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EMRA/AHDB Soft Fruit Day 2018

Timing is everything, as several speakers emphasised, and the timing of this year’s EMRA/AHDB Soft Fruit Day, at the end of a gruelling year for growers, proved to be popular as the attendance of 200 was the largest ever. The main themes were water and nutrient management and updates on pest and disease control. Spotted wing drosophila (SWD) required five speakers, showing the breadth of ongoing work to understand this pest and ultimately provide sustainable control. Topics covered included the difference the colour of growbags makes to coir temperatures, whether growing garlic alongside a strawberry crop really does work, and how often the push/pull model is being used in pest control research.

There was a high proportion of younger faces in the audience and amongst those presenting their research work. NIAB EMR actively encourages young scientists and for several years posters showcasing their work have been a feature of this event and a competition for the best posters has been running for a few years. This year PhD student Rory Jones won with his poster on developing attractive yeast strains for the attraction and control of SWD.

Water and nutrient management

Georgina Key of AHDB Horticulture kicked off the day with a summary of a cross sector initiative that has attracted EU-funding on the transfer of innovative techniques for sustainable water-use in fertigated crops. Concerns about water scarcity and quality across Europe are set to grow.

One of the outcomes of the work is ‘The Fertigation Bible’ which is free to download at www.fertinnowa.com/the-fertigation-bible.This helps growers by giving an overview of the technology available, costings, advantages and disadvantages, whether solutions have been commercialised, supporting systems and legislation. Georgina outlined the sections that will be of most interest to soft-fruit growers, including the provision of water, optimising water quality, fertigation equipment and management, reducing costs and environmental impact.

Dr Mark Else of NIAB EMR reported on progress at the Water Efficient Technologies (WET) Centre. The aim of the project is to create a UK centre of excellence bringing together leading irrigation equipment and sensor suppliers with researchers at NIAB EMR. This is leading to the development and commercialisation of an integrated portfolio of leading-edge irrigation technologies for horticulture. As Mark explained, using a demonstration area on a commercial scaleat NIAB EMR, his team has shown how applying these technologies can enable growers to improve their water-use efficiency, yields, berry quality and financial returns.

The WET Centre can provide growers with crop-specific workshops, training and one-to-one technical support to enable them to successfully adopt these technologies. With eight commercial-scale polytunnels covering 0.34ha divided into a ‘commercial’ area and an ‘advanced’ area, using Precision Irrigation involving high performance sensors, data-loggers and automated irrigation to ensure optimal coir moisture availability, the research is investigating improved coir water availability. Precision fertiliser applications are also being tested and there is a comparison of drippers. Rainwater-harvesting and water re-use is under investigation together with automated polytunnels and environmental control.

Results from the 2018 commercial area, using a 60-day crop of Malling™ Centenary, showed some interesting effects of bag colour and position in the row. Class 1 yields from white coir bags were significantly higher than from black bags: 393g/plant from black bags and 433g/plant from white bags. There was a highly significant 10% difference in fruit yields on three picking dates in June which was not due to a positional effect. Coir temperatures in white bags were 4C lower in the late afternoon during hot weather. Night temperatures were similar, but the rate of warming at sunrise was greater in black bags.

Comparing the ‘leg row’ with the middle and centre rows showed some differences. The ‘leg row’ was slower to develop, so there was a highly significant effect of row position on the rate of flower and crop development and a two-fold difference in first-pick Class 1 yield between the leg and middle and centre rows. However, there were no significant effects on total Class 1 yield or Brix values. Due to the slower development of the leg row, there was a 50% difference at the start of picking, which does have implications for harvest managers. Crop positioned in the centre of tunnels also shows limitations due to light interception. Work looking at the light intercepted by plants shows that it is likely to be limited in the centre of commercial tunnels and that this is exacerbated by ageing plastic.

Managing botrytis and powdery mildew in strawberries  

Changes in strawberry production methods have led to powdery mildew becoming the main strawberry disease, so an update on Dr Angela Berrie’s work looking for alternative treatments is a feature of the Soft Fruit Day. Many of the varieties grown today are susceptible to powdery mildew, but the current control approach, based on fungicides, is unsustainable. Everbearers are even more challenging for the control of both mildew and Botrytis as they are flowering, fruiting and being harvested on a continuous basis from June to November. Also, the risk of mildew and rots is generally higher from July onwards as humidity and temperatures are higher. This all means there is a pressing need for a new range of fungicide products, biological control agents (BCAs) and alternative chemicals to maintain disease control.

Angela’s 2017 trial results for managed programmes showed considerable promise. The untreated crop was compared with a routine fungicide programme and three managed programmes, where fungicides were alternated with BCAs, plant strengtheners and elicitors, and with or without routine applications of a silicon product, Sirius or B204. The variety used in the trial was Amesti grown in coir growbags. Disease risk, growth stage and rate of growth, plus mildew monitoring, were used as the criteria for management decisions and the NIAB EMR mildew risk model was employed. All the programmes gave almost complete control of mildew. By contrast, mildew on untreated fruit after three picks rose rapidly to over 90%. On treated plots, fruit mildew did not rise above 3% over the 14 picks, with programmes based on BCAs performing as well as standard fungicide programmes. Angela commented: “It’s a very interesting result” and was very upbeat about last year’s results. The mildew model had shown that powdery mildew was a high risk throughout trial period, but despite this, mildew development on leaves was very low on untreated plots, which is typical of Amesti.

This year, Angela has concentrated on decision criteria for managing mildew and Botrytis. For instance, forward-forecasting of disease risk is useful, but direct crop observation is imperative. Certainly, high temperatures combined with high humidity favour mildew, so in this last summer of hot dry weather, where temperatures were often over 30C, the mildew risk was down. For botrytis cold nights are good news, but generally it’s humid at night. One positive measure for SWD control has been the removal of botrytis inoculum from crops, as the waste is taken off site. The 2018 results reflected the lack of disease due to the weather, so they weren’t as dramatic in terms of yield as 2017.

The model predicted a powdery mildew risk in late May/early June and a continuing risk from mid-July onwards. However, the incidence of mildew in the trial was negligible, probably due to using clean plants at the start and the very high temperatures in July and August, meaning that the disease failed to establish. On managed plots, only four fungicides and five BCAs were applied compared to 14 fungicides and two BCAs on the routinely treated plots. For botrytis, the model showed a similar risk in late May/early June and then a continuing risk from mid-July onwards. High temperatures in June and July resulted in a low risk for this period. The opportunity to reduce the use of fungicides in the managed plots was much less with botrytis because of the high risk in late summer. On managed plots, 11 fungicides were used, compared to 14 on the routinely treated plots. Angela also admitted that the number of fungicides on the routinely treated plots would have been greater, but “we ran out of products and had to use BCAs for the last two treatments”.

The overall conclusion, after this unusual year, is that managing the two diseases enables reductions in fungicide use when the risk is low in the early part of the season, ensuring that products are available for the high-risk period in late summer. A total of 27 fungicides were applied to the routine-treated plots over the trial period, but it is important to remember that during the trial there were 20 harvests and that the fruit at each harvest would only have received around three or four sprays in the period from flowering to harvest.

Evaluating latent crown rot control in strawberry

In AHDB Horticulture project SF 157, Xiangming Xu’s plant pathology team at NIAB EMR is looking at managing Phytophthora diseases. As Xiangming outlined, the project started with sampling cold-stored plant material to determine the level of latent Phytophthora. It went on to assess the effects of pre-inoculating plants with AMF/PGPR on the development of Phytophthora. A large experiment was carried out to study the effects of dipping before planting, and drenching after planting, on crown rot development. Both biological and chemical products were included.

In 2015, the sampling of bare-root runners of several cultivars at planting time from several growers found very low levels of crown rot symptoms (≤5%) observed at a few sites, but crown rot (P. cactorum) DNA was detected in all samples. No red-core (P. fragariae)symptoms were seen and no red-core DNA was detected. In 2016 crown rot was only looked forin six cultivars and no symptoms were found in one sample and only 11% in the highest. Using molecular tests up to 22% was found. These results from sampling showed that the incidence of crown rot varied greatly, was mostly present asymptomatically and was not associated with specific cultivars.

Growers can be reassured that a high incidence of crown rot does not necessarily indicate crop losses in the field. In only one consignment in two years was plant mortality high, at 12%, and the incidence of molecular detection at 8%. The plants can either grow out the latent infection of crown rot or the detected DNA was from dead tissue.

Regarding red-core, the team was asked whether we could treat with AMF and/or PGPR to reduce the severity of disease? Inoculating plants with AMF or PGPR generally did not lead to reduced disease development. Crown wounding led to a higher disease incidence and crown tissue browning was associated with the presence of red-core. One experimental product increased plant death, but for some products, dipping before planting can reduce red-core development. However, the benefit of additional drenching after planting appears not to be significant.

New approaches to raspberry pest and disease control 

Sam Brown of ADAS asked, “Can we reduce the harmful effects of SWD sprays on spider mite predators?” He looked at these predators, both released and natural, and reported work from 2015 and 2017 monitoring the effects of SWD sprays on commercial crops, and a summary of a 2018 pollen trial on a commercial crop.

Firstly, naturally-occurring predatory mites were found to be valuable in controlling two-spotted spider mite (TSSM) when insecticide programmes to control SWD and other pests were used. In the 2017 trial, sprays applied by the grower reduced both species – Decis used against SWD and Calypso against blackberry leaf midge applied on 2 August reduced numbers of Phytoseiulus by 90% and A. andersoniby 75%. However, spider mite numbers had already been reduced by various predators and through the removal of old canes. Sam’s message following this trial was: “Get your predators established and your TSSM under control before you have to spray for SWD”.

A novel idea for boosting predator survival was trialled this year: boosting natural predatory mite populations with pollen before flowering so that there are more to survive insecticides! Although not yet tested on raspberry, work on peppers had shown that A. andersoniwill survive on Nutrimite®. In a very hot year for tunnel-grown raspberries, results were mixed. Spider mite numbers were reduced byPhytoseiulusand possibly A. andersonibefore SWD treatments were needed. Sam reported: “On some assessment dates, significantly more A. andersoniwere found in tunnels where we released and/or fed them with pollen than in tunnels with only natural andersoni.” However, there was no evidence that releasing andersoniand/or feeding them with pollen benefits spider mite control. Possibly more andersoniand TSSM may have been found if only leaves with TSSM damage had been sampled, as in the 2017 trial, and further work is needed.

Raspberry root rot (Phytophthora rubi)is a perennial problem and Ruth D’urban Jackson of ADAS reported on the latest work examining the effect of the cold storage of long cane raspberries on root rot and the potential for protection using biofungicides. The 2017/2018 trial used inoculated long cane Tulameen. Ambient or cold storage was followed by applications of biopesticides Prestop and Serenade ASO and were compared with Paraat and an untreated control.

Across both trials no significant treatment differences were seen, following either storage regime. Real-time PCR indicated that both P. rubi & P. idaeiwere present. Autumn-treated plants were assessed in October. There were significantly more plants with red roots among the cold-stored canes than the ambient. Spring-treated plants were assessed in October. There were significantly more Phytophthora symptoms in cold-stored plants than ambient-stored. It seems that coming out of cold-store stimulates the shedding of spores. Going from cold to warm conditions without acclimatization is particularly harmful. Ruth’s advice following the trial is:

  • Thorough root inspection is necessary to detect early rubiinfection.
  • Cane symptoms only show when the disease is well established.
  • Check water sources, plant material, used irrigation tape, matting and soil.
  • Use Phytophthora LFD kits and/or Fera PCR testing, if you are concerned.

Does garlic intercropping help with controlling pests?

This question has rarely been tested experimentally before. In 2018, Adam Walker of NIAB EMR set out to answer the main questions regarding garlic-use. He assessed the effects of using garlic intercropping compared to untreated plots, determined if the technique could deter the main strawberry pests without adversely effecting beneficial insects, and evaluated whether this method of intercropping garlic is a viable control option for everbearer strawberries.

The regular breaking of garlic leaves and leaving them in the crop releases active substances.

On the Hampshire farm where this technique has been used for some years, plots treated with one garlic clove per growbag were compared with untreated plots for first- and second-year plantings, and the results were encouraging. Garlic intercropping was shown to reduce strawberry aphid populations in all five full-assessments, and significantly in four of them. There were fewer aphids in first-year strawberry plantings than second-year plantings. Garlic intercropping had no effect on the numbers of the predatory mite N. cucumeris.The mean number of spiders was higher in garlic-treated plots than untreated plots. The estimated cost of applying this garlic treatment was £263 to 395/ha/year.

Strawberry pests update

New approaches to pest control in strawberry, looking at improving integrated pest management (IPM), specifically capsids and rose thrips(Thrips fuscipennis), were covered before turning to the biggest problem, with five presentations on SWD.

Jude Bennison of ADAS reported how gaps in our knowledge of rose thrips can help growers to avoid damage by thrips. This newly-identified thrips species causes fruit-bronzing symptoms in strawberries but is not controlled by the IPM measures to overcome western flower thrips (WFT). It’sa native speciesand they’re much darker than WFT but differentiating between the two species is very difficult!

This new pest of strawberry has been the subject of a countrywide survey by the ADAS fruit team. This is not easy, as the two thrips species occurred in mixed populations in 14 of the 22 sites where rose thrips were found, out of the 25 sites monitored in 2017. WFT was not present at many of these sites. More rose thrips were found in southern Britain, but it is widespread throughout the UK and it seems to overwinter in weeds, hedgerows and the crop. As N. cucumeriscan’t deal with adult thrips, but only the larvae of WFT, and so far, the monitoring didn’t find many rose thrip larvae in strawberry flowers or fruit, this may explain why IPM programmes based on N. cucumerisare not controlling rose thrips.

Other thrips species found in strawberry flowers where fruit damage was seen prior to this project were: Thrips major(rubus thrips), Thrips tabaci(onion thrips), Thrips vulgatissimus, Frankliniella occidentalis(WFT) and Frankliniella intonsa(flower thrips). However, rose thrips have always been predominant in mixed species or sometimes the only species present. In the latest 2018 field survey work, rose thrips were the main species found in June and Frankliniella intonsawas the main species found in July, as it is more tolerant of high temperatures. Other species found were mainly onion thrips and rubus thrips. Thrips larvae were found in July and August but in lower numbers than the adults. Fruit damage was low but peaked in July when flower thrips numbers peaked.

Jude concluded that several thrips species could potentially damage strawberry fruit as well as WFT, but the damage by these other species could be mainly caused by adults invading the crop. Oriusconsume thrips adults and larvae, but not every year is good for their establishment and they are very sensitive to pesticides. Released and natural predators, and the pesticides used against SWD, are likely to have kept thrips numbers and damage low. Work in 2019 will probably include testing for the best trap colour for thrips and whether mass trapping will be of help.

Francesco Maria Rogai of NIAB EMR and Jon Marcar of Berry Gardens Growers are working on improving IPM in strawberry.In general, the results of trials have been positive. Firstly, the establishment of N. cucumeriswas not affected by a single application of Hallmark or Calypso used to target spring aphids. Newly emerging folded leaves showed very little or no target pesticide residue. New folded leaves provide a refuge and potentially a source of food for tarsonemid mites. Newly emerging leaves are less likely to have plant protection product residues on them. This can all contribute to enabling predatory mites to establish and reproduce, which is demonstrated by the presence of eggs and nymphs. Francesco was pleased to announce good news: Hallmark, which in the laboratory is suggested to have a persistence against N. cucumerisof 8 to 12 weeks, did not appear to impact the release and establishment of N. cucumerisin the crop in this one-year trial.

The push/pull model to control crop pests is being used for a range of pests, and Adam Walker of NIAB EMR outlined its principles before describing its use for capsid control. Push/pull is the integration of stimuli that act to make the crop unattractive to pests while luring them toward an attractive source, from where they are subsequently removed. The components are generally non-toxic and are integrated with methods of population reduction, preferably biological control.

In 2017 the team tested the effect of hexyl butyrate (HB) as a ‘push’ and phenylacetaldehyde (PAA) and Lygus sex pheromone as a ‘pull’ on capsids, both adults and nymphs. After some promising results in 2017, this year the trial was extended to determine whether a significantly improved push of capsid species L. rugulipennisand L. pabulinus(adults and nymphs) can be achieved when using HB in combination with a second repellent (RIPM). The trials also aimed to determine whether capsid damage is reduced where treatments are applied and whether the additional repellent will attract natural enemies into the crop.

The results were disappointing; there was no push/pull effect in 2018 as in 2017. Capsid numbers recorded were less than half that of the previous year. Combining HB with RIPM, did not achieve a significantly improved push, but the RIPM repellent did appear to attract adult earwigs, although at low numbers. Adam commented, “Potentially, we could increase the number of point sources of HB and repeat the trial with a higher number of replicates.”

Spotted Wing Drosophila (SWD)

Last year, senior entomologist Dr Michelle Fountain of NIAB EMR gave an overview of the work in progress and, whilst admitting there was no silver bullet, said, “We are starting to put together a toolbox for SWD control”. Five strands of work were showcased at the Soft Fruit Day, most forming part of the PhD studies of young researchers at NIAB EMR and supported by AHDB funding.

The national monitoring of SWD across England and Scotland continues with a sixth year’s trap catch to assess. The 2018 graph of trap counts has been much the same as 2017, although numbers were down in the spring, due to the late winter cold snap catching adults just as they were becoming more active after the relatively mild winter. Francesco warned that mild autumns encourage stable winter populations. Concerns over resistance to the chemical controls that we have currently have led to work to reduce inputs.

After trials this year, the fortnightly spray programme of SWD control products proved as effective as the growers’ programme. Very low numbers of SWD were found in fruit from both programmes. There is a need to ensure adequate leaf coverage to ensure good contact with the flies. We didn’t run out of effective products and fruit damage was prevented up to harvest. Post-harvest is a good opportunity to use non-pesticide controls such as crop hygiene and precision monitoring. Mesh designed to exclude insects reduced the numbers of SWD inside the crop compared to the perimeter.

Ralph Noble has been examining the use of bait sprays for SWD. The idea is that we might be able to improve the efficacy of existing SWD insecticides, such as Tracer, and make other insecticides effective, thereby broadening the range of available products. Baits could be sprayed onto foliage and attract SWD away from fruit, thus reducing the risk of pesticide residues and resistance. Baits could be used for the control of summer and winter morphs of SWD, and the bait most readily available is fermented strawberry juice from the waste fruit in covered bins.

Bethan Shaw has been investigating the use of sub-lethal doses of control products, a technique known as chrono-toxicity. At different times of the day an organism can have an increased or decreased tolerance to toxins and environmental conditions control these internal clocks. For SWD, the aim was to identify whether there was any difference in insecticide susceptibility when the fly is most or least active in terms of adult mortality and egg-laying. Exirel, Hallmark, Pyrethrum and Tracer were trialled. There was no difference in susceptibility to the four products tested between ‘trough’ and ‘peak’ SWD activity levels. Typically, egg-laying recovered over time from sub-lethal doses via direct spray application. Exirel and Pyrethrum had no trans-generational effects, while Tracer and Hallmark did. Some females were not killed by a 100% dose of Tracer but, to date, the eggs laid did not survive. It should be noted that resistance to Tracer has been found in populations of SWD in California. These tests looked at the direct application to adult flies in the laboratory. It would be relevant to examine the effects of dried residues on leaves and fruit.

Bethan says that it is now essential to monitor baseline resistance to the main plant protection products in SWD wild populations and rotate products to prevent resistance build-up. From her work it looks as though ‘the best time to spray’ is not so important but that products should be applied at ‘peak activity’.

The ‘push/pull’ control strategy for SWD is under investigation by Christina Faulder. Her laboratory work is identifying attractants and repellents for the summer and winter morphs that will be tested in the field next year. Christina wants to hear from growers and has designed a survey to help researchers understand the viewpoint of growers; this can help shape and influence the way the technology develops. Growers are invited to contact Christina at C.E.Faulder@gre.ac.uk to get involved.

In his PhD, Rory Jones is concentrating on developing yeast strains for the attraction and control of SWD. All fruit flies have an attraction to over-ripe fermenting fruit with plenty of yeasts present. In previous trials, the yeast H. uvarum, Combi-protec and fermented strawberry juice increased the efficacy of insecticides used. Rory is now looking at selecting a highly attractive strain of H. uvarumand searching for other highly attractive yeast species because SWD is also attracted to ripening fruits. Current baits are both non-specific and uncompetitive with ripening fruit for capturing SWD. The aim is to develop species-specific blends to exploit as part of an attract and kill strategy. It may be possible to use lower doses of plant protection products to kill SWD.

This year, of the eight strains of H. uvarumtested in the laboratory, six were attractive and two additional yeast species attractive to SWD were found. Although three yeasts were significantly more attractive than water in the field, H. uvarumwas the only one that was more attractive than strawberry juice. More work is required to identify which other Drosophila species in the field are attracted to the potential SWD yeasts and test the attraction of SWD to blends of yeast species. It will be necessary to characterise yeast communities on ripening fruit to further investigate optimum blends. By combining the most attractive blends with plant protection products, the efficacy of a lure-and-kill control method can be tested.