Abstract
A field trial was conducted in 2012–2013 at Pukekohe to determine the efficacy of wettable sulphur, applied as a foliar treatment within a reduced insecticide programme, to control tomato-potato psyllid (TPP). Three of the four treatments included wettable sulphur (Kumulus® DF; at 6 kg/ha), either alone or in combination with other insecticides. The remaining treatment was a standard 7 day insecticide spray programme. Spraying for all treatments began at 60% plant emergence on 29 November 2012, and continued until 14 March 2013. The study demonstrated that although sulphur applications may reduce TPP nymph numbers on potato foliage, especially when used in combination with insecticides, sulphur alone is not an adequate TPP management strategy. The field trial results for sulphur-only treatments support previous work suggesting that although sulphur may slow the build-up of TPP populations by deterring egg-laying, the lack of repellence or anti-feeding properties still results in the transmission of Candidatus Liberibacter solanacearum by TPP. Given the ZC results, it is difficult to draw conclusions from this trial about the efficacy of sulphur/insecticide combinations for TPP management.
Introduction
Tomato-potato psyllid (TPP), Bactericera cockerelli(Sulc) (Hemiptera: Triozidae), is a vector for Candidatus Liberibacter solanacearum (Lso), a phloem-limited bacterium that causes severe production problems within the New Zealand potato industry (Anderson et al. Citation2013). Lso interferes with the potato plant’s transportation of sugars into tubers, resulting in mottled browning discoloration (‘zebra chip’ [ZC]) of cooked chips or crisps, rendering affected potatoes unmarketable (Munyaneza et al. Citation2007; Butler & Trumble Citation2012). Potato tuber yield and quality can be severely reduced by the ZC disorder, resulting sometimes in non-harvest of entire fields (Munyaneza et al. Citation2007). Feeding TPP also cause ‘psyllid yellows’ (PY), a physiological disorder (thought to be caused by a toxin released by TPP during feeding) that affects the entire potato plant and causes a reduction in plant growth, chlorosis or reddening/purpling of leaves, basal cupping of leaves, shortened and thickened internodes, aerial tubers, premature senescence and plant death (Teulon et al. Citation2009; Butler & Trumble Citation2012). Tuber yield is greatly reduced by PY, and affected potato tubers are typically small and misshapen, with rough skins (Butler & Trumble Citation2012). Above-ground symptoms of ZC are similar to PY, although with ZC the vascular systems of infected plants are brownish, and severely affected foliage appears scorched; tubers with ZC display browning of the vascular ring and necrotic flecking of internal tissues (Munyaneza et al. Citation2007).
The TPP has been associated with potatoes and other solanaceous crops in parts of North and Central America for many years, and was first found in New Zealand in an Auckland tomato crop in 2006 (Liefting et al. Citation2008; Anderson et al. Citation2013). Since then TPP has become widespread in New Zealand, occurring in all potato growing regions of the country (Teulon et al. Citation2009). TPP reproduces on a range of plant species, mainly from the Solanaceae family (Butler & Trumble Citation2012). In New Zealand, TPP is known to reproduce on five commercial crop species from the Solanaceae family (eggplant, capsicum, tomatoes and potatoes), Convolvulaceae (sweet potato) families, and a number of wild Solanum species (including the native species, poroporo [Solanum aviculare]) (Martin Citation2008). TPP-induced diseases (PY and ZC) have caused losses of between 20% and 50% in some potato crops in the USA (Cranshaw Citation2007; Wen et al. Citation2009). In New Zealand, the estimated cost of TPP to the potato industry during the 2010–11 season was NZ$28 million (Kale Citation2011); the increased costs were largely a result of additional insecticide applications and reduced tuber yields and quality (Walker et al. Citation2013b).
Since TPP was first detected in New Zealand in 2006, potato growers have been forced to rely heavily on chemical insecticides for crop protection. However, there is general recognition of the need for more sustainable control methods that can effectively reduce disease incidence in potato crops. Walker et al. (Citation2012) reported that insecticides may not be required for early-crop potatoes in northern New Zealand (Pukekohe) because flights and numbers of TPP entering these crops are usually below economic injury thresholds until late December. However, in many northern locations in New Zealand, weekly insecticide spray programmes may be required after this date, as TPP flights and numbers rapidly increase to levels that can cause concern (Walker et al. Citation2011). The current commercial insecticide programme for TPP control in northern New Zealand typically involves the application of ‘blocks’ of two to four applications of different mode of action (MoA) insecticides every 7 to 10 days from about mid December onwards. In general, foliar applications of abamectin (Avid®), spirotetramat (Movento®), spinetoram (Sparta®) and pyrethroid and organo-phosphate insecticides are used. The sequence of insecticides aims to maximise the impact of natural insect TPP predators by using more selective (non-lethal to natural enemies) products early in the season, and the rotation of different MoA insecticides is part of an insecticide resistance management strategy (Walker & Berry Citation2009). TPP pressure is lower in southern New Zealand and adequate control can often be achieved with fewer insecticide applications than described above (Robin Oakley, pers. comm.).
Lso-infective (Lso positive or ‘hot’) TPP adults can transmit Lso through feeding (Buchman et al. Citation2011), consequently there is increasing interest in products that have the potential to repel TPP and/or reduce feeding behaviour. Products with good TPP repellency, low mammalian risk and low environmental danger are of particular interest (Berry et al. Citation2009). A number of botanical oils and clays have been investigated (Butler et al. Citation2011; Wright et al. Citation2013a,Citationb). Sulphur is another product that may have value for TPP management. Sulphur has been used as a pesticide for many years, and was registered for pesticidal use in the United States in the 1920s, although formulations have varied considerably (DeOng Citation1924; Anon. Citation1991). Lime-sulphur (calcium polysulphide), a mixture that is considerably more toxic to mammals than sulphur (Anon. Citation2010), was used in the 1930s and 1940s to control TPP. It was effective in killing both immature and adult stages of TPP, as well as repelling adult TPP from the crop, but was phytotoxic (Butler & Trumble Citation2012). Wettable sulphur (Kumulus DF; 800 g/kg sulphur) is currently registered in New Zealand for use as an insecticide against erineum mite (Colomerus vitis) on grapes, and as a fungicide on a wide range of crops. Wright et al. (2Citation013c) reported that weekly applications of wettable sulphur significantly reduced TPP nymph numbers in potato foliage and had no phytotoxic effect on plants. In a series of laboratory tests where TPP were given a choice of plants with or without wettable sulphur on the foliage, Gardner-Gee (Citation2013) found that TPP laid fewer eggs on plants sprayed with sulphur compared with control plants. These studies suggested that wettable sulphur warrants further investigation. This paper reports on the results of a field trial to determine the efficacy of wettable sulphur, applied as a foliar treatment within an insecticide programme, for the control of TPP and the subsequent impacts on potato yield, tuber dry matter content and incidence of ZC.
Materials and methods
Crop management
The trial was conducted in spring/summer 2012–2013 at The New Zealand Institute for Plant & Food Research Pukekohe Research Centre (37°12′S, 174°51′E). The soil type was described as a Patumahoe clay loam with a pH of 6.6. Potato seed tubers ‘Moonlight’ were planted on 1 November 2012 at an inter-row spacing of 300 mm and to a depth of c. 250 mm. Seed potatoes were between 50–100 g in size and had been certified to New Zealand Seed Potato Certification Authority standards as second generation pathogen tested (potatoesnz.co.nz). Soil fertility was amended with an application of ‘Potato Mix’ (N:P:K = 6:6:6) at 2.5 t/ha at planting. For weed control, metribuzin (0.5 L/ha) and linuron (2 L/ha) were applied prior to crop emergence, and for late blight control, mancozeb was applied at recommended rates at 7–10 day intervals throughout the growing season. The plants were irrigated on two occasions (22 February and 2 March 2013).
Treatments and trial layout
The trial was laid out in a randomised Latin square design with four treatments of four replications each. Plots were 10 m long and 3 m wide, comprising four rows of potatoes spaced 750 mm apart. The two inner rows were the datum rows, and the two outer rows of each plot were ‘guards’. Plots within each bed were separated by non-planted buffer regions, 1.5 m long.
Starting from 60% plant emergence on 29 November 2012, the foliar insecticide treatments () were applied at 7–9 day intervals with a tractor-mounted plot sprayer using hollow-cone nozzles at a water rate of 500 L/ha at a pressure of 8 bar (120 psi). Treatments 1 to 3 included the application of wettable sulphur (Kumulus® DF; at 6 kg/ha), either alone or in combination with other insecticides (). Treatment 1 (‘sulphur only’) comprised of sulphur (plus the wetting adjuvant Contact Xcel at 100 mL/ha) throughout, except for the last two applications on 7 and 14 March 2013, when methamidiphos was added to the tank mix to control potato tuber moth (Phthorimaea operculella [Zeller]). Treatment 4 was a ‘standard commercial’ TPP insecticide programme (Walker et al. Citation2013b). Treatments 2 and 3 were similar treatments, with blocks of insecticides + sulphur alternating with sulphur (no insecticide), the treatments differing only in insecticide application dates.
Table 1 Insecticide treatments for control of tomato-potato psyllid (TPP) in a main crop potato trial at Pukekohe Research Station, 2012–2013 season.
TPP assessments
TPP nymphs (all stages) present on 25 fully expanded compound leaves, selected at random from the middle of plant stems, following recommendations in Walker et al. (2Citation013a), in each plot’s datum rows, were assessed at 7 day intervals after TPP nymphs were first seen in the trial vicinity (20 December 2012).
Tuber yields and specific gravity
Senescing plant tops were sprayed with the desiccant diquat (Reglone®) on 20 March 2013 and the trial was machine harvested on 26 March. The tubers from a 5 m zone in the centre of the middle two rows in each plot were weighed and graded as being ‘marketable’ (>60 g) or ‘reject’ (<60 g). On 27 March, the specific gravity of harvested marketable tubers was determined by calculating the difference in weight of approximately 8 kg of randomly selected tubers in air and in water. Slices (1.5 mm thick) from 50 randomly selected marketable tubers per plot were deep-fried in canola oil at 185 °C for 3 min. The ZC symptom severity of individual crisps was determined using a 0−9 colour score (degree of browning) (Anderson et al. Citation2013). An equivalent number of randomly selected reject tubers from each plot were also fried and similarly scored for ZC.
Statistical analyses
Data were analysed with analysis of variance (ANOVA), using Genstat (version 15, 2013, VSN International Ltd, UK), with rows and columns of the field layout as blocking factors, and the treatment as the treatment factor. The TPP count data was log transformed (replacing zeros with 0.5 s) and analysed using a repeated measures ANOVA on the four treatments.
Results
TPP nymphs on the foliage
TPP nymphs were first seen in the control plots on 20 December 2012 but were not observed in the trial until 22 January 2013 (). The ‘standard commercial’ spray programme (Treatment 4) received 16 conventional (excluding sulphur) insecticide applications. Treatments 2 and 3 each received eight conventional insecticide applications and 16 sulphur applications. Treatment 1 (‘sulphur only’) received 16 sulphur applications and two methamidophos applications at the end of the growing season for potato tuber moth control. In the insecticide-treated plots (Treatments 2–4) TPP nymphs on potato foliage were almost completely controlled throughout the growing season (0–0.12 nymphs per leaf). In contrast, TPP nymph numbers steadily increased in Treatment 1 (‘sulphur only’), reaching 6.5 nymphs per leaf on 13 March. The numbers of nymphs observed in Treatment 1 were significantly greater (P < 0.05) than those in Treatments 2, 3 and 4 after 22 January.
Table 2 Effect of spray treatments on tomato-potato psyllid (TPP) nymph numbers in potato foliage (mean number of nymphs per middle plant height compound leaf). Within an assessment date, means with the same letter are not significantly (P < 0.05) different.
Tuber yields and specific gravity
Marketable tuber yields in the control plots were significantly (P < 0.05) lower than in the ‘sulphur only’, treatment (Treatment 1), which was, in turn, significantly (P < 0.05) lower than the three insecticide treatments (which were not significantly [P > 0.05] different compared with each other) (). There was a near-significant (P = 0.080) difference in tuber specific gravity, with the sulphur-only treatment having a significantly (P < 0.05) lower specific gravity than the standard insecticide programme.
Table 3 Summary of the number of foliar-applied conventional insecticide applications, average marketable and reject tuber yields (kg/plot), and tuber specific gravity of harvested potatoes following different spray regimes in a main crop spring/summer trial at Pukekohe during 2012–2013. See for details of each treatment.
Zebra chip
ZC incidence and mean ZC scores were lowest in the ‘standard insecticide programme’ (Treatment 4) for both marketable and reject tubers, but the results were not significant (P > 0.05) compared with the other treatments (). There were no significant differences (P > 0.05) between all treatments in the levels of severe ZC (score > 4) in the marketable tubers and, although there was less than half the number of reject tubers with severe ZC (score > 4) than in the other treatments, the result was not statistically significant (P > 0.05).
Table 4 Summary of number of foliar-applied conventional insecticide applications, and zebra chip (ZC) incidence (%), ZC score (scale 0–9) and ZC severity (score > 4) of harvested potatoes following different spray regimes in a main crop spring/summer trial at Pukekohe during 2012–2013. See for details of each treatment.
Conclusion
The results from this study demonstrated that sulphur applications can reduce TPP nymph numbers on potato foliage, especially when used in combination with insecticides. Compared with the weekly insecticide spray programme, the two sulphur/insecticide treatments used in this study (two consecutive 7 day applications of sulphur sprays followed by two consecutive 7 day applications of sulphur plus insecticide) reduced the number of insecticide applications from 16 to 8, while providing equivalent control of TPP nymphs. Weekly application of sulphur (no insecticides) produced lower yields than the treatments where insecticides were included. The combined sulphur/insecticide treatments achieved marketable yields equivalent to the weekly insecticide programme, but did not have tuber specific gravity levels that were above the 1.075 level required by potato processors for cultivar ‘Moonlight’ tubers. The weekly insecticide spray programme was the only treatment that gave specific gravity results that could be regarded as acceptable to the process potato industry.
The incidence of severe ZC was also high (>20%) across all treatments, including the standard weekly programme. The reasons for the high ZC rates in the standard weekly programme are not clear. In previous trials with similar design, the standard weekly programme has performed better, resulting in lower severe ZC rates (11% or less), compared with 24% in this trial (Wright et al. Citation2013c). However, in a previous trial (Wright et al. Citation2013c), treatments with alternating applications of sulphur and insecticide (sulphur alone for 1–3 weeks, followed by insecticide alone for 1 week) failed to consistently achieve ZC levels as low as those seen in the weekly insecticide treatment, suggesting that it is difficult to manage TPP with sulphur/insecticide treatments (Wright et al. Citation2013c). Furthermore, both trials have generated cautionary observations about the practicality of sulphur/insecticide treatments. It is generally recommended to avoid applying sulphur to plants within 3 weeks of applying oils (Young Citation2013), which limits the range of insecticides that can be used with sulphur, as many insecticides are applied with oils (e.g. Movento OD utilises an oil dispersion formulation, and an oil adjuvant is recommended for Avid). Some growers are currently utilising oils that have TPP repellency (e.g. Organic JMS stylet oil) (Dohmen-Vereijssen, unpubl. data) as part of their spray programmes, and sulphur may be not be compatible with these oil products. As a result, sulphur may not be a practical option when multiple products are applied to the potato crop regularly during the growing season.
This study also provides further evidence that sulphur alone is not an adequate TPP management strategy. Applying sulphur only (with just two methamidophos applications at the end of the season) did not achieve effective control of TPP nymphs. Nymph numbers in the ‘sulphur only’ treatment in this study were higher than in the other three treatments, and this treatment had lower tuber yields and tuber specific gravity levels that were well below the 1.074 level preferred for process potatoes. In both this study and the previous study of wettable sulphur (Wright et al. Citation2013c), the sulphur-only treatment reduced nymph numbers, but the incidence of severe ZC did not differ between treatments.
The results presented here for sulphur-only treatments are in agreement with Gardner-Gee (Citation2013) who concluded (on the basis of laboratory and glasshouse studies) that, although sulphur may slow the build-up of TPP populations by deterring egg-laying, the lack of repellence or anti-feeding properties means that sulphur treatments alone may not be sufficient to prevent commercially detrimental levels of Lso transmission by TPP.
Acknowledgements
Funding for this project was provided by the Ministry of Business, Innovation and Employment (previously Ministry of Science and Innovation). Contract number no. C06X0811 and Plant & Food Research Core funding. The authors thank John Anderson for his support and Moe Jeram for field assistance.
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