Cornish blight control might be a lesson for all
By Andrew Swallow
CROPS under polythene, high rainfall, and susceptible early varieties mean blight control has to be spot on for growers in the south-west.
But the risk management principles applied there are equally valid in any part of the country, says Greenvale APs south-west regional manager David Cliffe.
"Keeping blight out completely is of paramount importance. The cornerstone of any control programme is to start early and stick to the intervals."
For Gavin Rodda, of Rosemorran Farm, Gulval, near Penzance that means getting on with a contact protectant product within 12 to 18 hours of the polythene coming off.
"We give it half a day to freshen up and then we are in there with the start of the spray programme."
Shirlan (fluazinam) is used for the first pass. "It protects that very tender foliage for the first five to seven days," says Mr Cliffe. "But once new leaves start unfolding and there is rapid new growth a fully systemic product is needed."
That is typically Trustan or Ripost (both cymoxanil + oxadixyl + mancozeb), applied at seven to ten day intervals depending on the disease pressure until the crop reaches 80% canopy cover.
One eye is kept on the weather at all times as spraying opportunities can be limited. "By and large we manage to get applications on within that three-day window," he says.
For the final two or three blight sprays a semi-systemic or "leaf-systemic" product such as Curzate (cymoxanil + mancozeb) is used. A zero harvest interval means protection can be continued right up to harvest, notes Mr Rodda.
"The cost of spraying that last spray is minimal compared to not getting the right market. If lifting is delayed because of a change in the market or the weather then you know you are covered."
Total cost of the programme is £170-£200/ha (£70-80/acre), giving 60-70 days cover. But because an increasing proportion of produce is washed and sold as plastic pre-packs to the supermarkets no chances are taken, says Mr Cliffe.
"If we arrive at a situation where a crop is under threat because of blight pressure in the locality then we remove the haulm early and wait for it to stabilise before lifting."
Before lifting Mr Roddas crops, and others in the Univeg Grower Group of which he is a member, crops are scored for the risk of rots by Greenvale AP, the groups marketing agent. The risk assessment takes key agronomic factors into account; seed source, presence of ground keepers, variety, chitting regime, and level of deterioration of seed tuber. "If there is a perceived risk of disease in a crop then that stock wont go to our prime retail accounts.
"That can impact on individual grower returns, but the group members appreciate the importance of customer confidence, especially with branded produce such as Cornish King," says Mr Cliffe. *
Financial pressure increases disease risk
Financial pressure on growers has increased the blight risk in the south-west, says Mr Cliffe. "Because of the market situation growers have been forced on to their earliest ground more often. That means there is a far greater risk of groundkeepers introducing blight into the crop." Added to that, more are using non-certified seed, either multiplied up specifically for seed use, or replanting "casualty" crops that did not meet target ware market specifications.
Mr Rodda maintains that is an unreasonable risk, and puts all crops in the area under increased blight pressure. "If you are home-saving seed then you have got to do the job properly, and I do not believe you can home-save seed in west Cornwall." His 40ha (100 acres) of potatoes are on a one in five or six rotation and all seed is secured from known sources in Devon, Shropshire, Scotland and Holland by Greenvale AP. As a result produce has complete traceability from seed to supermarket shelf.
"At the end of the day you will get what you pay for when it comes to the end product," says Mr Rodda.
DATES FOR DIARIES
May 17 BPC storage event, Sutton Bridge Experimental Unit. Seminars, demonstrations, displays and trade stands concentrating on post-harvest crop management.
Aug 9 SAC/SCRI Potatoes in Practice event, Invergowrie, Scotland. Field demonstrations of latest research findings.
Sept 4-5 British Potato 2001, Newark showground. Agronomy, harvesting and handling workshops, seminars and trade stands.
Sept 12-13 3rd International Potato Show, Villers-Saint-Christophe, Aisne, France. Growing, harvesting, handling, storage, packaging, marketing. Everything European to do with potatoes.
• Varieties: Rocket, Alcmaria, Sprint, Premiere, Colmo, Maris Peer, Charlotte.
• May-July lifting.
• Supermarkets target market.
• Blight risk cut with rotation and quality seed.
• Zero tolerance spray regime.