Enterprise, Esopus Spitzenberg, Fréquin Rouge, GoldRush, Grimes Golden, Golden Russet, Kingston Black, Michelin, Muscadet de Dieppe, Medaille d’Or, Porter’s Perfection, Sweet Alford, Stoke Red, Tydeman Late, Tolman Sweet, and Yarlington Mill.

Trees were provided with supplemental trickle irrigation and the the orchard soil consisted of a Brady sandy loam with imperfect drainage and soil textures consisting of mainly lacustrine sand and sandy loam over glaciolacustrine clays at depths greater than 1.5 m. Pests and diseases were managed with conventional pesticide (OMAFRA, 2021). Trees were fertilized annually with muriate of potash based on leaf tissue analyses but were not fertilized with nitrogen. Daily minimum and maximum temperatures at 1.5 m above the ground were recorded at the Simcoe Research Station.  Further planting details, cultivar descriptions, early yield, and juice characteristics have been previously reported (Plotkowski, 2020; Cline et al., 2021; Plotkowski and Cline, 2021a, 2021b).

Horticultural Measurements

In the spring of 2017-2021, phenology was rated three to four times per week from late April to the end of flowering in June. The rating system used herein was developed at Washington State University and covers the progression from the silver tip stage immediately after dormancy to petal fall. Tree mortality was recorded every autumn from 2015-2021.

Fire Blight Management

Fire blight was managed with applications of streptomycin during bloom and prohexadione-calcium (Apogee™) pre- and post-bloom. The incidence of FB infections was low to non-existent between 2015 and 2020. However, FB symptoms began to appear in vegetative 1-year old shoots in mid-June, 2021 almost exclusively in the cider apple cultivars, while other adjacent fresh-market cultivar blocks of Gala, Honeycrisp, Ambrosia, Golden Delicious, and Cortland were largely unaffected. The FB moved quickly in the tree to subtending older tissue. An aggressive spray program to contain its spread was immediately implemented with an initial application of streptomycin (24-June) followed by fixed copper (Cueva) on 28-June, 5-July, 19-July and 26-July. Unfortunately, the FB continued to spread despite active removal of affected shoots using recommended sterile methods, and in some cases removal of entire trees when there were multiple strikes on a tree. The infections continued unabated, and in many instances moved to the rootstock, causing collapse of the entire tree. By November 2021, there was widespread tree mortality in the orchard, to the extent that the difficult decision to remove the entire orchard was made to prevent further spread to adjacent  research and commercial orchards. Historically, the incidence of FB has been low over the past 30 or more years of production. This led us to suspect that the primary infection inoculum had migrated from adjacent commercial orchards in the region.

Cultivar Sensitivity to Fire Blight (E. amylovora)

Prior to 2021, all-cause tree mortality was relatively low, but was as high as 8% in Dabinett and Tydeman. Late in 2015, mortality was 8% in  Michelin and Breakwell in 2017, and ranged from 8 to 24% in Yarlington Mill and Michelin, respectively, in 2021 (Figure 1). All-cause tree mortality did not exceed 33% for any cultivar in 2020, and all but four cultivars (Dabinett, Yarlington Mill, Michelin and Tydeman Late) showed no tree mortality in the same year. In 2021, despite the application of pesticides to suppress and reduce the spread of E. amylovora, FB symptoms began to appear in mid-June and continued to spread through the cultivar block. By the end of the growing season in November, 42% of all trees in the experiment (of the initial 560 trees) displayed FB symptoms sufficiently severe that the trees had or would succumb to E. amylovora bacteria. Although the cultivars were randomly located in each block, all blocks were affected, indicating the FB pressure appeared uniform throughout the block.

There was a wide range but significant differences in cultivar tree mortality (P<0.001) ranging from no tree mortality for Enterprise (n=20 for all cultivars over four replications) and 100% tree morality for Ashmead’s Kernel and Tolman Sweet.  Overall, the incidence of FB was zero for Enterprise (0%), low for  GoldRush and Porter’s Perfection (< 20%); low for Binet Rouge, Kingston Black, Cline Russet, Dabinett, Grimes Golden, Frequin Rouge, Crimson Crisp®, Cox Orange Pippin and Muscadet De Dieppe (20-40%); intermediate for Calville Blanc d’Hiver, Bramley’s Seedling, Yarlington Mill, Michelin, Bulmers Norman, Stoke Red, Golden Russet, Breakwell, Esopus Spitzenberg (50-90%) , and high for Brown Snout, Medaille d’Or, Michelin, Brown’s Apple, Sweet Alford, Tydeman Late, Ashmead’s  Kernel and Tolman Sweet (90-100%)..

Tolerance for FB infection in commercial orchards is extremely low. Even when low FB symptoms and/or tree mortality caused by FB exist, cider growers should be concerned about further tree mortality in subsequent years if the bacteria continue to overwinter and spread, which is likely at specific temperatures, humidity, and precipitation conditions.

This study indicates that the majority of apple cultivars grown for cider in this study are susceptible to FB under ideal conditions for the causative agent E. amylovora. Cider producers should take preventative measures by using more FB tolerant cultivars and resistant rootstocks when there is a history of FB within or adjacent to the planting location, or when environmental conditions are often ideal for the proliferation for E. amylovora. Many apple cider cultivars flower later than fresh-market culinary apples and have secondary bloom or protracted bloom periods (Plotkowski and Cline, 2021a). These attributes increase the risk of FB incidence and severity (Dupont, 2019), indicating that selecting early flowering cultivars may be a best practice when growing cider-specific apple cultivars.

Figure 1. All cause tree mortality (%) of select apple cultivars cultivars from 2015 (year of planting) to 2021 (year 7) grown for cider at the University of Guelph Simcoe Research Station, Simcoe, ON. Mean values represented by the vertical bar with the same letter are not significantly different according to the Tukey-Kramer test at P=0.05 (for 2021 data only)

Literature Cited

Cline, J.A., Plotkowski, D. and Beneff, A. 2021. Juice attributes of Ontario-grown culinary (dessert) apples for cider. Can. J. Plant Sci. 101(4):536-545. https://doi.org/10.1139/cjps-2020-0223.

Dupont, T. 2019. Fireblight. [Online]. http://treefruit.wsu.edu/crop-protection/disease-management/fire-blight/ [Accessed 26 August 2022]

OMAFRA, 2021. Crop protection guide for apples Publication 360A. Queens Printer of Ontario. Online. http://www.omafra.gov.on.ca/english/crops/pub360/pub360A.pdf&nbsp; [Accessed 12 September 2022]

Plotkowski, 2020. Horticultural and juice attributes of cider apple (Malus domestica Borkh.) cultivars grown in Ontario, the endogenous development of yeast assimilable nitrogen in apple juice, and the effects of exogenous nitrogen supplementation on the fermentation of apple juice. Doctoral Dissertation. University of Guelph, Guelph. Available https://atrium.lib.uoguelph.ca/xmlui/handle/10214/21368

Plotkowski, D.J. and Cline, J.A., 2021a. Evaluation of selected cider apple (Malus domestica Borkh.) cultivars grown in Ontario. I. Horticultural attributes. Can. J.  Plant Science 101(6):818-835. https://doi.org/10.1139/CJPS-2021-0009

Plotkowski, D. and Cline, J.A. 2021b. Evaluation of select cider apple (Malus domestica Borkh.) cultivars grown in Ontario. II. Juice Attributes. Can. J. Plant. Sci. 101(6):836-852. https://doi.org/10.1139/CJPS-2021-0010

Dr. John Cline
Dr. John Cline

Department of Plant Agriculture, University of Guelph

Mandy Beneff
Mandy Beneff

Department of Plant Agriculture, University of Guelph