Abstract
Susceptibility to low-temperature injury and diseases is a major concern associated with ornamental camellia production. To comprehensively understand their growth, cold-hardiness, flowering, and disease resistance, 24 camellia (Camellia spp. and hybrids) cultivars and selections were evaluated in McMinnville, TN, USA (USDA plant hardiness zone 7a). During Mar 2011, camellias were planted in the field plots. Plant height and canopy width were measured annually from 2011 to 2019, and low-temperature damage was recorded in 2014 and 2023. The flowering duration was recorded each year from 2011 to 2020. The Camellia Yellow Mottle Virus, monochaetia leaf spot (Monochaetia sp.), edema, flower blight (Ciborinia camelliae), and flower spot (Botrytis cinerea) severity (% affected) were evaluated from Oct to Nov in 2016 and 2017. The season-long area under the disease progress curve (AUDPC) was calculated. Cultivars Arctic Snow and Pink Icicle exhibited the greatest height, and Autumn Spirit, Elaine Lee, Arctic Snow, and Survivor had the widest canopy width, whereas Shishigashira had the lowest height and canopy width. ‘April Remembered’, ‘April Rose’, ‘Arctic Snow’, ‘Ashton’s Ballet’, ‘Autumn Carnival’, ‘Autumn Spirit’, ‘Elaine Lee’, ‘Survivor’, and C. chekiangoleosa selection were least affected by winter low temperatures, whereas ‘Korean Snow’, ‘One Alone’, C. sasanqua selection, ‘Pink Icicle’, and ‘Shishigashira’ were severely damaged. ‘Arctic Snow’ flowered most reliably (6 of 8 years), whereas ‘April Remembered’, ‘April Rose’, ‘Ashton’s Ballet’, ‘Autumn Spirit’, and ‘Survivor’ flowered five times. ‘Korean Fire’, ‘Classic Pink’, ‘Maroon Mist’, and ‘Spring’s Promise’ displayed the highest virus severity and AUDPC. ‘Arctic Snow’, C. sasanqua selection, and the C. chekiangoleosa selection had no viral symptoms. C. sasanqua selection and ‘Red Aurora’ were significantly impacted by edema disorder, with severity ratings of ∼43% and 26%, respectively. Monochaetia leaf spot severity was highest in ‘Red Aurora’ and ‘Spring’s Promise’, whereas ‘Anacostia’, ‘Arctic Snow’, ‘Ashton’s Ballet’, ‘Autumn Spirit’, ‘Classic Pink’, ‘Kuro Delight’, ‘One Alone’, ‘Pink Icicle’, ‘Shishigashira’, and ‘Survivor’ exhibited the least monochaetia leaf spot severity and AUDPC. Flower blight and flower spot were observed only in ‘Arctic Snow’ and ‘Survivor’. These findings will aid landscapers and nursery growers with selecting and managing camellia cultivars effectively.
Camellia, the largest genus within the Theaceae family, comprises approximately 250 known evergreen shrub species (Pereira et al. 2022; Vijayan et al. 2009). Native to Eastern Asia, various Camellia species are used for tea, oil, cosmetics, and as ornamental landscape plants (Gao et al. 2005; Teixeira and Sousa 2021). The United States is popularly known for producing ornamental camellia, which are commonly grown in the warmer regions. According to the American Camellia Society, ornamental camellia nurseries are mostly located in California, Georgia, North Carolina, Florida, Alabama, Virginia, Louisiana, and Mississippi (https://www.americancamellias.com/publications-and-library). The most common ornamental camellia species include C. japonica, C. reticulata, C. sasanqua, and a group of yellow-flowering species called golden camellia, which includes more than 52 species (Garcia-Jares et al. 2017; Manh et al. 2019; Vela et al. 2013; Vijayan et al. 2009). Among them, C. japonica cultivars and hybrids are the most widely grown ornamental camellia, with more than 18,141 registered cultivars of horticultural use (Wang et al. 2021).
Ornamental camellia cultivars are flowering shrubs that exhibit a wide range of shapes and sizes and typically grow to heights of up to 3.7 m, with some even reaching a height of 7.6 m (Kole 2011). They are valued for their attractive flowers, which range in color from pure white to various shades of pink and dark red (Gu 2009). The flowers are 1 cm to 15 cm in diameter and are found in diverse forms, such as single, semi-double, double, formal double, or full peony (Trehane 1998). Depending on the cultivar, the flowering period of camellia could be year-round (Jiyin et al. 2016).
Most camellia cultivars thrive best in shaded areas within United States Department of Agriculture (USDA) hardiness zones 7 to 9, and possibly in zone 6b (Cathey 1990; Niemiera 2018; Pooler 2011). Low winter temperatures, especially hard freezes, can negatively impact crop establishment, growth, as well as flowering time and duration for sensitive cultivars (Trehane 1998). Additionally, severe frost during flowering can damage buds and flowers, resulting in a shortened bloom period or smaller flowers. In Tennessee, there has been a growing interest among nursery growers and landscapers in producing and planting more ornamental camellia. Although much of middle Tennessee has a temperate climate likely suitable for camellia growth, hard freezes with temperatures reaching approximately −15 °C or lower may pose limitations on production (Parajuli et al. 2023).
Initiatives have been undertaken to breed cold-hardy camellias (Ackerman 2007). These cold-hardy camellias were developed using cold-hardy C. japonica and C. oleifera selections. The cold-hardiness of ornamental camellia cultivars and hybrids has been documented in International Camellia Society reports (https://camellia.iflora.cn/Cutivars/Country). They are mostly hybrids and C. japonica cultivars. Nevertheless, our understanding of the relative sensitivity and tolerance of flowering camellia cultivars and hybrids to USDA plant hardiness zone 7a is lacking.
Camellias are also susceptible to various diseases, including monochaetia leaf spot (Monochaetia sp.), flower blight (Ciborinia camelliae), flower spot (Botrytis cinerea), viral disease (Camellia Yellow Mottle Virus), and edema disorder (Trehane 1998). These diseases can pose a threat to the survival of camellia and negatively impact their marketability. Monochaetia leaf spots manifest as large round spots with pale green to yellowish centers, surrounded by reddish-brown margins. Flower blight causes the petals to turn brown and leads to premature flower drop (Saracchi et al. 2022; Taylor and Long 2000). Flower spots, also known as gray mold, appear as light brown water-soaked lesions that later develop into dark brown necrotic lesions (Pereira and Mio 2020). Camellia Yellow Mottle Virus results in irregular yellow mottled patterns on the leaves and whitish blotches on the blossoms (Gailhofer et al. 1988; Milbrath and McWhorter 1946). Edema, a physiological disorder, is characterized by small, water-soaked, greenish-white raised areas on the leaves. It occurs when the roots absorb excess water but are unable to use it for photosynthesis or lose it through transpiration.
Identifying disease-resistant, cold-hardy camellia cultivars and characterizing their growth and flowering patterns when produced in USDA hardiness zone 7a would expand marketing opportunities for Tennessee growers and landscapers. This is the USDA zone where most camellia can grow, and the zone where only those with better cold-hardness can grow. Therefore, the main objective of this study was to thoroughly assess ornamental camellia cultivars for their growth, flowering characteristics, cold-hardiness, and disease resistance in McMinnville, TN, USA.
Materials and Methods
Camellia cultivars and selections were evaluated for growth, flowering, cold-hardiness, and disease resistance in USDA plant hardiness zone 7a and American Horticultural Society Plant heat zone 7 at the Otis L. Floyd Nursery Research Center, Tennessee State University, McMinnville, TN, USA. The details of the cultivars and selections are provided in Table 1. In Apr 2010, rooted cuttings of camellia were obtained from a North Carolina camellia nursery and a local nursery; then, they were potted in 3.8-L nursery containers. In Mar 2011, 24 camellia cultivars were planted in full sun in Waynesboro silt loam soil. Plants were planted in rows spaced 3 m apart, with 2.5 m between plants within rows. Five single-plant replications of each cultivar or selection were arranged in a randomized complete block design. Camellias were top-dressed with 15 g/plant of 19N–2.2P–7.5K controlled-release fertilizer (Osmocote, Scotts Miracle-Gro Company, Marysville, OH, USA) 1 month after planting, and then every year (controlled-release fertilizer product or with 15N–6.6P–12.5K) based on University of Tennessee soil test recommendations. Plants were irrigated via drip emitters as needed. Weed control in the planted row was managed by postemergent herbicides and occasional hoeing. Dead limbs were pruned out of the camellias every year. Monthly minimum temperatures were recorded and are presented in Table 2. Because of the damage caused by the hard frost in 2014, only 21 cultivars were evaluated for growth, flowering duration, cold-hardiness, and diseases. Cultivars One Alone, Korean Snow, Pink Icicle, Shishigashira, and C. sasanqua selection were severely damaged in 2014, but they were rejuvenated and included for further evaluation.
Available parentage details of ornamental camellia (Camellia spp. and hybrids) cultivars and selections included in a multiple-year (2011–23) evaluation of growth, cold-hardiness, flowering, and disease resistance in McMinnville, TN, USA.
Monthly minimum temperature recorded from 2011 to 2022, in McMinnville, TN, USA.
Initial plant height and width were measured to develop a baseline for plant growth, and then annually in Dec from 2011 to 2019. Height was measured from the base of the stem at the soil level to the top of the highest terminal bud on the main stem. Plant width was the average of the widest horizontal spread, from leaf tip to leaf tip, and the spread perpendicular to the widest spread. The plant growth index (GI) was determined as the mean of plant height and width [(height + widest width + perpendicular width)/3].
Flowering duration data were collected every year from 2011 to 2020. Flowering duration was defined as the period between which ≥20% of flowers had emerged and >80% senesced. Based on this range, the number of days that a cultivar or selection flowered was determined for each individual year. Yearly values were used to calculate the mean across all the years.
Plants were evaluated in Jun 2014 and 2023 to determine postwinter low-temperature damage because heavy low-temperature damage was observed during those years. Low-temperature damage was recorded as overall plant and stem damage and leaf rejuvenation. Overall plant damage was rated using the following scale: 1 = no damage, 2 = light damage, 3 = moderate damage, 4 = severe damage, and 5 = >90% dead. The scale used for stem damage was as follows: 1 = no dieback, 2 = twig dieback, 3 = stem dieback, and 4 = dead. Twigs were defined as the outermost and smallest-diameter branches on a given plant, representing growth from the previous one to two growing seasons, and the other larger branches were considered stems. The scale used for leaf rejuvenation was as follows: 1 = no damage, 2 = regrowth from exterior stems, 3 = regrowth from stems only in the lower third of the canopy, and 4 = no regrowth. Plants were evaluated for defoliation caused by low temperature in Apr 2014 and Feb 2023. In 2014, defoliation was rated using the following scale: 1 = no damage, 2 = mild bronze tips, 3 = bronze tips on half of the plant, 4 = bronze tips on all of the plant, and 5 = defoliation. During the 2023 evaluation, defoliation was recorded as the percentage of defoliation (0%–100%). The low-temperature damage in 2014 was much milder than that in 2023. The reason we used the percentage of defoliation in 2023 was because there were very few “bronzed leaves” left on the plant.
Cultivars were evaluated for Camellia Yellow Mottle Virus, monochaetia leaf spot (Monochaetia sp.), and edema using a scale of 0% to 100% coverage of the total foliage area on 29 Aug, 21 Sep, and 5 Oct in 2016, and on 29 Aug, 29 Sep, and 5 Oct in 2017. Additionally, cultivars were evaluated for flower spot (Botrytis cinerea) and flower blight (Ciborinia camelliae) coverage of the total flower using a scale of 0% to 100% on the aforementioned dates. Season-long disease progress was calculated using the following area under the disease progress curve (AUDPC) formula: ∑{[(xi + xi−1)/2](ti – ti−1)}, where xi is the disease severity rating at each evaluation time and (ti − ti−1) is the number of days between evaluations.
Final plant height and width recorded in Dec 2019 were analyzed using a one-way analysis of variance (ANOVA) with the PROC GLM procedure (SAS Inc., Cary, NC, USA). Tree height and width for each year were graphed using JMP Pro 17 (JMP Statistical Discovery LLC., Cary, NC, USA) to show annual growth patterns. A repeated-measures ANOVA using SAS software 9.4 (SAS Inc.) was performed to determine the main and interactive effects of year × cultivar on severity and AUDPC for virus, monochaetia leaf spot, and edema. When the interaction was significant, simple effects of the cultivar were separately analyzed by the evaluation year using a general linear mixed model with a logit link and beta distribution (PROC GLIMMIX). Severity data were converted from 0 to 1 to meet the PROC GLIMMIX assumption before analysis. Because only two cultivars had flower spots and flower blight, these data were presented without analysis. The winter low-temperature damage data and 2014 defoliation were analyzed using the Kruskal-Wallis test (SAS 9.4, SAS Inc.); 2023 defoliation was analyzed using the mixed model. The mean flowering duration was calculated based on the multiple years of data and presented graphically. Means were compared using Fisher’s least significant difference test or least mean square with α = 0.05. A repeated-measures ANOVA was used to compare yearly changes in the plant growth index of individual cultivars or selections. The repeated-measures analysis was accomplished via covariance structure modeling (Wolfinger 1993), in which the most appropriate covariance structure was selected by fitting data to various homogeneous and heterogeneous covariance structures available using JMP Pro 17 and subsequently comparing corrected Akaike information criterion (AICc) values. According to lowest AICc values, the first-order autoregressive [AR(1)] covariance structure was used for all repeated-measures analyses.
Results
During the final evaluation (9 years after planting), cultivars and selections ranged in height from 49.3 to 377.6 cm and in width from 54.0 cm to 249.3 cm (Table 3). Cultivars Arctic Snow and Pink Icicle were significantly taller than the rest of the cultivars, followed by Survivor and Autumn Spirit. Cultivar Shishigashira was significantly shorter compared with the other cultivars followed by Red Aurora. The final width of the cultivar Autumn Spirit was significantly wider compared with the other cultivars, except Elaine Lee, Arctic Snow, and Survivor, which had similar widths. Cultivar Shishigashira had the smallest width among the evaluated cultivars.
Mean final height and width of camellia (Camellia spp. and hybrids) cultivars and selections recorded in 2019, 9 years after planting, in McMinnville, TN, USA.
Plant height and width of camellia selections and cultivars were greatly impacted by low temperatures (Figs. 1 and 2). Overall, severe cold temperature impacts on GI were observed from 2011 to 2012, 2013 to 2014, and 2017 to 2018, for most of the cultivars (Table 4). From 2011 to 2012, the GI decreased for ‘Autumn Carnival’, ‘Classic Pink’, ‘Korean Fire’, C. sasanqua selection, ‘Red Aurora’, and ‘Shishigashira’, increased for ‘Arctic Snow’ and ‘Elaine Lee’, and did not change for the others. From 2013 to 2014, GI decreased for ‘Korean Fire’, ‘One Alone’, ‘Pink Icicle’, and ‘Red Aurora’, increased for ‘April Rose’, ‘Autumn Carnival’, ‘Autumn Spirit’, the C. chekiangoleosa selection, ‘Elaine Lee’, and ‘Survivor’, and remained the same for the others. From 2017 to 2018, an increase in GI was observed for cultivars April Rose, the C. chekiangoleosa selection, Elaine Lee, Korean Fire, the C. sasanqua selection, and Survivor; however, it remained the same for the other cultivars.
Annual mean height (cm) of camellia (Camellia spp. and hybrids) cultivars and selections planted in McMinnville, TN, USA. The initial plant height was measured in Mar 2011 as the initial plant height, and then annually in Dec from 2011 to 2019. Because multiple replicated plants were dead, growth data for C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not reported.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Annual mean height (cm) of camellia (Camellia spp. and hybrids) cultivars and selections planted in McMinnville, TN, USA. The initial plant height was measured in Mar 2011 as the initial plant height, and then annually in Dec from 2011 to 2019. Because multiple replicated plants were dead, growth data for C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not reported.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Annual mean height (cm) of camellia (Camellia spp. and hybrids) cultivars and selections planted in McMinnville, TN, USA. The initial plant height was measured in Mar 2011 as the initial plant height, and then annually in Dec from 2011 to 2019. Because multiple replicated plants were dead, growth data for C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not reported.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Annual mean width (cm) of camellia (Camellia spp. and hybrids) cultivars and selections planted in McMinnville, TN, USA. The initial plant width was measured in Mar 2011 as the initial plant width, and then annually in Dec from 2011 to 2019. Width = (widest width + perpendicular width) ÷ 2. Because multiple replicated plants were dead, growth data for C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not reported.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Annual mean width (cm) of camellia (Camellia spp. and hybrids) cultivars and selections planted in McMinnville, TN, USA. The initial plant width was measured in Mar 2011 as the initial plant width, and then annually in Dec from 2011 to 2019. Width = (widest width + perpendicular width) ÷ 2. Because multiple replicated plants were dead, growth data for C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not reported.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Annual mean width (cm) of camellia (Camellia spp. and hybrids) cultivars and selections planted in McMinnville, TN, USA. The initial plant width was measured in Mar 2011 as the initial plant width, and then annually in Dec from 2011 to 2019. Width = (widest width + perpendicular width) ÷ 2. Because multiple replicated plants were dead, growth data for C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not reported.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Mean growth index of camellia (Camellia spp. and hybrids) cultivars and selections in McMinnville, TN, USA.
Cultivars differed significantly in low-temperature damage ratings (Table 5). Based on the overall plant evaluation, ‘Korean Snow’, ‘One Alone’, C. sasanqua selection, ‘Pink Icicle’, and ‘Shishigashira’ in 2014 and ‘Arctic Snow’, ‘Autumn Carnival’, ‘Autumn Spirit’, ‘Kuro Delight’, ‘One Alone’, C. sasanqua selection, and ‘Shishigashira’ in 2023 were >90% dead because of low temperatures. In 2014, the C. chekiangoleosa selection and cultivars April Remembered, April Rose, Arctic Snow, Ashton’s Ballet, Autumn Carnival, Elaine Lee, and Survivor were least affected by low temperatures. In 2023, the C. chekiangoleosa selection, ‘Elaine Lee’, and ‘Survivor’ were least affected, whereas the others were moderately to severely damaged. The stems of ‘Korean Snow’, ‘One Alone’, C. sasanqua selection, and ‘Pink Icicle’ in 2014 and ‘Arctic Snow’, ‘Autumn Carnival’, ‘Autumn Spirit’, C. sasanqua selection, and ‘Shishigashira’ in 2023 were completely dead. Stems of the C. chekiangoleosa selection, ‘Elaine Lee’, and ‘Survivor’ were the least affected in both years. In 2014, ‘Korean Snow’, ‘Pink Icicle’, ‘Shishigashira’, and the C. sasanqua selection had leaf regrowth only from the base of the plant. In 2023, ‘Anacostia’, ‘Arctic Snow’, ‘Autumn Carnival’, ‘Autumn Spirit’, ‘Kuro Delight’, C. sasanqua selection, and ‘Shishigashira’ had regrowth only from the base of the plant. No leaf damage was observed in the C. chekiangoleosa selection and cultivars April Remembered, April Rose, Arctic Snow, Ashton’s Ballet, Autumn Carnival, Elaine Lee, Classic Pink, Maroon Mist, Red Aurora, and Survivor in 2014. In 2023, the least leaf damage was observed in the C. chekiangoleosa selection and cultivars April Rose, Ashton’s Ballet, Classic Pink, Elaine Lee, Red Aurora, Stellar Sunrise, and Survivor. For the other cultivars, leaf regrowth from the stems was observed. In 2014, the highest defoliation was recorded for ‘Shishigashira’. ‘Pink Icicle’, C. sasanqua selection, ‘Kuro Delight’, ‘Korean Snow’, and ‘One Alone’ had bronze tips on all of the plant. ‘Arctic Snow’, ‘Survivor’ and the C. chekiangoleosa selection had no damage, whereas ‘April Remembered’, ‘April Rose’, ‘Autumn Carnival’, ‘Autumn Spirit’, and ‘Survivor’ had mild bronze tips (Table 6). The other cultivars had bronze tips on half of the plant. In 2023, ‘Survivor’ and ‘Elaine Lee’ had the least defoliation, with 31% and 40%, respectively. The C. chekiangoleosa selection, ‘April Rose’, and ‘Korean Fire’ had 67%, 56%, and 79.8% defoliation, respectively. Other cultivars were completely defoliated.
Effects of low temperatures on the mean overall plant, stems, and leaves of camellia (Camellia spp. and hybrids) cultivars and selections evaluated in Jun 2014 and 2023, in McMinnville, TN, USA.
Mean defoliation caused by winter low temperatures of camellia (Camellia spp. and hybrids) cultivars and selections evaluated in Apr 2014 and Feb 2023, in McMinnville, TN, USA.
During the 8-year evaluation, ‘Arctic Snow’ flowered most dependably (six times), whereas ‘Maroon Mist’, ‘One Alone’, and ‘Shishigashira’ did not flower during any year (Table 7). ‘April Remembered’, ‘April Rose’, ‘Ashton’s Ballet’, ‘Autumn Spirit’, and ‘Survivor’ flowered five times. ‘Autumn Carnival’ flowered four times, and the C. chekiangoleosa selection and C. sasanqua selection flowered three times, whereas the others flowered one to two times. The cultivars also differed in the flowering season and duration (Fig. 3). ‘Arctic Snow’, ‘Ashton’s Ballet’, ‘Autumn Carnival’, ‘Autumn Spirit’, C. sasanqua selection, and ‘Survivor’ flowered during the fall. Cultivar Elaine Lee flowered in the summer. The rest of the cultivars flowered in the spring. Flowering continued for 61 d for cultivar Pink Icicle, 50 d for Spring’s Promise, 27 d for Stellar Sunrise, 47 d for April Remembered, 41 d for April Rose, 39 d for Autumn Spirit, and 34 d for Elaine Lee (Fig. 3). The cultivar Anacostia flowered for only 15 d, and Red Aurora flowered for only 16 d.
Flowering (F) or no flowering (N) over the course of each year from 2011 to 2020 of camellia (Camellia spp. and hybrids) cultivars and selections planted in McMinnville, TN, USA.
Mean flowering duration of ornamental camellia (Camellia spp. and hybrids) cultivars and selections planted in 2011 in McMinnville, TN, USA. Dashed line represents the flowering duration. The number of days during which a cultivar or selection flowered was determined for each individual year, and the mean was calculated across all the years (2011–19). Flowering duration was defined as the period during which ≥20% of flowers had emerged and >80% senesced. The mean number of days when cultivars flowered is displayed above the flowering duration line. Because multiple replicated plants were dead, the C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not included in the growth information. Cultivars One Alone, Maroon Mist, and Shishigashira did not flower during any of the evaluation years.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Mean flowering duration of ornamental camellia (Camellia spp. and hybrids) cultivars and selections planted in 2011 in McMinnville, TN, USA. Dashed line represents the flowering duration. The number of days during which a cultivar or selection flowered was determined for each individual year, and the mean was calculated across all the years (2011–19). Flowering duration was defined as the period during which ≥20% of flowers had emerged and >80% senesced. The mean number of days when cultivars flowered is displayed above the flowering duration line. Because multiple replicated plants were dead, the C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not included in the growth information. Cultivars One Alone, Maroon Mist, and Shishigashira did not flower during any of the evaluation years.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Mean flowering duration of ornamental camellia (Camellia spp. and hybrids) cultivars and selections planted in 2011 in McMinnville, TN, USA. Dashed line represents the flowering duration. The number of days during which a cultivar or selection flowered was determined for each individual year, and the mean was calculated across all the years (2011–19). Flowering duration was defined as the period during which ≥20% of flowers had emerged and >80% senesced. The mean number of days when cultivars flowered is displayed above the flowering duration line. Because multiple replicated plants were dead, the C. cuspidate selection and cultivars Korean Snow and Longwood Valentine were not included in the growth information. Cultivars One Alone, Maroon Mist, and Shishigashira did not flower during any of the evaluation years.
Citation: HortScience 58, 12; 10.21273/HORTSCI17430-23
Diseases such as viruses, monochaetia leaf spots, flower blight, flower spot, and physiological disorder edema naturally occurred. There were no interactive effects of year and cultivar on the severity and AUDPC of viral infection, monochaetia leaf spot, and edema (Table 8). The main effects of year and cultivar on virus, monochaetia leaf spot, and edema severity and AUDPC were significant. The severity and AUDPC of viral infection were significantly higher in 2015 than in 2016. The severity and AUDPC of edema and monochaetia leaf spot were significantly higher in 2016 than in 2015. Flower blight and flower spot were observed only in two cultivars, Arctic Snow and Survivor (Table 9). Because there was no year × cultivar interaction, disease values were pooled across years. Virus severity ranged from 0% to 13.5% (Table 9). Cultivars Korean Fire, Classic Pink, Maroon Mist, and Spring’s Promise showed the highest virus severity (>7%) and AUDPC (Tables 9 and 10). The C. chekiangoleosa selection, ‘Arctic Snow’, and C. sasanqua selection had no sign of viral infection, and virus severity and AUDPC of the other cultivars were comparable to these three cultivars (<4%). C. sasanqua selection and ‘Red Aurora’ were highly impacted by edema disorder, with 43% and 26% edema severity, respectively, and AUDPCs of 1144 and 697, respectively; these values were significantly higher than those of the other cultivars (Tables 9 and 10). Edema severity and AUDPC of C. sasanqua selection were significantly higher, and ‘Red Aurora’. ‘Anacostia’, ‘April Remembered’, ‘One Alone’, ‘Korean Fire’, ‘Pink Icicle’, ‘Spring’s Promise’, ‘Stellar Sunrise’, and ‘Survivor’ exhibited the lowest edema severity and AUDPC. Monochaetia leaf spot severity was highest in ‘Red Aurora’ and ‘Spring’s Promise’, with 12% and 9% severity, respectively, and had the highest AUDPC (Tables 9 and 10).
P value for the main and interactive effects of year (n = 2) and cultivar (n = 21) of camellia (Camellia spp. and hybrids) cultivars and selections on severity (% of the leaf area affected) and the area under the disease progress curve (AUDPC) of virus (Camellia Yellow Mottle Virus), edema, and monochaetia leaf spot (Monochaetia sp.) in McMinnville, TN, USA.
Mean severity (% of the leaf or flower area affected) of virus (Camellia Yellow Mottle Virus), monochaetia leaf spot (Monochaetia sp.), flower blight (Ciborinia camelliae), flower spot (Botrytis cinerea), and edema of camellia (Camellia spp. and hybrids) cultivars and selections in McMinnville, TN, USA.
Mean area under the disease progress curve (AUDPC) of virus, edema, and monochaetia leaf spot of camellia (Camellia spp. and hybrids) cultivars and selections in McMinnville, TN, USA.
Discussion
This is the first study to provide comprehensive information regarding the growth, cold-hardiness, flowering, and disease resistance of ornamental camellia cultivars and selections. Establishing and sustaining the ornamental qualities of camellia cultivars are greatly challenged by the risk of low-temperature damage. Three of the 24 ornamental camellia cultivars or selections planted in 2011 (i.e., C. cuspidata selection, ‘Korean Snow’, and ‘Longwood Valentine’) were excluded from evaluation after 2014 because of severe stem dieback resulting from low-temperature injury.
The cultivars and selections varied significantly in growth, cold-hardiness, flowering duration, and disease resistance. Certain cultivars outperformed others across all evaluation parameters. Camellias were exposed to sub-freezing temperatures during Oct to Apr from 2011 to 2023, coinciding exactly with the camellia cultivars’ blooming season. Because Tennessee is in a transition zone, growth and flowering were also likely adversely affected by windy and/or dry conditions that accelerate desiccation, warm conditions immediately preceding extreme lows, or extended exposure time to cold conditions. Notably, most cultivars demonstrated reduced height, width, and GI after winters during which plants were exposed to temperatures below −10 °C (−10 to −19 °C), particularly in 2012, 2014, and 2018. Decreases in height, width, and GI resulted from low temperature-induced twig or branch dieback.
The International Camellia Society has compiled a comprehensive list of camellia cultivars and hybrids, along with their concise descriptions. Among these, ‘April Remembered’, ‘April Rose’, ‘Ashton’s Ballet’, ‘Autumn Spirit’, ‘Classic Pink’, ‘Elaine Lee’, ‘Korean Fire’, ‘Longwood Valentine’, ‘Maroon Mist’, ‘Red Aurora’, and ‘Survivor’ are classified as cold-hardy. According to the International Camellia Society report (available at: https://camellia.iflora.cn/Cutivars/Country), the cold-hardiness ratings for these cultivars are as follows: April Remembered, cold-hardy; April Rose, very cold-hardy; Ashton’s Ballet, cold-hardy to −23 °C; Autumn Spirit, offered as a cold-hardy selection; Classic Pink, cold-hardy; Elaine Lee, plant hardy to −24.5 °C; Korean Fire, can withstand temperatures down to −30.5 °C; Kura Delight, cold-hardy to −26 °C; Longwood Valentine, plant has exceptional cold-hardiness; Maroon Mist, cold-hardy to −20 °C; Red Aurora, very hardy; and Survivor, cold-hardy to −23 °C. In the current study, we found that most of these cultivars were the least impacted by low winter temperatures during the 2014 evaluation. Specifically, ‘April Remembered’, ‘April Rose’, ‘Ashton's Ballet’, ‘Autumn Spirit’, ‘Classic Pink’, ‘Elaine Lee’, ‘Maroon Mist’, and ‘Survivor’ demonstrated superior cold hardiness. Additionally, cultivars Arctic Snow and Autumn Carnival, as well as the C. chekiangoleosa selection, which are not classified as cold-hardy by the International Camellia Society, were also among the least affected by low temperatures during the current study. However, ‘Korean Fire’, ‘Kuro Delight’, ‘Longwood Valentine’, and ‘Red Aurora’ exhibited light to moderate damage caused by low temperatures in 2014. However, most of the cultivars were severely affected by the low temperatures in 2023, with only a few, such as April Rose, Ashton’s Ballet, the C. chekiangoleosa selection, Elaine Lee, and Survivor, being lightly to moderately damaged.
The results of the current study also indicated that low winter temperatures adversely affected flower initiation and flowering duration. Major low-temperature damage was evident in 2012, 2014, and 2018, during which most of the spring-flowering cultivars and selections failed to produce flowers. Only ‘April Remembered’ and ‘April Rose’ produced flowers during Spring 2018. Interestingly, fall- and winter-flowering cultivars were least affected by low temperatures, likely because, in contrast to the spring-flowering camellias, neither the flower buds nor the flowers of the fall- and winter-flowering cultivars and selections were exposed to extreme low winter temperatures. Moreover, these cultivars had ample time to recover (i.e., produce new shoots and flower buds) after low-temperature damage that typically occurs during winter and spring. The exceptions were in 2013 and 2014, during which none of the fall-flowering cultivars or selections bloomed. The absence of blooms during those years was a consequence of unseasonably cold temperatures in Nov.
During the 8-year evaluation of flower initiation, some cultivars did not flower at all, whereas others experienced only a few years of flowering, and some cultivars produced flowers during as many as six of the evaluation years. ‘April Remembered’, ‘April Rose’, ‘Arctic Snow’, ‘Ashton’s Ballet’, ‘Autumn Carnival’, ‘Autumn Spirit’, and ‘Survivor’ flowered most dependably over the course of the study, with flowers produced during at least five of the eight evaluation years. All these cultivars were least damaged by low temperature. Interestingly, most of these cultivars, such as Arctic Snow, Ashton’s Ballet, Autumn Carnival, Autumn Spirit, and Survivor, bloom during fall and winter before Dec 15, indicating that low winter temperatures played a role in limiting flower initiation of the winter- and spring-blooming cultivars and selections. Elaine Lee was the only cultivar that had an increase in GI during all three low-temperature years (2012, 2014, and 2018), and the C. chekiangoleosa selection had an increased GI during two of the three years and produced flowers three times over the course of the 8-year evaluation. ‘Elaine Lee’ flowers in summer and the C. chekiangoleosa selection flowers in spring. Despite displaying minimal low-temperature damage, the cultivar Maroon Mist did not flower during any year during the 8-year evaluation. Cultivars Pink Icicle, Spring’s Promise, and Stellar Sunrise had longer-lasting flowering periods, but they flowered only a few times.
Although cultivars exhibited varied responses to diseases and edema disorders, these issues were generally minor. Only a few cultivars were impacted by these diseases and edema. For instance, ‘Classic Pink’, ‘Korean Fire’, and ‘Maroon Mist’ showed the highest virus severity (range, 8%–14%) and disease progression. Only ‘Red Aurora’ was moderately impacted by monochaetia (∼12%). Flower blight and flower spots were observed solely in ‘Arctic Snow’ and ‘Survivor’, yet the severity of the problem was not substantial enough to cause significant damage. Edema severely affected the C. sasanqua selection and ‘Red Aurora’. Although careful monitoring of these diseases is necessary to prevent further damage to the crops, they do not currently appear to be major issues.
In conclusion, after conducting evaluations of growth, cold-hardiness, flowering, and disease resistance, certain cultivars and selections stand out as promising options for McMinnville, TN, USA (USDA plant hardiness zone 7a). Among them, the spring-flowering cultivars April Remembered, April Rose, and the C. chekiangoleosa selection and fall- and winter-flowering cultivars Arctic Snow, Ashton’s Ballet, Autumn Carnival, Autumn Spirit, Elaine Lee, and Survivor, showed great potential compared with the others. These selected cultivars and selections also offer diverse size options for nursery growers and landscapers.
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