Field Performance and Susceptibility to Fungal Pathogens of Eleven Blackberry Cultivars

ABSTRACT

Horticultural performance of eleven blackberry cultivars, including two primocane-fruiting cultivars, was evaluated over three years after fruiting began at the Musser Fruit Research Center in South Carolina, United States. Results showed differences in yield between cultivars for each year of the study. ‘Natchez’ had the highest cumulative yield after three years, followed by ‘Navaho’, ‘Von’, and ‘Osage’, whereas ‘Prime-Ark® Traveler’ produced significantly less than these four cultivars. Yield steadily declined for all cultivars over the three years, except for ‘Osage’, which remained consistent. ‘Prime-Ark® Freedom’ always had the largest fruit, although it had the greatest yield reduction after the second year. Multiple factors may explain yield decline for the majority of the cultivars studied, including freeze damage and tip dieback. Tip dieback was observed in 2016 in multiple cultivars, but dieback incidence was highest on ‘Prime-Ark® Traveler’. The dieback was associated with an outbreak of broad mites and thrips in the experimental block and with secondary pathogens isolated from the decaying tissues primarily Fusarium spp. and Alternaria spp. Cultivar differences in susceptibility to Alternaria spp. were confirmed in laboratory studies.

KEYWORDS:

• Yield
• fruit weight
• tip dieback
• thornless
• integrated pest management

Introduction

The horticultural quality and productivity of commercial blackberry cultivars (Rubus L. subgenus Rubus Watson) vary by climate and region as temperatures and humidity limit their geographic adaptability (Clark, 2016; Stanton et al., 2007). In the U.S., blackberry breeding programs have released several thornless cultivars, including primocane-fruiting blackberries as well as cultivars having early and late ripening seasons, and fruit qualities desired by consumers and growers such as firmness, high soluble solids, enhanced shipping and superior postharvest attributes (Clark and Finn, 2014, 2008; Fernandez et al., 2013; Salgado and Clark, 2016). Since 2005, production of blackberries has rapidly increased in the western and southern U.S. (Clark and Finn, 2014), yet few trials have determined which cultivars perform well in these areas (Fernandez et al., 2016; Kaps, 2019). Moreover, environmental conditions together with cultivar susceptibility to pathogens, influence disease prevalence (Schilder et al., 2017). Diseases such as ‘cane blight’ (Leptosphaeria coniothyrium (Fuckel) Sacc.) and ‘cane and leaf rust’ (Gymnoconia spp. and Arthuriomyces spp.) can significantly impact the productivity of the plant, whereas ‘gray mold’ (Botrytis cinerea) and ‘rosette’ (Cercosporella rubi) can impact yield (Schilder et al., 2017). The goal of this study was to compare the horticultural performance and incidence of pathogens of eleven thornless blackberry cultivars over three harvest seasons.

Materials and Methods

Horticultural Performance

A cultivar evaluation trial was carried out at Clemson University’s Musser Fruit Research Center (MFRC), located in Seneca, South Carolina, USA. Eleven thornless blackberry cultivars were planted in January 2015 and their horticultural performance was evaluated over three years, between 2016 and 2018. The following cultivars were evaluated: ‘Arapaho’, ‘Natchez’, ‘Osage’, ‘Ouachita’, ‘Navaho’, ‘Von’, ‘Apache’, ‘Triple Crown’, ‘Chester Thornless’, and two thornless primocane-fruiting cultivars: ‘Prime-Ark® Freedom’ and ‘Prime-Ark® Traveler’. There were 15 plants per cultivar, with groups of five plants replicated three times (i.e. three rows). Plants were grown 1 m apart within the rows. All plants were grown on a V trellis system (1.5 m tall x 1 m wide) with white plastic mulch and drip irrigated with 3.4 L/plant per hour for 3 h per week. Blackberries were fertigated with calcium nitrate weekly between April and July, then monthly in August and September to total 22.42 kg N/ha/year. Pruning and removal of floricanes occurred during the winter, following the standard practice of growers in the region. Lateral branches were trimmed to 20–30 cm on remaining primocanes, and all branches close to the ground (< 20 cm) were removed. Primocanes of all cultivars were soft-tipped at 1.5 m height during the summer. Soft tipping occurred at various times during the summer whenever primocane growth reached the tallest trellis wire (1.5 m). The two primocane-fruiting cultivars were double cropped, and primocanes grew alongside floricanes within the same planting during the summer months.

The block was maintained with a minimal disease control program to document any noteworthy susceptibility to pathogens. Insecticides (rotating spinetoram, acetamiprid, and zeta-cypermethrin) were applied once a month between the end of February and April, every two weeks during May and June, and once a month during July and August. Miticides were applied when broad mite (Polyphagotarsonemus latus) populations were above threshold levels in October 2016, and July 2017. Bloom (50% and 90% bloom) dates and fruit harvest window were recorded yearly. Commercially ripe floricane fruit were harvested twice each week throughout the production season whereas primocane fruit were harvested once each week. Harvested fruit weight was recorded and averaged across the five plants of each cultivar per row to calculate the total yield per plant. Mean fruit weight was calculated as an average of 75 fruit (25 fruit per row) harvested weekly for four weeks during peak production. Primocane fruit weight was recorded over three weeks during production.

Pathogen Isolations

Tip dieback was observed at the MFRC, as well as at a commercial farm near Pelzer, SC and at an experimental block at Virginia Polytechnical Institute and University, Blacksburg, VA. At the MFRC, the incidence of dieback was recorded using 20 primocanes of each cultivar in the three replicates during 2016 and 2017. Declining shoots with dead tips were collected from all three locations. Fungal isolations were conducted from the intersection between necrotic and asymptomatic tissue. Each segment, about 5 cm in length, was surface sterilized with 10% bleach for 30 s, rinsed three times, and placed on potato dextrose agar (PDA) petri dishes (90 mm) for incubation at 22 °C in darkness. Emerging colonies were then transferred to new PDA plates to obtain pure fungal cultures. The universal fungal barcode, ribosomal Internal Spacer Regions (ITS) 1 and 2, was sequenced from isolates with different colony morphologies for species or genus identification. DNA extraction and polymerase chain reaction (PCR) were conducted as described previously (Hu et al., 2011). Sequencing was conducted at Clemson University Genomics Institute (Clemson, SC).

Cultivar Susceptibility Assay

Non-symptomatic distal primocane (from primocane-fruiting cultivars) or floricane shoots about 20 cm in length were obtained from the MFRC cultivar trial in both years of assessment. The segments were surface sterilized for 30 s in 10% bleach and then rinsed three times with sterile water. Two 50 µl drops containing Fusarium spp. or Alternaria spp. spores at 1 × 10⁵ spores/ml were placed on each segment. The drops and cane surface were penetrated with a sterile needle with care not to cross contaminate. After 1 week of incubation in a moist chamber, the disease incidence and severity (lesion length) were determined. The size of developing lesions was assumed to be an indicator of cultivar susceptibility. The bigger the lesions, the more susceptible the cultivar.

Statistical Analysis

Annual yield and mean fruit weight data were analyzed using analysis of variance (ANOVA) and Tukey’s Honest Significant Difference (HSD) post hoc means separation tests using the statistical program JMP® (Version 14.1.0; SAS Institute, Cary, NC, United States). ANOVA tests were also used to calculate differences in cumulative yield over three years and by using Student’s Least Square Means contrast tests.

Results

Blooming and Harvest Timing

Bloom and harvest dates were similar in 2016 and 2017, and slightly delayed in 2018 (Table 1). The 50% bloom occurred between April 4/5 and May 10/11 in 2016 and 2017. In 2018, spring temperatures fluctuated with cycles of warm and cold weather, causing a delay of bloom and harvest dates of one week and up to ten days for some cultivars. Each cultivar went from 50% to 90% bloom in a period of 3–8 days, independently of year. Overall, the harvest window was about four weeks for early-season cultivars, and about six weeks for late-season cultivars. The length of the harvesting season increased during the second and third year of the study, with most cultivars producing over six to eight weeks. Primocane fruit were also harvested from ‘Prime-Ark® Freedom’ and ‘Prime-Ark® Traveler’.

Table 1. First and last dates of floricane and primocane harvest, and floricane 50% and 90% bloom dates for 11 blackberry cultivars over three years of production at the Musser Fruit Research Center in South Carolina, USA

	2016			2017			2018
Cultivar	Harvest	50%	90%	Harvest	50%	90%	Harvest	50%	90%
Floricane
Apache	6/16-8/11^z	04/21	04/25	6/8-8/9	04/20	04/22	6/15-8/1	05/04	05/08
Arapaho	5/25-6/21	04/18	04/20	5/22-7/5	04/15	04/20	5/25-7/11	04/27	05/02
Chester Thornless	7/1-8/11	05/10	05/13	7/5-8/22	05/08	05/11	7/3-8/29	05/22	05/30
Natchez	5/27-6/28	04/14	04/19	5/22-7/13	04/13	04/16	5/25-7/18	04/25	05/01
Navaho	6/9-7/13	04/26	04/29	6/5-8/9	04/24	04/28	6/8-8/14	05/04	05/09
Osage	5/31-7/6	04/14	04/21	5/25-7/25	04/11	04/19	5/29-7/24	04/25	05/01
Ouachita	6/6-7/13	04/20	04/25	5/29-7/25	04/20	04/25	6/4-7/25	04/30	05/04
Prime-Ark® Freedom	5/13-6/13	04/05	04/08	5/19-6/26	04/04	04/09	5/25-6/22	04/13	04/18
Prime-Ark® Traveler	5/19-6/16	04/14	04/19	5/22-7/5	04/11	04/17	5/25-7/18	04/16	04/24
Triple Crown	6/24-8/3	05/10	05/12	6/22-8/9	05/01	05/05	6/19-8/8	05/16	05/20
Von	6/13-8/3	04/25	04/28	6/19-8/15	04/25	04/28	6/12-8/14	05/06	05/11
Primocane
Prime-Ark® Freedom	7/7-8/11	-	-	7/5-8/9	-	-	7/11-8/8	-	-
Prime-Ark® Traveler	7/13-8/11	-	-	7/18-8/22	-	-	7/18-8/29	-	-

^zDates are shown as month/day

Fruit Yield and Weight

There were differences in floricane yield (P ≤ 0.01) between cultivars each year of the study (Table 2). In 2016, ‘Natchez’ was the top producer, with yield significantly higher than ‘Ouachita’, ‘Triple Crown’, ‘Prime-Ark® Traveler’ and ‘Chester Thornless’, which was the cultivar with the lowest yield. ‘Prime-Ark® Freedom’ was the second top producer, also with significantly higher yield than ‘Prime-Ark® Traveler’ and ‘Chester Thornless’. However, ‘Chester Thornless’ had the highest yield in 2017, followed by ‘Navaho’ and ‘Von’, whereas ‘Prime-Ark® Freedom’ and ‘Prime-Ark® Traveler’ had the lowest yields. Yield declined for all cultivars in 2018, with the exception of ‘Osage’, which remained fairly consistent, and was the cultivar with the highest yield in the third year (significantly higher than cultivars such as ‘Triple Crown’, ‘Prime-Ark® Traveler’, and ‘Apache’). Cumulative floricane yield was also different (F = 4.54, P ≤ 0.01) between cultivars; ‘Natchez’ had the highest yield in contrast to other cultivars (F = 8.3, P ≤ 0.01) after three years, followed numerically by ‘Navaho’, ‘Von’, and ‘Osage’. ‘Prime-Ark® Traveler’ produced significantly less than these four cultivars. Cultivars such as ‘Apache’, ‘Arapaho’, ‘Natchez’, ‘Prime-Ark® Freedom’ and ‘Prime-Ark® Traveler’ had their highest yield in 2016 and declined steadily after until all had lower yields by 2018 compared to 2016 (P ≤ 0.01). ‘Osage’ also had its greatest yield in 2016 but was consistent from year to year, and yield was numerically the highest among all cultivars in 2018. Primocane fruit yield from ‘Prime-Ark® Freedom’ and ‘Prime-Ark® Traveler’ were similar in 2016 and 2017. In 2018, ‘Prime-Ark® Traveler’ had more primocane fruit than ‘Prime-Ark® Freedom’, but this small difference did not result in differences for the total primocane fruit yield. When yields of primocane and floricane fruit from ‘Prime-Ark® Freedom’ and ‘Prime-Ark® Traveler’ were combined together and then compared to the other cultivars, the total yield after three years of all 11 cultivars were statistically equal (P > 0.05, data not shown).

Table 2. Annual yield per plant (kg, n = 15) and cumulative yield (2016-2018) of 11 blackberry cultivars at the Musser Fruit Research Center in South Carolina, USA

Cultivar	2016	2017	2018	Total
Floricane
Apache	7.8 abc^z	4.1 bcd	1.8 c	13.7 ab
Arapaho	9.0 ab	3.6 bcd	3.3 abc	15.9 ab
Chester Thornless	2.7 d	12.0 a	4.1 abc	18.8 ab
Natchez	11.8 a	5.3 abc	5.2 ab	22.3 a
Navaho	7.3 abc	9.5 ab	5.0 ab	21.8 a
Osage	7.1 abc	6.5 abc	6.1 a	19.7 a
Ouachita	6.3 bc	6.5 abc	3.4 abc	16.2 ab
Prime-Ark® Freedom	9.8 ab	1.2 d	3.0 abc	14.0 ab
Prime-Ark® Traveler	4.6 cd	3.3 cd	2.2 bc	10.1 b
Triple Crown	5.9 bcd	8.5 abc	2.8 bc	17.2 ab
Von	7.1 abc	9.4 ab	4.8 ab	21.3 a
Primocane
Prime-Ark® Freedom	0.3 a^y	2.1 a	0.3 b	2.7 a
Prime-Ark® Traveler	0.4 a	1.7 a	0.6 a	2.7 a

^zFloricane values within a column which do not share a similar letter are significantly different (α < 0.05) by Tukey’s HSD test.

^yPrimocane values within a column which do not share a similar letter are significantly different (α < 0.05) by Student’s LSD test.

There were differences in fruit weight each year and the three-year average (F = 37.1, P ≤ 0.0001). Independent of yield, ‘Prime-Ark® Freedom’ consistently had large fruit each year and was statistically higher (P ≤ 0.001) than all other cultivars over three years (Table 3). Fruit weight declined significantly for ‘Ouachita’ (P ≤ 0.05) and ‘Triple Crown’ (P ≤ 0.01) over the three years while other cultivars including ‘Osage’ and ‘Von’ declined numerically even while maintaining similar yield between different years. Others including ‘Apache’ and ‘Natchez’ were similar in fruit weight across the three harvest seasons, yet yield decreased throughout the trial.

Table 3. Mean fruit weight (g, n = 75) during each year of harvest and across the three years at the Musser Fruit Research Center in South Carolina, USA

Cultivar	2016	2017	2018	Mean
Floricane
Apache	7.7 bcd^z	8.0 bc	7.4 abc	7.7 b
Arapaho	5.8 de	5.3 de	5.3 cd	5.5 de
Chester Thornless	4.8 e	5.3 e	4.7 cd	5.0 e
Natchez	8.4 bc	6.9 cd	7.1 abc	7.6 b
Navaho	5.5 e	5.3 de	5.0 cd	5.2 de
Osage	6.0 cde	5.7 de	5.3 cd	5.6 de
Ouachita	6.8 cde	6.2 de	5.9 cd	6.1 cde
Prime-Ark® Freedom	9.7 ab	9.7 ab	9.8 a	9.7 a
Prime-Ark® Traveler	7.1 cde	5.9 de	6.2 bcd	6.5 bcd
Triple Crown	7.8 bcd	6.7 cde	6.6 bcd	7.1 bc
Von	6.2 cde	5.5 de	5.1 cd	5.7 de
Primocane
Prime-Ark® Freedom	11.7 a	10.6 a	7.8 abd	10.0 a
Prime-Ark® Traveler	5.3 de	5.2 de	4.2 d	4.8 e

^zFloricane and primocane values within columns which do not share a similar letter are significantly different (α < 0.05) by Tukey’s HSD test.

Pathogens

Severe tip dieback was observed in 2016 on ‘Prime-Ark® Traveler’ at the MFRC trial (Figure 1, A) and some other cultivars including ‘Apache’, ‘Arapaho’, ‘Navaho’, ‘Osage’, ‘Ouachita’, ‘Prime-Ark® Freedom’, ‘Triple Crown’, and ‘Von’ (Table 4).

Table 4. Percent incidence of tip dieback on different blackberry cultivars (2016)

	Tip dieback incidence (%)
Cultivar	Rep 1	Rep 2	Rep 3	Mean ± SE
Apache	25	35	15	25 ± 8.2
Arapaho	25	50	15	30 ± 14.7
Chester Thornless	0	0	0	0
Natchez	0	0	0	0
Navaho	10	10	15	11.7 ± 2.4
Osage	20	25	50	31.7 ± 13.1
Ouachita	10	40	30	26.7 ± 12.5
Prime-Ark® Freedom	40	15	15	23.3 ± 11.8
Prime-Ark® Traveler	65	70	80	71.7 ± 6.2
Triple Crown	5	5	0	3.3 ± 2.4
Von	55	5	0	20 ± 24.8

Figure 1. Primocane tip dieback (a) and fungal spores obtained from dieback samples (b) (Photos by Lauren Darnell)

Most frequently affected was ‘Prime-Ark® Traveler’ (mean incidence was 71.7%), but other cultivars including ‘Apache’, ‘Arapaho’, ‘Osage’, ‘Ouachita’, and ‘Prime-Ark® Freedom’ had disease incidence above 20%. On the contrary, ‘Chester Thornless’ and ‘Natchez’ were not affected. No tip dieback was observed in the same trial in 2017. Fungal pathogens including Alternaria spp. and Fusarium spp. were consistently isolated from affected tip tissues. An example of spores of fungal pathogens recovered from tips is shown in figure 1B. But occasionally, Botryosphaeria spp. and Pestalotiopsis spp. were identified as well at this location (Table 5). An outbreak of broad mites (Polyphagotarsonemus latus) and flower thrips (Frankliniella sp.) was also observed in 2016 in all locations sampled, including the affected canes. In addition to the aforementioned fungal pathogens, Phomopsis spp. was isolated from declining canes from an experimental field at Virginia Tech, VA. The segments from declining tips collected at a commercial farm in South Carolina only revealed Alternaria spp. and Fusarium spp. (Table 5).

Table 5. Plant pathogens isolated in 2016 from symptomatic blackberry tips^a.

Fungi	MFRC, SC	Pelzer, SC	Experimental field, VA
Alternaria spp.	x	x	x
Botryosphaeria spp.	x	-	x
Fusarium spp.	x	x	x
Pestalotiopsis spp.	x	-	x
Phomopsis spp.	-	-	x

^aFungi were identified either by spore morphology or by ITS sequencing; ‘x’ indicates the presence of the pathogen in at least some of the cane segments examined.

Cultivar susceptibility to an Alternaria sp. isolate obtained from MFRC segments was assessed by artificially inoculating detached primocane tips. Similar to our field observation, ‘Prime-Ark® Traveler’ was one of most susceptible cultivars in this assay in 2016. ‘Navaho’, ‘Osage’, and ‘Von’ were also highly susceptible to Alternaria (Table 6). The experiment was repeated in 2017 with the same Alternaria isolate, however, no symptoms were recorded on any of the cultivars inoculated. Inoculations conducted in 2017 with a Fusarium sp. isolate obtained from the MFRC field the previous year also did not yield symptoms (data not shown).

Table 6. Disease incidence (%) and severity expressed as lesion length (cm) of detached primocanes collected at the Musser Fruit Research Center after inoculation with an Alternaria isolate collected in 2016

Cultivar	Disease incidence (%)	Lesion length (cm)
Apache	40	3.8
Arapaho	50	3.3
Chester Thornless	12.5	0.6
Natchez	0	0
Navaho	100	4.5
Osage	80	4.6
Ouachita	8.3	1.0
Prime-Ark® Freedom	50	4.6
Prime-Ark® Traveler	90	5.1
Triple Crown	20	1.3
Von	80	4.8

Discussion

Differences between the floricane fruit yield each year were not maintained consistently throughout the three years of the experiment, and all cultivars except ‘Prime-Ark® Traveler’ showed similar statistical yield after three years. Large variations in yield between years of study most likely account for the inability of the HSD test to identify significant yield differences between other cultivars than ‘Prime-Ark® Traveler’. Student’s Least Significant Difference means post hoc test, revealed ‘Navaho’ and ‘Natchez’ had significantly higher yield over the three years in contrast to ‘Apache’, ‘Prime-Ark® Freedom’ and ‘Prime-Ark® Traveler’. Variation of annual yield by cultivar has also been observed in a Missouri trial, and their two highest yielding cultivars where ‘Chester Thornless’ and ‘Triple Crown’ after five years of harvest (Kaps, 2019). The MFRC trial plants also outperformed average yield per plant values recorded at the Missouri trial. Whereas Kaps (2019) reported the yield of ‘Chester Thornless’ and ‘Triple Crown’ average to be 3.9 kg/plant annually, both ‘Navaho’ and ‘Natchez’ at the MFRC averaged nearly double the amount, 7.35 kg/plant after three years.

Yield reduction after the second year of growth has been observed previously for some cultivars in China (Wu et al., 2012) and fluctuations of yield between years is likely due to environmental reasons such as chill hours or freeze damage (Clark, 2016; Fernandez and Ballington, 2010). A reduction of primocane growth has been observed on ‘Natchez’ as a result of high floricane yield (Strik, 2017), however measurements were not taken on primocane height or node number and all canes were tipped each year at 1.5 m. Similarly, the practice of double cropping has shown to either negatively affect (Strik, 2017) or have no effect on the following season yield (Drake and Clark, 2003). It is possible that double cropping influenced floricane yield of the primocane-fruiting cultivars, but the primocane yield was usually lower in comparison to the floricane yield, with an exception for ‘Prime-Ark® Freedom’ in 2017. However, our observations suggest ‘Prime-Ark® Freedom’ had some damage from a freeze in 2017 and this is thought to partially explain why yield was reduced. ‘Prime-Ark® Traveler’ was planted three weeks later than the other cultivars in 2015 and this may have contributed to the low yield in 2016, although it also showed the highest percentage of tip dieback in 2016.

Fruit weight decline over the three years was observed on several cultivars regardless of total yield and was significant for ‘Ouachita’ and ‘Triple Crown’ and nearly significant (P ≤ 0.1) for ‘Osage’. Other cultivars had consistently high fruit weight, such as ‘Apache’, ‘Natchez’, and ‘Prime-Ark® Freedom’, yet the inconsistent or low yield per plant may not justify planting these cultivars depending on labor costs and production goals. Although the trial was drip irrigated, weight decline may be a result of seasonal variation of humidity and water status (Strik, 2017). Consistent, large fruit weight for ‘Apache’ and ‘Natchez’ was also reported in Missouri (Kaps, 2019). Although yield per plant of a particular cultivar may remain consistent or increase, decreasing fruit size over time results in greater labor cost of picking if not performed mechanically.

Additionally, the severe tip dieback observed on many cultivars in 2016 may have also reduced yield the following season, although tip dieback was found regardless of pinching. The pruning method of double-tipping on primocane-fruiting cultivars has previously been shown to increase yield in both field and greenhouse environments (Thompson et al., 2009), but this practice was not followed for this experiment as the trial replicated the practices used by the growers in the region. Pruning practices should be explored in future studies in regions with humid subtropical climates where disease pressure may be high. In any case, it is still unclear what caused the tip dieback in 2016 at the MFRC. Without exception, the fungal isolates obtained from diseased tips were not primary pathogens, but they are known to cause cane cankers (Ellis et al., 1984). They are opportunists, dependent on injury to enter the tissue. Artificial inoculation of uninjured cane segments with spore suspensions, for example, did not yield infection (data not shown). Broad mites have been associated with terminal and lateral damages on multiple blackberry cultivars, including ‘Prime-Ark®’ cultivars (Demchak and Johnson, 2017). Such damage may have provided a gateway into the plant tissue for the opportunistic cane pathogens identified in this study (i.e. Alternaria spp. and Fusarium spp.). The fact that miticides were only applied to the MFRC blackberries later in the year of 2016 and that broad mites were observed support this hypothesis. Moreover, insecticide and miticide applications in 2017 appeared to have managed both thrips and mites and may have accounted for the lack of tip dieback that same year. Therefore, it is possible that the secondary pathogens identified were enabled to enter the cane tips in 2016 due to insect and/or mite damage.

The cultivar susceptibility assay conducted in 2016 proved that Alternaria spp. can colonize and destroy cane tissue following tissue injury. We did not expect, however, a failure of the same isolate to produce disease on cane segments the following year. The Alternaria sp. isolate was stored on filter paper for 12 months prior to the experiment, which may have affected its pathogenicity. Alternatively, it is possible that the 2016 canes were somehow predisposed to infection due to thrip and/or mite colonization. That may have caused stress to the plant that could have allowed otherwise weak pathogens to enter and colonize (Schoeneweiss, 1975). Observations of tip dieback on ‘Arapaho’ in California did not reveal signs of cultural or pathogen involvement. Instead, an interaction of the environment and cultivar was suggested (Bolda, 2012).

In summary, blackberry production was variable by cultivar during each year of the study. Several cultivars such as ‘Apache’, ‘Natchez’, and ‘Triple Crown’ had an ideal 7–8 g average fruit weight (Clark and Finn, 2008), and ‘Natchez’ had the highest numerical yields over three years. While ‘Prime-Ark® Freedom’ consistently had the largest fruit, it had the greatest yield reduction after the second year. Furthermore, after observing no tip dieback in 2017 following miticide and insecticide applications at MFRC, this research establishes the importance of insect and mite management to control tip dieback of some blackberry cultivars.

Declaration of interest statement

The authors certify that they have no affiliations with or involvement in any organization or entity with financial or non-financial interests in the subject matter discussed in this manuscript.

Acknowledgments

The authors would like to thank the North American Raspberry and Blackberry Association for their financial support, James E. Cooley and Andy Rollins for sharing field equipment and for their knowledge and assistance for setting up the trial, and the MFRC staff for their maintenance and help during harvest.

Additional information

Funding

This work was supported by the North American Raspberry and Blackberry Association. This material is based upon work supported by NIFA/USDA, under project number SC-1700530 and Technical Contribution No. 6824 of the Clemson University Experiment Station.