28 April 2001

French put

model to test

Blight forecasting systems stood up to their biggest French challenge yet. David Millar reports from a special seminar in Paris

ZERO tolerance is the only approach acceptable to the French potato grower in dealing with blight – even if that means applying a couple of dozen treatments on a "long season" variety.

So imagine the delight of growers in the Champagne region when trials using a blight prediction system based on weather data and computer modelling brought average savings in 1999 and 2000 of £40/ha in spray costs and as many as half the usual number of spray passes in 1999.

Last year provided a supreme test for blight prediction systems in France. There were three major periods favourable to blight spread: early June, the whole of July and the last two weeks in August. Agronomist Eric Poittevin, of the CETA de Champagne group of 40 potato growers, installed three Adcon weather stations on farms. They managed to save up to £72/ha, depending on variety, by producing blight predictions using the Ullrich-Schrîdter modelling system.

In each of the trials, blight was well controlled by applications of a dithiocarbamate. Normally, the growers would apply these every seven days or after 20mm of rainfall. In one of the trials, the system predicted the same spray requirement as the normal farm practice (16 sprays costing £116/ha in total) so no saving was made. The same starch variety in a 21-week growing cycle needed 24 spray treatments by the farmer, costing £218/ha, but just 18 treatments worth £194/ha using the prediction system.

Less infection

The final variety Franceline was sprayed 16 times by the farmer at a cost of £224/ha but 11 times under the trial system at a cost of £152. Just 1% mildew infection was recorded and the reduction by five sprays saved £72/ha in fungicide alone. The few blight spots seen were blamed on a four-day hold-up in spraying because of rain.

Farmer Jacques Pussemier, from the Ferme de Vignay, at Gironville sur Essonne, directly south of Paris, was pleased with his use of a similar weather station and the AddVantage prediction system. He tried it out on a crop of Agata, irrigated using either a rain gun or an underground system, and saved himself two treatments, bringing total blight control costs back to £260/ha. Although he tries to use mancozeb most of the time, he invariably has to back this up with more expensive, top of the range blight fungicides.

All 113ha of potatoes he grew in 2000 were grown at his merchant buyers request under the "production raisonné" system. That means he has to justify all inputs on varieties such as Charlotte, Monalisa, Agata and Liseta. Like most large-scale French growers, daily use of the sprayer against blight has been the norm in the past but he accepts that such routine treatments must change to meet the requirements of the trade, consumers and a society becoming increasingly concerned about the environment.

Farming around 50 miles from the nearest major agricultural weather monitoring station, he welcomed the independent decision-making ability gained from having his own weather station. "The first thing you have to do each day is check the results on the weather station and spray as necessary," he emphasised.

The regional experience of growers and agronomists with the prediction systems is backed up by the results of trials by blight expert Denis Gaucher of the ITPT (Potato Technical Institute) with both the Ullrich-Schrîdter and the similar Negfry modelling systems. Both calculate an index of blight risk with a built-in threshold to trigger spraying based on weather data. Three research station sites and a range of varieties with varying susceptibility to blight were used. Farmers fields, including those of Mr Pussemier, were also included in the trials.

The biggest reduction in the number of treatments was made on the research sites. Compared with routine weekly treatment, spray passes were cut by between 30-47%, depending on the chosen trigger level. Blight control with all the treatments was broadly similar but worthwhile savings underlined the potential of investing in the blight prediction systems.

On the monitored farms, average saving in treatments came to nearly £42/ha in 2000 and £68.50/ha in the lower pressure year of 1999.

Mr Gaucher argued that the need for growers to take quick action against blight meant it was best for producers to use the systems themselves, although a group sharing an agronomist could reduce costs.

Spray cuts

However, Serge Duvachelle, of the SPV (the State-run Vegetable Protection Service), said its subscription blight warning service based on regional weather stations and telephone or fax warnings could also help potato growers fight blight and meet consumer and environmental requirements. Last year, the service, which uses Guntz-Divoux and Milsol modelling systems to predict blight, cut up to three sprays from the routine treatments otherwise used by growers.

In his no-nonsense approach to tackling blight in 2001, growers should be prepared to hit the first signs of infection hard with a robust product, following up with two more sprays at three-day intervals. However, he also urged growers to use varietal resistance in their anti-blight strategies, highlighting resistant varieties such as Santé and Eden and intermediate types such as Nicola.

NB. An exchange rate of 10 French francs to the £ has been used