28 April 2001

Chose your weapons

Many rape crops might appear sparse and open but they are still perfect targets for disease and pests. We review the threats

IT takes just a few sunny days for crops to put a spurt on growth. But with the new leaves, and even the odd flower, come diseases and pests.

  HGCA-funded fungicide trials in Scotland and England are revealing light leaf spot infection, particularly in susceptible varieties Apex and Synergy – despite autumn treatments.

  In backward crops which have not yet received an autumn fungicide there might just be time to apply an early fungicide prior to applying the stem extension spray, says SAC. In any crop which has yet to receive a fungicide this season, SAC advises a single spray as soon as green bud is reached – to replace planned autumn and spring sprays.

  Richard Elsdon, technical manager with United Oilseeds gives the English picture: "Most winter rape crops should have received an approved triazole in the autumn, with a spring follow up in February as soon as low levels of phoma were seen on the lower leaves."

  On forward crops he recommends applying Folicur if they are about "welly high": "The fungicide will not only control light leaf spot but have a growth regulatory action too. Watch dose rates; where light leaf spot is well established the spray will need to have maximum curative activity," says Mr Elsdon.

  ADAS confirms that a triazole fungicide is worthwhile where crops are at early stem extension and have 25% or more plants affected.

  Sclerotinia is the other major disease threat this year. About 30-40% of a crop can be lost with severe infections.

  Mr Elsdon advises checking fields with a history of the disease and look the fruIting bodies (apothecia): "They are a useful pointer of local high risk conditions."

  The spores will germinate quickly if temperatures warm up. Product choice is Compass (thiophanate-methyl + iprodione) or Konker (vinclozolin + carbendazim): "Both are good but application timing is crucial – as crops come into flower but before first petals are falling. Thats because the petals must be used to carry the chemical down to the lower parts of the crop," explains Mr Elsdon.

  A further disease threat this season if temperatures warm up might be alternaria: "We havent seen it for years, but a warm and wet early summer with a mininum temperature of 14íC day and night is perfect for the disease. But if you have to spray against sclerotinia anyway, the same fungicides should be sufficient to control developing alternaria.

  "The key is to protect the developing pods and beware cutting rates because there will be no chance to get back into the crop," warns Mr Elsdon.

  Turning to pests, cabbage stem flea beetle larvae may be present in some crops not treated earlier: "Slice open the leaf petiole and bottom part of stem to spot telltale burrows and even a maggot," he advises.

  April may have been a little on the cool side for pests, but ADAS are reporting first flights of pollen beetle into crops which are now in flower. The beetles are foraging for pollen in these crops, so they present a big risk to varietal associations which have just 20% of plants that bear pollen. They can be particularly damaging when crops are in green bud and very early yellow bud development," warns Mr Elsdon.

  CPBTwyford advises spraying Gemini or Synergy as soon as one beetle per plant is found, to ensure maximum pollen production. However, in conventional normal developing crops the treatment threshold advised by ADAS and the SAC is 10 to 15 per plant. On backward crops the threshold comes down to five pollen beetles per plant. Treatment is an approved pyrethroid.

  Seed weevil and bladder pod midge tend to be more of a problem in pockets of the country, such as north of Humber into Scotland. "One spray against seed weevil will have a good enough effect on bladder pod midge, since the midge comes in on the exit hole made by the seed weevil on the pod."

  Seed weevil threshold is one per plant and treatment is an approved pyrethroid when the crop is in full flower.

  Aphids are rarely a problem, but can come into crops during the early green pod stage. Sometimes a headland spray with pirimicarb may be necessary, but rarely is a full field treated, says Mr Elsdon.