I was fortunate in being able to spend much of a day in cherry orchards with Dr Steven Roberts, a plant bacteriologist, last week here in Herefordshire.
Steven has published excellent work on bacterial canker of cherry and plum – recently focussing on problems affecting nursery stock used for the establishment of farm woodlands but now with funding from commercial fruit growers, through the Horticultural Development Company.
A lot of his work has involved distinguishing between pathovars of the bacteria responsible for canker, Pseudomonas syringae. For most people this is arcane stuff but in important ways the pathovars behave differently. Steven complained about some of the assumptions made in reports about such things as resistant varieties so a blog is probably not a wise place to pretend knowledge.
Infected fruit buds
I am particularly interested to understand the biology of Pseudomonas syringae pathovar syringae, which until recently wasn’t thought to be important for UK cherries. I have the impression that it has been responsible for serious blossom and bud blight in Herefordshire orchards though making such a statement will invite complaints that I am ignoring Monilinia and have no proper laboratory evidence for such a claim.
Bacterial diseases of fruit crops are an old problem but still damaging for orchards. Some of the descriptions of the rate of development and spread of Pseudomonas syringae given by Dr Roberts were sobering!
I had already been ‘softened up’ by listening to a presentation by Marcel Wenneker at the recent ISFC conference in Holland (http://www.softfruitconference.com/)
entitled “Erwinia as a New Pathogen of Strawberry” and arising from a study of recent
problems under glass in the Netherlands.
Fortunately, summer covering with tunnels almost eliminates the post-blossom spread of bacterial canker in cherries unless carried on pruning tools but we should always remember how easy it is to spread infection on secateurs. There is also the possibility that pollinating insects play a role.
It is worth remembering that bacteria can also be the good guys! We are now familiar with the benefits of Bacillus subtilis in Serenade and anyone interested in the wonderful world of phylloplane ecology will know that we are only just beginning to understand and properly exploit the power of beneficial bacteria. I am now going to take a break and drink my Yakult and watch a video about ice nucleation that Dr Roberts kindly mailed.
Windbreaks play a key part in the successful deployment of tunnels.
Poplar windbreak - Haygrove Newent
Even when a tunnel has been constructed according to the
strongest possible specification, windbreaks may still important – reducing
problems with the management of doors and venting during the growing season and
minimising visual impact on the landscapes.
There are a few important things to remember:
Natural windbreaks take a while to establish so
Windbreaks should not completely block the wind,
otherwise air will tend to accelerate as it flows over the barrier and, like a
ocean wave, roll down on to the tunnels with considerable down-force.
Windbreaks will interfere with cold air drainage
during frost events. The lower canopy of natural windbreaks should be removed.
Haygrove chose to plant poplar windbreaks at their farm in
Gloucestershire. This species is probably the fastest growing windbreak tree
for northern Europe. Haygrove helped the trees by installing drip irrigation –
connected to the header pipe used to irrigate the berry crops being grown
beside the windbreak. The trees were planted 1m (3’) apart. A good variety is ‘Balsam
Spire’ (TT32). Trees can be started using 2m whips.
Another commonly used species has been Alder. The roots are
less invasive but they are slower growing and phytophthora can be a problem.
In the UK, as in many countries, we are struggling to control western flower thrips (Frankliniella occidentalis) on many crops – perhaps especially strawberries here.
Western flower thrips (photo courtesy of Bioplanet)
Glasshouse salad crops and flowers seem to have less of a problem because of stable onditions for the establishment of biological control. Glasshouse salads yield more produce per square meter so growers can justify bigger investment in biological control inputs and monitoring than is feasible for most strawberry crops.
The thrips predator -Neoseiulus (Amblyseius) cucumeris. Photo courtesy of Bioplanet
In the UK we work mostly with Neoseiulus cucumeris syn. Amblyseius cucumeris as the basis for thrips bio control but it hasn’t been enough to cope with severe cases of spinosad resistant thrips so we have started to focus more on the use of predatory bugs from the genus Orius.
An adult orius in action! (Photo courtesy of Bioplanet)
The increasing incidence of spinosad resistant thrips and our interest in orius will have a significant impact on growing practices. With a majority of growers now farming at least a part of their strawberry area on soil-less systems it will be important to learn how to manage pest populations without the option of ‘running away’ to a safe patch of fresh ground.
Investment in better, stronger and more automated tunnels has had the same effect but may also open up opportunities to create the more stable conditions required for early establishment of biological control systems.
Insect nets may also become an essential part of our management plans – to prevent the ingress of certain other pests that would need to be treated with products that are harmful to the thrips bio control system.
One of the big questions facing British growers is: “When to introduce orius?”
We know that it is working well at sites in Spain and Italy. How can we create the right environment in our tunnels. It is so very helpful for agronomists to have good environmental data for tunnels – not just for crop forecasting but also for bio-control decisions. Be sure to include a weather station system in any future tunnels purchase!
Grateful thanks to Bioplanet s.c.a for use of their images.
English growers are probably as confused as their plants by the weather they are currently experiencing.
Our rivers have flooded and there have been VERY few frosts. A meteorologist told a recent grower conference that the jet stream and storm track have been sitting on top of each other for much of the winter, a fact that probably explains why most of the contents of the Gulf of Mexico has ended up in our fields.
Another rainy day at Haygrove Ledbury
In some countries growers are worried about leaving strawberry plants in situ for the winter because of frost kill whereas in the south of England, this year, our plants may not have accumulated optimum chill units under tunnels in some districts. We may regret not having left them outside! Having said that, it is important to understand that although it hasn’t been freezing, it hasn’t been warm either so some of the chill totals are higher than growers might have feared.
There also concerns that where new frigo plants have been planted into coir bags, they may be making top growth in the mild conditions but not developing sufficient root.
Oh well…… At least it was sunny today!
I will go an drink a glass of wine from last years tunnel crop – see below - and remember what warm weather feels like.
Wine grapes protected by tunnels
Corn at harvest, England
Growers in Britain can feel the autumn coming, on some nights
it has been very Autumnal. Apple growers
would like some good day-night differentials to accelerate fruit colouring but
berry growers are probably keener to accumulate as many growing degrees as possible
because they all add up to extra yield.
Knowing that our North American cousins refer to the autumn
as ‘fall’ – which makes a lot of sense – is a reminder that we hope that fruit
prices will do exactly the opposite at this time of year. Unfortunately prices
haven’t been so good in recent Septembers with the late crop of glasshouse
strawberries clashing with field grown day neutrals. We are not sure what this
September will bring – some day-neutral varieties were badly hit by a British
heat wave in July. That is several days above 30oC here, so nothing
very exciting for most fruit growers in other countries, but enough to cause some
UK adapted varieties to go into a kind of exhaustion.
Tired leaf = tired plant and probably no flower coming
However some other varieties are looking fantastic right
now. If we can keep botrytis out so that picking can be extended well into
October, many growers will have a good Autumn.
A late everbearer variety - pushing a lot of fruit for September on table tops
Too hot for an Englishman but what about strawberries?
A physiologically confused Fruit Doctor is writing this
item: Having spent time in the Yarra Valley in Australia at 37oC at
the beginning of March and last weekend in Scotland admiring the snow capped
mountains but worrying about the likely impact of a late season on the UK strawberry
market. Isn’t it amazing that we can grow berries in such diverse environments?
In Australia the light is so intense that ripening strawberry
fruits are often scorched in the field before they can be picked. Growers have
found that diffusing poly films can prevent this scorching so that even in
temperatures above 30oC they are reluctant to vent. This contrasts
with standard practice in Britain where strawberry and raspberry growers are exhorted
to avoid more than two hours per day above 25oC, venting to avoid
Sun scorched strawberries
The University of California bred strawberry variety ‘Albion’
has now been grown successfully by growers at farms ranging from Aberdeenshire,
Scotland to Victoria, Australia. Wow!
A slow start to Spring (England, on the official first day of Spring 2013)
If there are problems, these are often the result of
variations in humidity (low and high) or raised night temperatures. Measuring
these variables and understanding their effects is important because it isn’t
just variations in temperature that cause physiological problems in plants.
British growers have been skinning tunnels for some weeks
now. A few growers skinned in the first days of January or kept the tunnels
covered for the whole of the Winter after adding some strengthening systems
(ranging from bracing wires through to steel tubing or even the new Gothic
shaped hoops). We have been coping with almost continuous precipitation until
recently but the rain turned to snow in January.
Growers who had already skinned were able to use available
labour to get on with planting and other tasks whereas at other sites, people
returning to work after Christmas, often from abroad, were faced with a
frustrating and expensive wait for a gap in the weather. While skinning early
remains a ‘calculated risk’ we have become more confident both in our
understanding of how to specify tunnel structures and in our management of
those structures once built.
Clery coming into flower, Cornwall UK, 3rd week of February
Right now there are some open flowers on the early June
bearer variety ‘Clery’, down in Cornwall under fixed tunnels over fleeced table
tops whereas outdoor fields in Herefordshire have hardly moved. Most of the
early fields now have tunnels with doors and the crops underneath are protected
by one or two layers of fleece (USA = “row cover”) or a layer of fleece plus a
layer of perforated clear film.
But…. that doesn’t mean they can be ignored! Agronomists
will be crawling around under the fleece, looking for aphids and it won’t be
long before the crop needs ‘managing’ again. Fleece and the perforated film
heats the crop but cuts out a lot of light. If left on too long or if tunnels
aren’t vented during sunny weather plants become ‘stretched’. The foliage
becomes leggy and soft, collapsing easily later on.
For outdoor crops a week in May is worth more than a month
in February for growing degrees (see table below, for 2012) but under well sealed tunnels things will happen much faster. 2012 was a strange year for British growers because April was colder than March!
UK growing degree hours, Spring 2012
A visit to an impressive HAMK University of Applied Sciences school at Lepaa, Finland.
Raspberries under Haygrove Tunnels
The school is well equipped, even boasting a micro-propagation facility, and filled with a great bunch of students. I was fortunate in meeting a young grower, albeit no longer a student, who managed to grow 1.4 kg/cane of fruit from cold stored long cane raspberries imported from England (!!).
The Finnish consumers are said to prefer the flavour of more traditional varieties but these are grown because they have proven their ability to survive the near-arctic winter. However Glen Ample is also finding a good market. Tunnels are, of course, essential for growers who have chosen to invest in such expensive plant material.
On the subject of the arctic: It is worth reporting that we also met Pauliina Palonen, a widely respected small fruits researcher. Pauliina is currently working in a team trying to improve the stock and growing techniques of the arctic raspberry or nagoon berry (Rubus arcticus). This fruit has a remarkable, aromatic, flavour and is bought for use in drinks. It is exciting to see this kind of work: How many new or half forgotten fruits are waiting for an innovative grower to find?
Arctic Raspberry (source: Wikipedia)
Fruit Doctor attended an excellent event, entitled FOCUS ON LIGHT SPECTRUM at the Stockbridge House Technology Centre here in England on Tuesday of this week. The meeting kicked off with an excellent presentation by Professor Nigel Paul (Lancaster University) and included talks by several leading European researchers and finished with talks by three growers who have been pioneering polythene tunnel covers that exert special effects on the spectrum of light for the crop or with new generation LED lamps.
Much of the discussion was focused on the development of crop and variety specific light recipes to optimise the use of LED lamps but another important strand was the effect of ultra-violet light.
It seems that there are no UV generating lamps for commercial scale horticulture (hectares) on the market so the only way to raise the amount of UV reaching a crop is to change the type of glass of poly used to clad the production unit. The ultra-clear teflon related material known as F-Clean was discussed. F-Clean was the material chosen by the famous Eden Project here in the UK.
Conventional greenhouse glass and tunnel poly allow very little UV to the crop but UV has been shown to boost flavour, enhance fruit/leaf colour and encourage the production of a more compact plant.
Standard and UV window light transmission spectra
Fortunately for tunnel users a generation of UV ‘window’ films are available. Look out for Visqueen Lumisol, for example.
I think that anyone reading this will be feeling some
sympathy for our American friends who may have been affected by Hurricane
Sandy. It’s risky enough being a grower without “Super Storms”.
- Frost damaged Elsanta strawberry crown after Winter 2010-11, UK
Growers of table top strawberries face a particular challenge if they decide to over-winter a crop instead of replanting in the Spring. The compost filled troughs, pots or bags are exposed to un-relenting chill if left on the table structure.
There is no moderating effect of the surrounding soil – something that prevents severe crown injury in conventional fields in all but the coldest districts.
Furthermore, any blanket of snow does not prevent heat loss from beneath the containers. In Britain, growers are advised to move containers off from tables and onto the floor if night temperatures below -8oC (46oF) are expected.
Roots of Elsanta plants damaged by a combination of freeze injury and phytophthora
This is expensive on labour and exposes the plants to soil borne pests and diseases, perhaps especially Phytophthora fragrariae. If the plants are then covered with fleece, Botrytis and aphids can be difficult too.
- Wrap the containers in fleece (row cover) in situ. This has limited benefit if
freezing conditions persist for many days.
- Convert to 4-season tunnel structures. Under a
tunnel the wind chill factor is massively reduced and fleece is easier to
manage. Maybe a heat source can be made available for extreme events.
- Haygrove have developed a patented telescopic
table top system so that tables can be lowered and raised quickly.
Telescopic substrate system, Autumn 2011, in the "up position".