The Combination of Negative Prognosis and Fry Infection
We combined the negative prognosis model following Schrödter and Ullrich with the assessment for the spray interval by the FRY model this is called NegFry. This combination is used in Denmark and Northern Europe with success.
The negative prognosis model defines the date of the first spray in dependence of the last year pressure a threshold of 150 or 250 is used for the first spray against late blight. This first spray can be still made with a Metalaxyl containing product, knowing that with the first and only application of Metalaxyl we can expect efficacies of 75% to 80%. All the next sprays will be done with preventative products. This can be Mancozep or Chlorthalonil.
In Netherlands and Belgium there have been discussions not to use Metalaxyl containing products at all. In this case the use of the negative prognosis to estimate the date of the first spray might be problematic. As alternative solutions we would suggest in areas with covered early potatoes to start spray as soon as the plastic is removed from the early potatoes. In areas without early potato we suggest the use of watermark sensor to sence for water logging situation. As soon as we have an ambient temperature higher than 10°C and water tensions smaller than 10 cBar (100 mBar) for several hours we have to expect systemic growth of the pathogen starting from latent infected seed tubers. The sprouts of this potatoes will become covered by sporangia over night and epidemic will start with force. After the first water logging situation at temperatures higher than 10°C we have to start the preventative spray program.