The Negative Prognosis

The use of a negative prognosis means not to spray as long the prognosis answers the question about the presence of the pathogen in field with NO. This explains the term negative prognosis. The Schrödter and Ullrich Negativ prognosis have been published in the year 1972. It uses temperature, leaf wetness or high relative humidity and rain to assess the propagation of the pathogen in the field. A value in between 0 and 400 is indicating the propagation of P. infestans in the field. This value increases if air temperature is in between 15°C and 20°C, if relative humidity is higher then 70%. It increases faster for all times if relative humidity is higher than 90% and there is precipitation or if there is leaf wetness for more than 4 hours. If this situation is for longer than than 10 hours the increase is higher.

Thresholds: Schrödter and Ullrich are defining a value of 150 to correspond with a disease incidence in the field of 0.1%. A value of 250 corresponds with a disease incidence of 1%. They suggest that after a year with a low pressure of late blight in the seed producing area no sprays are needed before a value of 250 is reached. If a higher amount of inoculum has to be assumed sprays should start at 150.

Wheras the original model defines the start of calculation with the emergence in the specific field, we changed the start of calculation to a temperature based rule making sure that we calculate as soon as the first possible tomato will grow. We will calculate as soon as temperature from 10:00 to 18:00 is higher than 8°C and night temperature is never below 2°C.

PInfestans NoBlight The negative prognosis has been used very successful starting form 1972 until the nineties of the last century. This has been the time before we could find resistance against Metalaxyl. The first spray in this years was usually done by Metalxyl and with this the field could be cleared up from P. infestans. Now big areas have resistance against this compound and we do not have any fungicide showing a similar clearing up effect.
In areas where covered potato is grown beside of open field potato we suggest to start spraying as soon as the plastic is removed from the covered crop. The disease can develop under the plastic and the covered crop will become an inoculum source after uncover.
P. infestans grows systemic inside the sprout. This is important if we have latent infected seed. The systemic growth is much favoured by water over saturated soil. To be able to receive information over water saturation of the soil we suggest the use of watermark sensors. Watermarks are very economic and very helpful for potato irrigation. If we have a period of several hours after emergence where the water tension of the watermark sensor is below 10 cBar (100mBar) and more than 10°C air temperature we have to assume good conditions for systemic growth of the pathogen and we have to start with the sprays against late blight. The NoBlight model:Severity Values are determined, according to the model of Maine (By Steven B. Johnson, Ph.D., Extension crops specialist).

SEE: http://umaine.edu/publications/2418e/#tableImage  1

[]Table 1: Calculation of Severity Values
Temperature °FHours of 90% or higher relative humidity (RH)
At 45–54°F, >27 hrs. 90% RH                        At 55–59°F, >24 hrs. 90% RH            At 60–81°F, >21 hrs.
(Total hours – 1)                                             (Total hours – 1)                                 (Total hours – 1)
SV = —————— -4                                     SV = —————— -3                         SV = —————— -2
3                                                                      3                                                          3