If there was ever a good example of how critical timing can be in horticulture, this year's National Institute of Agricultural Botany (NIAB) variety trial of onions from seed at PG Rix Farms, Colchester, Essex, is it.
Sown on 3 April, compared with March last year, the trial was a little late going in so plots are slightly behind in maturity than usual. But had it been drilled three weeks earlier, the trial would have been subjected to intense rain, causing the soil to cap - and the plots would have suffered the same fate as the bordering commercial crop, which had to be ploughed in.
Weather apart, the season so far has also been a challenge in terms of downy mildew infection, although levels are lower than last year. NIAB vegetable consultant Mike Day says: "There has been a lot of mildew pressure and the brown varieties have done well to keep as clean as they have, although they have been subjected to an intensive spray programme."
Varieties with genetic resistance to mildew have been an exciting recent development, says NIAB vegetable specialist Bruce Napier. He believes Santero, a maincrop variety in its second year of trialling, and BGS237, in the demonstration plots, are just the start. "We will see resistance being bred into better varieties with higher yields," he says. "For instance, BGS237 is just too late maturing for most growers."
As with onion set varieties, breeders of onions from seed are also looking for increased earliness, so growers have a better chance of clearing crops in good weather, and improved storage ability. Two of the best performers last year for their ability to store to the end of April without sprout suppressants are Wellington - an early maincrop variety and one of five industry standards against which other brown varieties in the trial are compared - and Vision, in its second year of NIAB trials.
Reviewing their performance based on NIAB's full-season trial results from last year from two sites, in Essex and Norfolk, Napier says that of the early varieties Bennito and Vision scored better than average for mildew susceptibility with Vision producing good yields. Tangito yielded well in the Essex trial and stored to the end of March but then sprouted fairly rapidly. Sunnito is new in the main trial this year. "It is said to be early-maturing but doesn't look to be in that slot this year, based on the figures we have," says Napier.
Hypark was light on yield on the Essex site last year but among the top varieties for storage, while Hybing also yielded better in the Norfolk trial but did not store particularly well.
Of the mid-season varieties, Premito was "middle of the range" for yield based on one year's results and stood up well to mildew, although there is a spattering of disease in plots of the variety this year. Centro achieved high yields on the Norfolk site last year but has suffered from mildew infection this year. Norfolk-trialled bulbs of Sprinter, included as an industry standard, stored until April.
The maincrop variety ADV 00335 was below average for yield in the Essex trial last year and looked more susceptible to mildew, while Napoleon, also with maincrop timing, and Sunskin, of late maincrop maturity, stored well to April and were less badly affected by mildew.
In the red varieties, Napier says Redspark is probably the best-performing. "It can be earlier than Red Baron," he says. "It was higher-yielding than other varieties in the trial last year." Red Baron, an open-pollinated variety which is attractive on seed price, still has a place, adds Day, if you don't rely on it to store for long. "And, once graded, yields are not as high as the hybrids."
The numbered variety 301/5 is showing a similar maturity time to Redspark and, in Napier's small-sampled tests, produces a high percentage of single centres, important to consumers who value attractive onion rings. It is a specific trait on which its breeder, grower Alastair Findlay of Allium Farms, Bedfordshire, concentrates - along with resistance to internal rot and long natural storage. "With the increasing cost of bringing onions long distances, if it is possible to produce a good article really late then it is an objective breeders should work on," he says. "I have stored some of my varieties for 12 months."
Findlay, who also chairs the British Onion Producers' Association's R&D committee, works only on red varieties. "Reds make up 20 per cent of the national acreage of onions," he says. "It has gone up year on year and I think it will continue as people recognise that red varieties have high levels of fructose."
If growers can look forward to better control of mildew through genetic resistance - rather than by using fungicides that are coming under threat from EU proposals to change the way pesticides are approved - some are beginning to think how variety choice can assist mechanical weeding. "This year we were asked to assess the foliage more closely," says Napier. "We are looking at 'cranking' - when the tops lean over - thickness of foliage, and foliage density as a whole based on distinct, uniform and stable keys used for national listing." It is a question for growers, he says, of balancing the density of canopy, to discourage weed emergence, against humidity levels that will favour disease infection. "If disease gets into crops, it will romp away in certain foliage densities, so it will be useful to record plant habit." Assessments were made when plants were between leaf 10 and 12.
For instance, the red variety Reddawn, back in the main trial this year after a long absence, is characterised by particularly fine-leaved foliage which Napier says has kept plants relatively clear of disease and would accommodate mechanical weeding. "It had the least cranking of all varieties and less dense foliage but was flattened by recent heavy rain and strong winds," he says. "The bulbs had already filled out so hopefully there will be no yield penalty."
Trial plots will be lifted in early September, the bulbs graded and weighed to assess yield, and back on view on 6 November at NIAB in Cambridge. This will also be the time when varieties will be selected for next year's trials.
There were four red varieties and eight brown in the preliminary trial this year, when new varieties are entered for assessment. "Companies are putting a lot of work into their breeding programmes," says Napier.
The variety trial is funded by HDC and seed firms. Factsheets for spring-sown onions from seed and from sets 2007 are available to levy payers from HDC.
ADV 00335 Limagrain Advanta
Arlondo Limagrain Advanta
Arthur Limagrain Advanta
301/5 Allium Farms
Kamal Limagrain Advanta
Red Baron Bejo