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RXLR genes block host resistance
The late blight pathogen, Phytophthora infestans, deploys a number of effector proteins, known as RXLRs, some of which are able to knock out the resistance genes which have been bred into cultivated potatoes. There are many different RXLRs released from the haustoria as they penetrate the cells of the host plant. Among the first to be described in detail was AVR3a, certain forms of which have proved extremely effective against varieties which, until recently, were regarded as having high levels of disease resistance.
The key to understanding such interactions between host and pathogen came from an international project designed to produce a detailed map of the Phytophthora genome, as molecular biologist Paul Birch explained when he addressed a Potato Council blight forum (see Potato Review, May, 2010).
‘We now have a blueprint of everything that makes Phytophthora work and everything it needs to infect the plant,’ he began. ‘We’re starting to learn more about its biology, how it is recognised by resistance proteins and how, over the years, it has been so successful at overcoming this resistance.
‘Even today, P. infestans is a serious threat to global potato production,’ he observed. ‘We do have resistance genes, several of which have been deployed but have been overcome. We also have chemicals but in a bad blight season you can end up spraying every week. On top of that we have the fact that the blight population is changing with Genotype Blue 13 coming to dominate. It is more aggressive than other isolates and is capable of overcoming a range of resistances.’
He described receptors on the surface of plant cells which detect ‘exposed molecules’ produced by microbes and send signals back to the nucleus to switch on defence genes. He explained that an effective pathogen must produce a range of proteins which will stop these defences from working.
‘At this point we have a susceptible reaction but plants have evolved a second layer of resistance,’ he continued. ‘There are hundreds of resistance proteins and their job is to sense the presence of effectors and trigger a response – the plant cells commit suicide to prevent the pathogen from invading any further.
‘There is a phenomenal evolutionary battle going on with the effectors changing constantly to evade detection and the plant evolving to maintain resistance.’
For the full story see our July/August edition
Are blight models still relevant?
In contrast to 2007, the worst blight year in many growers’ experience, 2009 turned out to be a low-risk season, at least in the eastern counties. Independent agronomist David Hudson says good results were achieved from routine 7-day interval spraying but this presents a dilemma for the future.
‘That’s not easily defensible,’ he admits. ‘Decision support systems are a nice idea but difficult to put into practice. Most growers just don’t have the spraying capacity to respond and saving one or two blight sprays isn’t much against the frequent reorganisation of spray timings and product changes.
‘With the new blight you have to think long and hard about drifting away from the best products – Infinito, Ranman and Revus.’ He also questions the validity of the Smith period: ‘It is suspected that the more aggressive strains, Blue-13 and Pink-6, are active below and or above the Smith period criteria of 10oC and 90% humidity and get through the life cycle faster, but what are the new criteria?
‘We need answers to use DSS with confidence. How have the programmers decided on the new blight growth criteria when pathologists have yet to update Smith periods?’
Also in the east, Hutchinsons’ Darryl Shailes points out that when the EU Sustainable Use Directive comes in to force the industry will need to demonstrate its use of DSS. Last year he found it almost impossible to get blight into trials plots and this hardened his opinion that routine 7-day intervals are not the way forward. He uses Dacom’s Forecast-Xtra system to inform his decision-making. It provides reports three times a week that contain three-day forecasts for the weather, spraying conditions and late blight risk. Dacom agent Howard Hinds claims the computer model behind the system has been updated to take account of more aggressive strains.
For high-risk conditions, as in June and July last year, Darryl Shailes sticks to seven-days or less and uses the top performers in Euroblight rating system, Infinito, Revus or Ranman. If there has been an infection period since the previous treatment and spraying is delayed, he opts either for Infinito, which also has a strong rating for curative action, or he adds cymoxanil to Ranman or Revus for kickback.
When risk is low, as during the sustained dry period in August and into September last year, he says the system gives him confidence in extending intervals to 10–14 days and using fluazinam or mancozeb-based products.
‘You need to have a will to use the system, but it makes logistics much easier,’ he says. ‘For example, instead of stopping irrigation to get a blight spray on at seven days, you can confidently leave it a day or two and utilise the sprayer on other crops. This can help massively with scab control. Using DSS certainly brings a lot more science to decision-making than just looking at the weather outside.’
For the full story see our July/August edition
Cost cutting favours web separators
When Pearson’s Megastar was introduced in 1990 its novel approach to potato crop cultivation created a stir and split growers into two camps, those who liked the fine, even tilth produced by stars and those who felt that the resultant beds might lack structure. Twenty years later opinion is still divided and although polyurethane stars are widely used on soil separators throughout the UK, a significant number of operators have stuck stubbornly with web machines.
One of their principal concerns has always been running costs. Abrasive soils and those with a high stone content (particularly flint) can be tough on stars; webs last longer and are changed more easily with less down time – a major issue for contractors and those covering a large acreage or working with a range of different soil types.
In recent years manufacturers have moved towards compact, high-output machines incorporating various permutations of star modules and conventional steel webs but Standen Pearson and Grimme UK have, simultaneously it seems, taken a step back this year by responding to demand for an all-web design.
The results were on display practically side by side on ‘potato alley’ at the Cereals event last month. Grimme’s offering was based on the successful CS/CW150 and 170 de-sign, Standen’s version looked very similar to the company’s Unistar models. Both machines featured multi-blade shares and three short webs followed by a full length main web bringing up the rear.
As a result both are longer overall – 300mm extra on the Unistar chassis but almost a metre in the case of the Grimme machine – though the manufacturers claim that there has been no compromise on manoeuvrability. ‘You can’t get the separation on a web machine without length,’ explained Grimme’s Barry Baker. ‘We’ve not had an all-web model for some time and there are certain areas of the country where growers are just anti stars,’ he told Potato Review. ‘We’ve had three machines on trial in the UK this year, including one with a boulder box, and they have been well received.’
Grimme has retained large 16mm web bars throughout, with an overlap between each web to prevent stone loss. The belt drive system is split equally from a centre gearbox, the first and third webs powered from one side, the second and main web from the other. A fixed scrubber and automatic levelling are fitted as standard and overall sieving area has been increased by 19%.
Standen’s Uniweb will be a
vailable in 1500mm and 1600mm working widths and can also be specified as a three-row model (2400mm). A spiked diabolo roller provides electronic depth control and ‘extra-large’ serrated discs have been fitted to help ensure a smooth feed in, even when working downhill.
The first three webs are all interchangeable (to help even out wear) and the fourth is fitted with a clod crushing ‘draper’ web which is raised or lowered by means of a simple ratchet system. Hydraulic levelling and self-centring steering are fitted as standard equipment.
In common with other Standen-Pearson soil separators this latest design has no axle beams or cross members underneath the webs on which soil can collect. According to marketing manager Alex Mathias, bed quality is improved dramatically as a result.
For the full story see our July/August edition
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