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Final Report: Evaluation of alternative sprout suppressants

Ref: R438 a component of Defra programme FO/0217, Reporting Period: 2010-2014, Report Authors: Report by: Graeme Stroud & Adrian Briddon, Sutton Bridge Crop Storage Research

While AHDB, operating through its Potato Council division seeks to ensure that the information contained within this document is accurate at the time of printing no warranty is given in respect thereof and, to the maximum extent permitted by law the Agriculture and Horticulture Development Board accepts no liability for loss, damage or injury howsoever caused (including that caused by negligence) or suffered directly or indirectly in relation to information and opinions contained in or omitted from this document. No part of this publication may be reproduced in any material form (including by photocopy or storage in any medium by electronic means) or any copy or adaptation stored, published or distributed (by physical, electronic or other means) without the prior permission in writing of the Agriculture and Horticulture Development Board, other than by reproduction in an unmodified form for the sole purpose of use as an information resource when the Agriculture and Horticulture Development Board is clearly acknowledged as the source, or in accordance with the provisions of the Copyright, Designs and Patents Act 1988. All rights reserved.

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Content

s1.0 Summary.......................................................................................................................................................... 3

2.0 Introduction....................................................................................................................................................... 4

3.0 Methods and Materials..................................................................................................................................... 4

3.1 Small scale experiments................................................................................................................................... 4

3.2 Large scale (semi-commercial) experiments....................................................................................................7

4.0 Results.............................................................................................................................................................. 9

4.1 Small scale experiments................................................................................................................................... 9

4.2 Large scale (semi-commercial) experiments..................................................................................................17

6.0 Conclusions.................................................................................................................................................... 23

7.0 Appendicies.................................................................................................................................................... 24

7.1 Additional sprouting data................................................................................................................................ 24

7.2 Records of application to small scale chambers.............................................................................................26

7.3 Product labels................................................................................................................................................. 30


1.0 Summary

Cultivars Russet Burbank and Saturna were held in small-scale chambers at 6°C and 9°C for periods up to approximately 3 months and 6 months, over three storage seasons. A range of sprout suppressants were applied, as cold mists, and efficacy of sprout control assessed in comparison with CIPC applied once, as a liquid, directly to tubers at store loading.

CIPC resulted in effective, commercially acceptable sprout control of both cultivars, at both storage temperatures and in all seasons. Of the alternative sprout suppressants, over three seasons, caraway oil was relatively effective, but control was sometimes incomplete during storage at 9°C for 6 months. Spearmint oil was effective at 6°C, but sprout control was inconsistent at the warmer storage temperature.

On the basis of small-scale work, caraway oil and spearmint oil were selected for assessment in semi-commercial trials (16 tonnes, in boxes). With caraway oil applied non-positively and spearmint oil applied using positive ventilation, sprout control of cultivars Russet Burbank and Saturna was incomplete during storage at 9°C, under the conditions tested.


2.0 Introduction

After an initial dormant period, sprouts grow from stored potatoes. Unchecked, sprouting gives rise to internal changes that eventually render the crop unsaleable. Such changes include excessive weight loss, shrinkage and deterioration in processing quality. Suppression of sprout development during storage, especially at the warmer temperatures used for processing, is critical. CIPC is the main active substance for the control of sprout growth, and in 2008 was used on around 47% of stored potatoes in Great Britain (Garthwaite et al. 20091 ) and made up 94% of postharvest treatments to stored potatoes.

Alternative sprout suppressants are being sought and treatments are becoming available. To date, only post-harvest treatment with ethylene has had significant commercial success (www.pesticides.gov.uk/approvals.asp?id=271). It was used on 4% of stored crops in 2008 (Garthwaite et al. 2009). Treatments with ethylene currently are limited to the fresh potato sector, because of the associated potential deterioration in processing quality.

Essential oils have been recognised as sprout suppressants for many years and are available formulated as products for sprout control (and other applications) elsewhere. Little independent information is available on the alternatives. This project (R438) is designed to give comparative information on the efficacy of new active substances that are available or expected to become available in the near future. An alternative method of using CIPC (GroStop Ready) is also included. This product is a liquid formulation which is applied directly to tubers at store loading. Although available for some years, this method of applying CIPC has never been widely adopted in the UK, unlike mainland Europe where it is frequently used.

These studies were funded by AHDB Potato Council as part of a larger programme, primarily funded by DEFRA (FO/0217: Reducing post-harvest losses and wastage in UK potato storage due to sprouting).

3.0 Methods and Materials

3.1 Small scale experiments

Experimental work was conducted using 120 litre plastic chambers (Fig. 1), inside controlled environment rooms at 6 and 9 °C (±0.5). Humidity was not controlled. Netted samples (total weight ~40 kg) were placed in the chambers and sealed. Ventilation was by vacuum pump drawing 2 air exchanges per day as 15 minute bursts spaced evenly through the day. Contaminated air was exhausted outdoors to prevent cross-contamination. Samples of cultivars Russet Burbank and Saturna shared each chamber

Figure 1: Small scale storage chamber, showing ventilation pipe.


Season 2010-11

Chambers were loaded, made up to 40.0 kg, sealed and all initial applications made on 16 December 2010. Untreated samples (not in chambers) were held in separate stores, at similar target temperatures. Four sprout suppressants were tested.

CIPC (GroStop Ready) was applied at a nominal application rate of 18 g tonne-1 directly to tubers on a roller table using a Microstat ULV spinning disc applicator (Horstine Farmery, Gainsborough, UK) at store loading. The electrostatic function of the applicator was not used. No further applications of CIPC were made during storage.

Essential oil applications were cold misted by airbrush and compressed air at 0.75 bar, through a port in the chamber, into a void beneath the tubers. Ventilation was discontinued for 24 hours following in-chamber treatments. See Appendix 7.2 for application details.

Spearmint oil (Biox-M) was applied initially at a rate of 90 ml tonne-1 , followed by applications at 30ml tonne-1 at 21 day intervals.

Clove oil (Biox-C) was applied on four occasions, the first two occasions at 48 ml tonne-1 and then the rate was increased to 84 ml tonne-1 . During the final application a fan was used to recirculate the applied mist through the crop from the application port, beneath the crop, to a similar port directly above.

Caraway (Talent) was applied at the proposed weekly application rate for Talent on ware potatoes (initially 30 ml/tonne, reducing by 5 ml every 2 weeks until 15 ml/t and then at that rate until unloading).

Assessments were carried out in April and July after 16 and 29 weeks storage respectively. On the first sampling occasion sprout growth (25 tubers) was assessed and then at unloading sprouting, processing quality (crisps 30 and chips 20 tubers), pathology and defects (25 tubers).

For crisping, 300 g of slices between 1.22 and 1.47 mm thick were taken from 30 mechanically peeled tubers and washed in water for 45 seconds. Each sample was then fried for 3 minutes in oil heated up to 177 °C at the start of frying. After frying the sample was weighed and then crisps with defects (a dark discolouration larger than a 5 mm diameter circle) removed and weighed. The remaining blemish free sample was then assessed objectively three times using a HunterLab D-9000 colour quality meter fitted with a D25-L optical sensor [Mountsorrel, Leics., UK].

Chips were processed as single 3/8th inch square longitudinal sections from each of 20 sound tubers and fried for 90 seconds in oil heated up to 190°C at the start of frying. The fry colour of individual strips was assessed subjectively by comparison with a USDA standard colour chart [Munsell Color, Baltimore, Maryland, USA] under standard artificial white light. The USDA assessment scale used for assessing chips (light to dark - 000, 00, 0, 1, 2, 3 & 4) was linearized 1 to 7 (SBCSR scale) and reported as a mean. Scores of 1 to 3 are good; scores of 4 and 5 acceptable and higher scores rejected.

Season 2011-12

In the second year of the study, the same treatments were applied as previously but with the addition of 3-decen-2-one (SmartBlock). All chambers were made up to 42.0 kg of tubers and from here onward the untreated samples were also stored in ventilated chambers. Chambers were loaded on 9th November 2011.

CIPC (Gro-Stop Ready) was applied similarly to last season but, from 2011-12 forward, by Mafex Mantis spinning disc applicator, over a conveyor belt. Spraying took place on 18th November 2011.


The additional treatment of 3-decen-2-one (SmartBlockTM) was applied at a rate of 115 ml tonne-1 when either cultivar showed a sprout incidence of approximately 75%. The application method was the same cold misting technique used for caraway, clove and spearmint oils. One SmartBlockTM application was made to the 6 °C crop on 27th January 2012 and four applications were made to the 9°C crop on 9th December 2011, 27th January 2012, 16th March 2012 and 14th May 2012.

Assessments were carried out in April and July after 14 and 29 weeks storage, respectively. On the first sampling occasion sprout growth (40 tubers) was assessed and then at unloading sprouting, processing quality (chips 20 and crisps 30 tubers), pathology and defects (40 tubers).

In year 1, the use of clove oil did not result in sprout control. This was discussed with approval holders in the USA (PIN/NIP and Pace). In year 2, modifications were made to improve ‘contact’ between mist and sprout tissue by recirculation and by increasing volatility (through raising temperature). However, after two applications, at 48 ml tonne-1 , control was still not evident so applications and assessment of this treatment was discontinued after the first sampling occasion. This treatment was not tested again in this study.

Season 2012-13

Chambers were loaded to a combined crop weight of 40.0 kg on 6th November 2012. This season’s trial was a repeat of the previous setup with the addition of an irradiation treatment of 15 kJ of short wave ultra violet light or UV-C (100 - 280 nm).

Custom made irradiation equipment was supplied by Cranfield University. Tubers designated for this treatment were removed from their chambers, placed rose end up, and exposed to the light for 11 minutes on 5 th December 2012.

CIPC was applied on 22nd October 2012. SmartBlockTM was applied twice to the 9°C crop on 31st January 2013 and 21st March 2013, respectively and once to the 6°C crop on 8th April 2013. Caraway and spearmint were applied regularly as before.

Assessments were carried out after 12 and 26 weeks storage. On the first sampling occasion sprout growth (40 tubers, as previously) was assessed and then at unloading sprouting (40 tubers), processing quality (chips 20 and crisps 30 tubers), pathology and defects (25 tubers).

Due to an extended cold period, moist air drawn out from the chambers in the 9 °C store condensed excessively and ran back into store accumulating in the u-bends formed by fastening the pipework. This inhibited air flow enough to damage tuber quality with anaerobic conditions resulting in dark fry colours and blackheart. These data were discarded and, consequently, it was decided to repeat the 9 °C experiment the following season (2013-14).

Season 2013-14

This experiment was intended to be a repeat of the 9 °C part of the previous season’s work. However, the UV-C irradiating equipment failed irreparably during normal operation and this treatment was not pursued further. CIPC was applied and chambers loaded on 11th October 2013. SmartBlockTM was applied three times on 17th January 2014, 17th March 2014 and 3rd April 2014, respectively. Assessments were carried out after 16 and 25 weeks storage. All other variables replicated the previous season.


3.2 Large scale (semi-commercial) experiments

Season 2011-12 Caraway oil (S-carvone)

Treatments and experimental design

Cultivar Saturna was stored in 16 one tonne pallet boxes in a single experimental store at 9.0°C. Netted sub-samples of cv Saturna and Russet Burbank were placed within the bulk crop in defined box locations (front/back, left/right, top/uppermiddle/lower-middle/bottom). Eight nets were buried within designated boxes for each cultivar and for each assessment occasion. The experimental design was an unreplicated comparison of treatments with variation measured by eight in-store replicates.

Store set-up and control

The 16 tonne capacity Controlled Environment Room (CER) was configured with air discharged above boxes (overhead throw) and recirculated back through pallet spaces beneath boxes. Boxes were stacked 4 high as a solid block of 4 columns. No chlorpropham (CIPC) sprout suppressant had ever been used in the store.


At the beginning of storage, crop temperatures were pulled-down at a rate of 0.5 °C per day until the holding temperature of 9 C was achieved (tolerance ± 0.5 °C). Humidification, by wet cell, was then enabled at 95% RH (tolerance ± 5%) on 16th November 2011.

Chemical application

Caraway oil (Talent, Makhteshim Agan - Benelux) was applied using a Dyna-Fog (Curtis Dyna-Fog, Dayton, Ohio, USA) cold fogging system, located in the store headspace. At the time of application refrigeration was turned off, and store air recirculated continuously for a period of 24 hours before returning to automatic control. Talent was applied at the proposed EU ware label rate every 7 days beginning on 18th November 2011 and ending on 27th April 2012 (final application). The first two applications were made at 30 g/tonne, the second two at 25 g/tonne and the third two at 20 g/tonne. All subsequent applications were at 15 g/tonne.

Assessments

The longest sprout length and number of sites of sprouting were measured on 25 tuber sub-samples at intake and after 12 and 24 weeks of storage (unloaded 14th February 2012 and 10th May 2012 respectively). Fry colour was measured as chips (French fries) for Russet Burbank and crisps for Saturna at intake and 24 weeks using the same frying methods as in the small scale experiments. Pathology and defects were scheduled for assessment at 24 weeks only.

Season 2012-13 Spearmint oil (R-carvone)Treatments and experimental design

Cultivar Russet Burbank was stored in 16 one tonne pallet boxes in a single experimental store at 9.0 °C. Netted sub-samples of cvs Russet Burbank and Saturna were placed within the bulk crop in defined box locations (front/back, left/right, top/upper-middle/lower-middle/bottom). Eight nets were buried for each cultivar and for each assessment occasion. The experimental design was an unreplicated comparison of treatments with variation measured by eight in-store replicates.

Store set up and control

A 16-tonne box capacity Controlled Environment Room (CER) was configured for positive ventilation, with alternate pallet slots blocked and air discharged from a plenum chamber. Boxes were stacked 4 high as a solid block of 4 columns. Air was recirculated through a conditioning duct for refrigeration, heating or humidification as necessary. No chlorpropham (CIPC) sprout suppressant had ever been used in the store.

After store loading, crop temperature was pulled-down at a rate of 0.5 °C per day until the holding temperature of 9.0 °C was achieved (tolerance ± 0.5 °C). Humidification by wet cell was then enabled at 95% RH (tolerance ± 5%) on 13th November 2012.

Chemical application

Spearmint oil (Biox-M, MAPP No. 16021) was applied using a Cedax EW 3000 Electrofog (Xeda International, Saint-Andiol, France) discharging directly into the store plenum chamber. At the time of application refrigeration was turned off, and store air recirculated constantly for a period of 24 hours before returning to automatic control. Biox-M was applied at the label recommended rates every 21 days, beginning on 15th November 2012 and ending on 11th April 2013 (final application). The first application was at 90 g/tonne with all subsequent applications at 30 g/tonne.


Assessments

The longest sprout length and number of sites of sprouting were measured on 25 tuber sub-samples at intake and after 14 and 26 weeks of storage (unloaded 29th February 2013 and 19th April 2012, respectively). Fry colour was measured as chips (French fries) for Russet Burbank and crisps for Saturna at intake and after 26 weeks using the same frying methods as in the small scale experiments. Pathology and defects were assessed at 26 weeks only.

4.0 Results

4.1 Small scale experiments

Sprouting

The mean longest sprout lengths per tuber (mm) are shown in Tables 3 and 4 for cultivars Saturna and Russet Burbank, respectively. Sprouting up to 10 mm might be acceptable for processing; lengths in excess of this would normally be rejected. Figures demonstrating excessive sprouting in the tables are shaded red.

First year (Season 2010-11)

At 6°C, sprout control in Saturna was effective using CIPC, caraway and spearmint. Sprout length was excessive on both sampling occasions using clove oil. With Saturna at 9°C, only treatment with CIPC maintained sprouting at an acceptable level at unloading.

Sprouting in Russet Burbank gave broadly similar results but with apparently lower sprouting pressure. At 6°C no sprout control was required to maintain acceptable sprouting at sampling occasion 1 (16 weeks). However, by unloading sprouting was only effectively controlled by CIPC, caraway and spearmint (although the latter was significantly less effective than the former two). Clove oil was never effective. At 9°C only CIPC and caraway were effective at controlling sprout growth. Even at the first sampling occasion sprout growth was excessive using spearmint and clove oils.

Second year (Season 2011-12)

During storage at 6°C, all treatments resulted in very effective control of sprouting in both cultivars. This was especially effective in Russet Burbank with mean longest sprout lengths less than 3mm at unloading (29 weeks). Sprouting in Saturna was also well controlled at 6°C across all treatments, with sprout lengths much less than the 10mm threshold


The potential for sprout growth was considerably greater during storage at 9°C, with untreated samples of both cultivars having excessive sprout growth at the first sampling occasion. In Saturna, sprouting was most effectively controlled using CIPC and caraway. There was little difference between the sprout control treatments for Russet Burbank with all treatments limiting sprouting to less than 3mm except spearmint at unloading (7.9 mm, but variation was high).


Third year (Season 2012-13) 6°C only

There was very low sprouting pressure at sampling occasion 1 (14 weeks) with low incidences (See Appendix 8.1, Table 22). However, by unloading (25 weeks) the mean longest sprout in untreated Saturna had grown to a commercially unacceptable 11.8 mm. All suppressant treatments were effective on Saturna except UV-C (11.2 mm at unloading). However, CIPC and caraway were especially effective with final values of 0.3 and 0.1 mm, respectively. The mean lengths of the longest sprout in Russet Burbank were all commercially acceptable at this storage temperature but CIPC was particularly effective (0.4 mm at unloading).

Fourth year (Season 2013-14) 9°C only

After 15 weeks (sampling occasion 1) sprouting in Saturna was only adequately controlled by CIPC, caraway and 3-decen-2-one. By unloading only CIPC was still commercially effective with a mean length of 2 mm.


At the first sampling occasion all Russet Burbank treatments adequately controlled sprouting, especially CIPC and 3-decen-2-one (0.6 and zero mm, respectively). As for Saturna, by unloading at 25 weeks, only CIPC effectively controlled sprouting in Russet Burbank with a mean longest sprout length of 1 mm.

Saturna crisp assessment

A weight of crisps with defects of <5% would be considered low and greater than 15% unacceptable. Where crisp fry colour is assessed by Hunter Lab, values greater than L 59 are considered commercially good, between L 59 and L 49 may be acceptable but less than L 49 would be rejected. The results in Table 5 are shaded green for commercially good and red for unacceptable.

The Saturna crisp processing quality assessment results are shown in Table 5. After storage at 6°C in season 2010-11, only CIPC resulted in low and acceptable levels of fry defects. Fry colours were all intermediate except for clove oil which was commercially unacceptable. At 9°C, CIPC and caraway resulted in high levels of fry defects but levels under clove and spearmint were acceptable. No fry colours were unacceptably poor but those of spearmint were maintained at a commercially good value.

In the season 2011-12, all fry defect levels were unacceptably high at both temperatures. Fry colours were intermediate where assessment was possible, except at 6°C under CIPC, which fried unacceptably dark. Only 6°C results are displayed for the 2012-13 season and these all had unacceptably high defects. It was only possible to assess the colour of the UV-C treatment and this was poor. During the season 2013-14, with storage at 9°C only, all treatments generally had low levels of defects. Those of untreated and CIPC were nil. Most defects were found under caraway with a borderline value of 5.8%. All fry colours were light and commercially good.


Russet Burbank chip (French fry) assessment

The USDA assessment scale used for assessing chips (light to dark - 000, 00, 0, 1, 2, 3 & 4) was linearised to a 1 to 7 scale (SBCSR score) as shown below (Table 6), and reported as a mean shown in Table 7. A score up to 3.9 is considered good; 4 to 5.9 borderline, whereas higher scores would be rejected.

Across all seasons at 6°C there were no significant differences in the fry colour of Russet Burbank chips due to suppressant treatment with all achieving an acceptable, intermediate colour, except for a commercially good chip score of 3.8 in the untreated for season 2012-13.

For the crops stored at 9 °C fry colour was generally lighter and commercially good but again there were no significant differences due to treatment. However, certain treatments did fall into the intermediate quality category. These were CIPC, spearmint and untreated in the second season 2011-12.


Pathology and defectsSeason 2010-11

An unavoidably small sample size (25 tubers) led to wide variation in the data. Silver scurf (Helminthosporium solani) severity (surface area affected) had particularly wide variation and incidences were generally high. However, the caraway treated Russet Burbank at 6 °C was notable for low severity and incidence. Black dot (Colletotrichum coccodes) severity and incidence ranged more consistently higher than silver scurf but it was not possible to identify a treatment effect. There were 4 sporadic incidences of dry rot (Fusarium spp rot) in Saturna and no incidences of any other store developing disease.

Lenticel discolouration was present on both cultivars at variable but generally high incidences, no trends emerged. Internal rust spot was found in Russet Burbank only but again no treatment trends were found. Vascular discolouration was found in four tubers, all in spearmint treated Saturna. Internal sprouting was frequently found, occurring in all Saturna treatments at 9°C ranging between 2 tubers with CIPC and 11 tubers with caraway. For the same variety at 6°C there were 2 tubers in both spearmint oil and clove oil treatments only. Russet Burbank seemed less prone to the defect with 2 incidences found in both caraway and clove oil treatments at 9°C only.

Skin damage resembling chemical scorch was seen at low levels under most treatment regimes on Saturna, especially 6 °C clove oil (7 incidences) and CIPC (5 incidences). None was found in 9°C untreated. However, Russet Burbank only showed damage on the 6°C CIPC treatment with an incidence of 40% (10 tubers). There were 3 incidences of glassiness in Russet Burbank each in different treatments including the untreated. No other defects were detected.


Season 2011-12

Small sample sizes (40 tubers) led to wide variation in the data. Silver scurf generally had wide ranging severity and high incidences. Little can be inferred from the data except that both incidence and severity tended to be higher on Russet Burbank. However, silver scurf severity was noticeably lowest after caraway treatment at both temperatures.

Black dot incidences and severity were consistently high in the whole trial and no treatments stood out from the rest. Skin spot (Polyscytalum pustulans) was found on Saturna as single incidences of very low severity for most treatments including the control. However, on Russet Burbank only 9°C CIPC had pustules, at 2 incidences (5%). There was a single incidence of dry rot in untreated Saturna and 4 of gangrene in Russet Burbank, 2 of which were untreated.

Slight lenticel discolouration was present on both cultivars at variable but generally high incidences, no trends emerged. Slight internal rust spotting was found at low incidences throughout the trial. Slight vascular discoloration occurred sporadically at high incidences, the greatest of which was in untreated Saturna at 13 incidences (34.2%). Internal sprouting occurred in untreated tubers of both cultivars. However, elevated incidences were seen in 9 °C Saturna treated with caraway, spearmint oil and 3-decen-2-one at 13, 28 and 30%, respectively

Skin damage as chemical scorch like symptoms was seen sporadically at low incidence and severity under all chemical treatments, including untreated, and on both cultivars. However the greatest incidences were found on CIPC treated Russet Burbank at both 6 °C (20%) and at 9 °C (8%). There were no severe defect symptoms and no other defects were noticed.


Season 2012-13

A small sample size of 25 tubers gave wide variation in the data. At 6 °C, silver scurf severity was generally low on Saturna and higher on Russet Burbank but further discrimination between sprout suppressant treatments was not possible. Incidence was very high on Russet Burbank but relatively low on Saturna except for untreated (68%) and SmartBlockTM (60%). Black dot was present at high incidences and generally high severities with broad variation enough to preclude further analysis. Also in the trial there was a single incidence of both skin spot and dry rot, 2 incidences of soft rot. No other storage diseases were detected

Swollen lenticels were found on 4 CIPC treated Russet Burbank tubers only. Skin damage resembling chemical scorch was found at low severity but high incidences on CIPC treated Saturna (72%) and Russet Burbank (40%). A few incidences also occurred with caraway and spearmint treatment. No other defects were detected.

Season 2013-14

Silver scurf severity, by surface area affected, was too variable to adequately compare sprout suppressant treatments due to small sample size (25). Incidences were generally high (over 50%) on Russet Burbank with all sprout suppressant treatments and Saturna treated with spearmint (68%) and SmartBlockTM (56%). Variation in black dot severity was too broad to distinguish between sprout suppressants but incidences were all high. There was a single incidence of skin spot in CIPC treated Russet Burbank. No other storage diseases were detected.

Skin damage as chemical scorch like symptoms were found in both CIPC treated cultivars, especially Russet Burbank with a 100% incidence. Internal sprouting occurred only in Saturna, 3 times with spearmint and once with caraway. No other defects were detected.


4.2 Large scale (semi-commercial) experiments

Season 2011-12

No sprouting was detected on either crop at intake. At 12 weeks, 99% of Russet Burbank was sprouting and then completely by 24 weeks. All Saturna were sprouting by week 12 (Table 12).

The mean length of the longest sprout (Table 13) for Russet Burbank at 12 weeks was 1.4 mm increasing to 32.8 by week 24. There were no differences in length due to placement within store except that, on the second sampling occasion, it was inhibited at the top position to a commercially tolerable mean of 5.3 mm. Saturna had a grand mean sprout length of 11.4 mm at 12 weeks and although variation was high there was a trend for increasing sprout suppression with higher placement in the store with only the upper and upper middle positions commercially acceptable.


At 24 weeks all placements were unacceptable. The mean number of sprouting sites (Table 14) in both cultivars was always greater on the second sampling occasion but sample placement did not appear to have an influence.

Fry quality at intake for both varieties was excellent. Russet Burbank chips scored 1.7 (1 light – 7 dark) and Saturna crisps had a very light mean Hunter L value of 61.1 with minimal defects, 1.6% by weight. By 24 weeks chip quality had deteriorated but was still acceptable with an SBCSR score of 3.4 (equivalent to USDA class 0 or 1). Crisp colour was intermediate but fry defect levels were very high (30.6%).


Due to the poor visual appearance of the extensively sprouting crop and high levels of silver scurf and black dot the pathology and defect assessment was considered irrelevant except for internal sprouting which was assessed. The incidence in Russet Burbank was low at 1% and high in Saturna at 12%.

Season 2012-13

At intake no sprouting was detected in either cultivar. Sprouting incidence in Russet Burbank averaged 56% at 14 weeks and sample placement made little difference except that the boxes against the plenum chamber (back) at 48% had a lower incidence of sprouting than the front stack at 63% (Table 17). By the second assessment occasion incidence was up to 99.5%. In Saturna mean sprouting incidence in week 14 at 92.5% was much higher than Russet Burbank.

The mean length of longest sprout at week 14 was acceptable at 2.2 mm for Russet Burbank and 3.8 for Saturna (Table 18). However, by 26 weeks both sprout lengths of 62.2 mm and 37.9 mm respectively were unacceptable. No differences were noticed due to sample placement.


The numbers of sites of sprouting increased consistently from 14 to 26 weeks in both cultivars but no trends were noticed due to placement.

Fry quality at intake was fair for both Russet Burbank chips and Saturna crisps which had few fry defects. After 26 weeks of storage the chips had deteriorated slightly but remained acceptable. Crisp quality was poor with an intermediate colour (L value 55.0) and a high level of fry defects (22%).


In Russet Burbank silver scurf incidence was very high and severe with a mean of 29.3% of the surface area of a tuber covered. Black dot incidence was also high but severity lower at 13.5%. The level of soft rotting was 2%. Skin damage does not refer to mechanical abrasion but any blemishing that might be induced by chemical contact. An incidence of 5% was found but the severity was low. Swollen lenticels were noticed during assessment and additionally recorded at 4%. There was no internal sprouting.

Saturna had comparatively much less silver scurf incidence at 36.5% and very low severity at less than 1%. However, black dot incidence was very high and of similar severity to Russet Burbank at 14.5%. No soft rotting was found. Skin damage incidence was low and of very low severity. Internal sprouting was found in this cultivar but at a low incidence of 1%. Swollen lenticels were more frequent than in Russet Burbank at 9.5%. No skin spot, dry rot, gangrene or blackheart was detected in either cultivar.

5.0 Discussion

In this study, the use of relatively cool (6°C) and warm (9°C) storage conditions, in combination with a short-dormant, vigorous growing cultivar (Saturna) and a longdormant, less vigorous cultivar (Russet Burbank), provided a range of sprouting pressures for the assessment of sprout suppressants. This information was then used to scale up experimental work to a more commercially relevant scale. Under conditions where sprouting pressure was most limited, sprouting was successfully controlled by most suppressant treatments, including CIPC, caraway, spearmint and 3-decen-2-one.

Data from the four seasons of small-scale trials indicate that sprout control was generally most effective using CIPC (in these trials applied hydraulically at store loading). It provided commercially acceptable sprout control for both cultivars at both temperatures in all four seasons. Caraway also gave relatively effective sprout control and performed the best of all the alternatives especially at 6°C. However, although it performed as well as CIPC, and frequently had shorter sprout lengths, control occasionally broke down and fell below the industry standard at 9°C by 6 months. Spearmint performed adequately on both 6 °C crops, inconsistently on Russet Burbank at 9 °C and poorly on Saturna at the warmer temperature.

Where 3-decen-2-one was used it performed well on both cultivars at 6 °C to full term, but only consistently up to the first sampling occasion at 9 °C. However, this chemical was applied as determined by a sprouting incidence of 75% (not length) and the final assessment may have fallen out of step with the application programme. In the 2011-12 season, 3-decen-2-one was used for the first time in the small-scale trial. With this compound, a difference in the number of applications due to storage temperature was notable.


Clove oil did not work when applied in the same way as the above alternatives, perhaps due to lower volatility, but cannot be discounted without testing a more appropriate method of delivery. Clove oil is registered and in commercial use in the USA. The UV-C irradiation treatment did not differ from the untreated control where tested.

On the basis of small-scale experimental work, caraway oil and spearmint oil treatments were selected for use in the large-scale (16 tonnes) studies in 2011-12 and 2012-13, respectively. Caraway oil was used with the store ventilation system in a non-positive (overhead throw) arrangement while for spearmint oil positive ventilation was used.

Cold misted caraway oil delivered in an overhead throw ventilation system resulted in overall poor sprout control. Control was initially good in Russet Burbank (12 weeks) but thereafter there was only acceptable efficacy evident in top boxes. With cv Saturna acceptable efficacy was only evident in top boxes at 12 weeks. Poor efficacy elsewhere in the store indicates limited transport of the sprout suppressant to the crop. With the overhead-throw ventilation principle, air (and any treatments applied using air) is not delivered to the crop, but through the pallet apertures of boxes, with the delivery to crop being effected by convection currents. This limits the effectiveness of such stores for drying and chemical applications (Potato Council Store Managers’ Guide, 2008). Results from the first large scale trial suggest that simply using a sprout suppressant with high volatility will not sufficiently overcome the limitations of overhead-throw store design. The likely availability of caraway oil (Talent) for commercial use on ware potatoes in the United Kingdom is currently unclear.

Large scale storage using spearmint oil delivered using positive ventilation at 9°C did not control sprouting well. Although sprout control of both cultivars was initially effective (14 weeks) it fell below commercially acceptable standards between 14 and 26 weeks. Sprouting was consistent between placements suggesting that distribution was even, but at this temperature, for these varieties, using the standard dose rate and application timings, control was not adequate in this store.

Processing quality in both large scale experiments were generally acceptable except for an excessive% weight of defective crisps in Saturna.

Processing quality in Saturna crisps after full term storage were generally spoilt by high levels of frying defects except in year 4 (season 2013-14, 9 °C). The only treatment to stand out from the rest was spearmint in year 1 at 9 °C with few defects and lighter fry colour, although, unusually, no defects were seen in year 4 with CIPC or untreated tubers (both having light fry colours). Russet Burbank chips had generally consistent fry colours between suppressant treatments, except in year 2 at 9 °C where caraway and 3-decen-2-one had better fry colour than the other chips

The high humidity in the small scale chambers were likely to have promoted storage diseases. Incidences of silver scurf and black dot in particular were generally very high throughout, as might be expected at warm storage temperatures. Despite this, no consistent effect due to sprout suppressants emerged, though a low severity of silver scurf was seen on a number of occasions using caraway oil.

Internal sprouting occurred sporadically in most treatments, including untreated crop, however 9 °C Saturna consistently had a high incidence, 6 °C Saturna and 9 °C Russet Burbank showed some, whereas 6 °C Russet Burbank, with the lowest sprouting pressure had none. CIPC generally showed much less or none of the defect, but it was observed following caraway oil, spearmint oil, clove oil and occasionally 3-decen-2-one.

Phytotoxicologial skin damage resembling a chemical scorch was noticed in the study as a sporadic ‘background noise’, such that it even occurred in untreated crops, probably confounded by a generally poor skin finish. However, CIPC was much more likely to be associated with the defect than the other suppressants and in 2013-14 the 9°C Russet Burbank had an incidence of 100% compared with nil in all other treatments. This is not usually a problem where CIPC is fogged but the hydraulically sprayed liquid formulation used in this study wets the tuber skin and delivers the dose directly. However, this might not be a problem in processing crops where skin is peeled and skin appearance is less commercially relevant. In certain circumstances the accurate distribution of CIPC throughout the stored crop might outweigh the risks of partially scorched skin.


6.0 Conclusions

A range of alternative sprout suppressants were assessed, under standardised conditions, in comparison with CIPC applied as a liquid, directly to tubers. Varieties (cvs Russet Burbank and Saturna) and storage temperatures (6°C and 9°C) were selected to give a wide range of sprouting pressure. CIPC, caraway oil and spearmint oil resulted in effective sprout control at 6°C, consistently over three years of study. But at a 9°C storage temperature, successful sprout control was influenced by cultivar, season and storage duration. CIPC was the only active substance to give consistently acceptable levels of sprouting in both cultivars, in all three seasons and for the full storage term.

Assessment at 9°C of caraway oil on a semi-commercial scale (16 tonnes) using non-positive overhead throw ventilation, and spearmint oil (using positive ventilation) indicate neither of these sprout suppressants were sufficiently effective under the conditions tested.

Although caraway oil and spearmint oil were the most promising alternative sprout suppressants from 3 years’ small-scale work, results suggest their use in commercial box stores (typically overhead-throw type stores in GB), or even in box stores which have been modified to improve ventilation efficiency, would not result in sufficiently effective sprout control. In the most demanding storage situations (e.g. late season processing varieties with short dormancy requiring a relatively warm storage temperature for long durations), such treatments are only likely to be of supplementary use, i.e. replacing some CIPC treatments, not replacing all CIPC treatments.


7.0 Appendicies

7.1 Additional sprouting data



7.2 Records of application to small scale chambers

2010-11


2011-12


2012-13


2013-14


7.3 Product labels

Biox-M label


Talent label from the Netherlands


Biox C label from USA


Documents

Microsoft - Final Report: Evaluation of alternative …€¦ · Web viewAcross all seasons at 6 C there were no significant differences in the fry colour of Russet Burbank chips due