Article taken from The Vegetable Farmer Magazine
By Richard Crowhurst
The Vegetable Farmer spoke to four leading agronomists to find out what their key concerns were in terms of potential crop production issues for the coming season, beginning this month with the challenge of disease control.
Vegetable growers saw high levels of virus in crops from salads to carrots and brassicas last year as large aphid populations spread the disease before they died off later in the summer. Now the number of overwintering aphids found by agronomists over the unusually mild winter suggests that there could be early aphid and virus pressure across most vegetable crops in the coming season.
""Cucumber mosaic and lettuce mosaic viruses in lettuce were a problem last year due to high levels of aphid numbers," confirmed David Norman of Precision Agronomy, who principally looks after salad and confirmed as the warmest winter for England and Wales since records began in 1910, and the third-warmest for the UK as a whole, has been an increase in rust diseases on winter allium crops. ""We've seen a bit more leek rust so far this spring, continued David.
""Usually the winter is cold enough that the disease doesn't progress very much, but with this mild winter we've certainly seen levels of rust about that we don't normally see, which I think is just down to the mild, damp autumn and winter.""
""You will probably hear the same message from everyone, but with such a mild winter you've got to think there's going to be a high population of pests and allium crops in the East of England.
""In contrast last year was a very low mildew year on salad crops as, for most of the summer, it was just too cold. Mildew needs warm, muggy conditions to spread and we never really got them last year. Who knows what we are going to get this year? Just because we've had a mild winter doesn't mean we will get a warm summer, but a warm wet summer will lead to mildew problems."" One legacy of the mild winter, which was recently provisionally diseases generally," said John Allan, Vegetable Technical Team Leader for Agrovista.
""Given the weather I think we need to be watching out for pests and diseases this season.""
This sentiment is shared by Howard Hinds of Root Crop Consultancy Ltd. ""Cavity spot was a big problem last year, in most of the carrot growing regions in the east. Cavity spot is continually top of the list of carrot disease issues, but last year was particularly bad and we saw an increase in symptoms in field stored crops over the winter due to the very mild December that we had."" Control of cavity spot has often relied on the use of metalaxyl-M, but the latest research, presented last month by Dr John Clarkson and PhD student Kat Hales of Warwick University, suggests that there is much more work required to understand both the species of fungi responsible for the disease, and also how it interacts with the crop, soils and other host plants.
""We're looking at all options for cavity spot control," added Howard. ""This includes some of the new biologicals which may help us, particularly where they are used alongside metalaxyl-M treatments."" Like David, Howard also cites aphids and the resulting virus diseases as a major problem last year, this time in carrot crops. ""One of the big issues last year was virus and aphid control and that's one of the things that's concerning everybody at the moment with the mild winter. Parts of East Anglia and the Midlands were hit quite badly by virus last year."" ""Although we were concerned about the population of pests last year, and we saw quite high populations of aphids in certain crops towards the end of June, the populations did crash," points out John Allan. ""It is worth remembering that because you get high populations of pests you also get high populations of beneficial and predators as well. Having said that, you do have to be a bit wary."" Both David Norman and Chris Wallwork, Agrii's Horticultural Technical Manager, said that they had seen more incidences of light leaf spot in leafy brassica crops, particularly in the autumn, ""I think that light leaf spot in vegetable brassicas is one trend which looks like it will continue," commented Chris. ""There are several different reasons for that.
One is the mild winter that we've had so far and there is a lot of light leaf spot out there at the moment in any crops which have been untreated."" Chris commented that there can be big differences in the susceptibility of different varieties to the disease and some of the most recent work funded by AHDB Horticulture suggests that there might be different strains of the disease in different areas. In a summary of work undertaken by Coretta Klbppel at the University of Hertfordshire as part of project FV422 emailed to levy payers, AHDB said; 'Varietal susceptibility varies by area suggesting that different races of the LLS pathogen dominate different growing areas. This has been highlighted in a trial which shows that oilseed rape varieties with good resistance in Cambridgeshire can have significantly less disease resistance in Scotland.' As part of the project the susceptibility of different crop types was also compared in glasshouse experiments using cultivars which were known to be susceptible for each crop. Infection of crops such as broccoli and cauliflower was observed, even though traditionally light leaf spot has not been considered to be a significant disease of these crops. This means that most growers will not apply fungicide to control light leaf spot on these crops and AHDB has suggested that this may mean that untreated fields could provide a source of disease inoculum, especially if left for long periods after harvest.
""I have seen severe light leaf spot symptoms on untreated crops of purple sprouting broccoli this winter, so it is not only a phenomenon in glasshouse experiments," stressed Chris. The increase in light leaf spot may partly be down to varietal susceptibility, but Chris also warns that over reliance on resistant varieties could lead to a bigger problem in the longer term. ""Perhaps we have been putting too much pressure on the varietal resistance that we have had available," he said. ""If growers use varietal resistance but don't protect it with suitable crop protection products then the end result can be just as bad as growing a very susceptible variety. You need a combination of both resistant varieties and effective crop protection products; that's what integrated crop management is all about."" For comparison Chris points to the evolutionary race between varietal resistance and disease pathogenicity which is already familiar to many salad growers. ""In salads there have been a whole series of resistances to downy mildew in varieties of lettuce and spinach. New resistance genes are identified by the plant breeders, but the pathogen is continuously playing leapfrog with the breeders and developing new virulence genes," he explained.
""Or perhaps we are selecting out particular virulence genes? The end result is the same: a breakdown of resistance."" It is also important to protect the activity of the fungicides that are available. ""There is an issue around changes in the sensitivity of the pathogen to certain fungicides," explained Chris ""We and other people have done various trials over the years, many of them with oilseed rape. Our first experiments looking at the sensitivity of light leaf spot to different fungicides in vegetable brassicas were in 2012, and included some biopesticides alongside conventional ones. They produced some interesting results, although back then the issues were only really identified up in the far north-west. I think everyone knows about them now."" One thing that all four agronomists agree on is that although growers have a reasonable choice of fungicides, the most successful growers continue to innovate and embrace new technology. ""There's a lot of activity in the whole field of bio controls, but there are challenges with some of the products in terms of application, for example the water volumes needed," commented John Allan. ""In general terms the people who are going to do well in the future are the ones who are willing to try new techniques and look at alternative technologies," concluded Chris. ""People who are prepared to reassess the end result that they are trying to achieve and the methods they use to get there given the changing circumstances and the reduced margins, will continue growing. The people who just do the same thing year after year are less likely to be here in the future.""