Archive Article: 1997/09/20
1 THERE is lots of stem canker spores around but what happens next depends on how much rain we get in September.
The potential is there for stem canker to be the No1 disease in English crops. Given high humidity and rainfall, those spores will be flying into crops.
Within a matter of five to seven days, the spores will produce leaf spotting. The infection grows rapidly down the leaf stem to the main stem at 5mm a day and could be down in the main stem in 10 days.
Therefore there is very little time between spores causing leaf spotting and the infection reaching the stem. Once the spots are there a few days, the fungus is probably already in the stem and it is too late to spray.
There is a clear link between the leaf spots and the stem canker you see six months later. Yield loss is caused by the premature ripening from the stem lesions. UK growers lose around £40m from stem canker each year despite spraying. Similar losses from light leaf spot add up to 1t/ha falling victim to these diseases.
Canker spores are normally released from late September through to March – May in 1997 – so fungicide protection should be maintained every six to eight weeks.
Get the dose rate right and Punch C or Plover are rather more flexible for canker control than other products in an ADAS Boxworth trial as long as leaf spots hadnt been there too long.
In a 1996 timing trial with Punch C, we sprayed in October and got a 1t/ha yield response but waiting another four weeks for the first spray reduced that by half – worth £20/ha.
Dr Peter Gladders,
Plant pathologist,
ADAS Boxworth, Cambs.
2 THE leaf spotting symptoms may not always be visible but the fungus is there. For early warning of infection, growers can take leaf samples and put them inside a plastic bag. Place them in an airing cupboard or on a window sill where the temperature will bring out signs of any hidden infection.
Cheap and cheerful is often the attitude of some growers and advisers to choosing fungicides. In some instances, in certain crops that will be the case but not for oilseed rape in this situation.
David Ellerton,
Procam Group, Herts.
3THERE is some good resistance to stem canker creeping into the new rape varieties such as Licrown. Contact is dropping back to a seven rating after this season, and Express stays rated at seven.
There are differences in the response from varieties to fungicide applications which reflect their inbuilt resistance to diseases such as canker, light leaf spot and downy mildew. It shows up particularly in the north where Lipton and Lightning gave the lowest response in trials. These are the varieties with the highest resistance to light leaf spot.
Growers should assess the risk of disease in the varieties they have drilled. Varieties susceptible to stem canker will benefit most from earlier spraying in high risk situations. However, several varieties have good resistance to light leaf spot but few have even moderate resistance to stem canker.
Trials carried out by NIAB for DuPont showed that spraying Punch C in October with a February follow-up gave much better yields than delaying the first spray until December. Express, Synergy and the very canker-susceptible Nickel were used in the trial. Early spraying is absolutely critical for a susceptible variety.
Dr Jane Thomas, Plant pathologist, NIAB, Cambridge.