DELAY costs money. Its a familiar theme but never more true than for winter oilseed rape where failure to catch infection this autumn can never be corrected.
The spray window for preventing the establishment of phoma stem canker is narrow after the first warning symptoms are seen – and sometimes there may be no obvious symptom spotting at all. Once in the stem, the usual array of fungicides are unable to prevent the fungus getting to work inside infected plants.
Later sprays may have some value on plants which were not infected first time around but trials evidence shows the benefit is markedly reduced from missing the first spray timing.
The HGCA issues forecasts each autumn warning growers of the need for sprays. Independent agronomists and some agrochemical companies and their distributors also carry out their own field walking to check for symptoms. Onset of infection can vary from October through to December.
The crop harvested this year showed less severe canker at harvest than in the previous two years but Dr Peter Gladders, plant pathologist at ADAS Boxworth, warns there will still be spores to respond to suitable weather conditions for spread this autumn. Dry weather, as in autumn 1996 when phoma leaf spot was delayed by at least two months, will be good news, high humidity or rainfall is not.
Why are disease levels so high when fungicide use is increasing? The complexity of the life cycle of phoma stem canker is partly to blame as it occupies most of the life of the crop. Airborne spores are spread from debris left by previous rape crops and produces phoma leaf spots with a few days. The fungus grows down the leaf stalk to the stem at up to 5mm a day.
However, development within the stem itself is slow and the cankers only appear about six months later.
In broad terms, light leaf spot infection is more common in the north and in Scotland, and stem canker is the most yield sapping disease in the east and south.
"Critical studies of fungicide dose and timing at ADAS Boxworth in 1994-95 indicated that fungicides have limited kickback activity," says Dr Gladders. "If the first spray is not well timed, canker can reduce yields by up to 1t/ha. Growers can do better with their canker control without spending any more money."
Substantial yield penalties are likely from delaying the first spray. Unreplicated HGCA plots at last years Arable Farming Event at Stamford highlighted this. The effects were more marked on Apex which had both light leaf spot and canker, than on Nickel which had canker alone.
Punch C (flusilazole +carbendazim) was applied at three different autumn timings, each with a February follow-up. The best response – 1t/ha from the Apex and 0.5t/ha from the Nickel – came from the two-spray programme started in October. Spraying first in November brought a 0.6t/ha response in Apex but delaying the first spray until December gained about 0.3t/ha only.
ADAS has also examined the best doses of four commonly used fungicides – Sportak Alpha (prochloraz plus carbendazim), Folicur (tebuconazole), Plover (difenconazole) and Punch C – for controlling canker. The trial in 1995 had unusually high infection with every plant showing leaf spotting symptoms.
Unsurprisingly, a full-rate, four-spray programme produced the best results for all four products, although Plover and Punch C gave markedly better control. Four half-sprays of these fungicides also gave 60-70% control, but two full sprays dropped control to 40% or less, and two half-rate sprays produced around 20% control at best when the crop was examined in the following July.
Dr Gladders highlights the results from the industry standard of two half-doses of Sportak Alpha – 10% control of canker in the 1995 Boxworth trial. There were better responses in other ADAS trials and there can be advantageous physiological effects from using Sportak.
Variety plays an important role in determining the likely response to fungicides, says Dr Jane Thomas, of NIAB. "An early spray is absolutely critical for canker susceptible varieties," she emphasises.
NIAB compared the yield response of the very susceptible Nickel with that of Synergy, which has a NIAB stem canker resistance rating of five, and Express which is more resistant. Early application of Punch C in October gave better disease control and yields than delaying the first spray until December.
Dr Thomas highlights the arrival of new stem canker resistant varieties such as Licrown to join Contact and Express at the top of the resistance ratings on the Recommended List. She also points out that early work by NIAB is showing gains for the grower in extra oil content from obtaining good disease control throughout the season.
This could be worth an extra £10/ha from crops harvested in 1997, according to Nick Myers, an agronomist with the ProCam Group, who also underlines the importance of catching the fungus early. In the groups survey of 20,250ha (50,000 acres) of commercial crops, the highest gross margins for rape come from those crops which received both autumn and stem extension sprays.