Summary

Forbedret mulighed for reduktion af fungicidforbruget i kartofler

Potato late blight is caused by the fungus Phytophthora infestans and is the pest most damaging to yield and quality in Danish potato production. The fungus occurs every year in potato fields and necessitates several fungicide applications during the season to ensure a profitable potato production.

With financial support from the Danish Environmental Protection Agency, the project “Improved basis for reduced fungicide use in potatoes” started in 2005 for a duration of two years. The objective of the project is to improve the knowledge basis for reducing fungicide use in potatoes. Spore dispersal, survival and infection of potato late blight were studied in field trials in 2005 and 2006 and new methods were developed that will improve the quality of weather forecast data for calculating of the risk of potato late blight development and dispersal. Based on these studies, decision support models for control of potato late blight were developed and tested under field conditions. The project was carried out in collaboration between the University of Aarhus, Dansk Landbrugsrådgivning (the Danish Agricultural Advisory Service) and Danmarks Meteorologiske Institut (the Danish Meteorological Institute).

The results show that the fungus forms its spores (sporangia) during the night and that the spores are released in the morning when there is a substantial drop in relative humidity. Infection can occur in the morning if there is an overlap between the remaining dew drops and the release of sporangia. The formed sporangia can only complete infection if free water is present within a few hours, either from dew or precipitation. The results also showed that newly formed sporangia can remain in the lesions and be dispersed over at least 2-3 days. This means that infection events are not necessarily due to spores that had been produced on the same day but up to several days prior to the infection event.

An index of infection pressure (HSPO) was developed in the project. It is based on sporangia formation as calculated from weather conditions. By coupling the sporangia formation with a model of survival based on global radiation, a better correlation was achieved between infection in the field (on catch plants) and spore formation, than if only infection pressure was used. The model has not yet been tested in field trials, but it is expected that the involvement of the survival model will improve the identification of periods with small risk of new infections. Fungicide use can thus be reduced.

As part of the project, DMI has improved the forecast of relative humidity (RH) during the growing season. Previously, the forecast of RH was 5-10% too low. With the adjustment of RH, the calculation of the daily level of sporangia formation in the forecast has become slightly too high, but as the infection pressure is calculated with weather data dating back 2 days and ahead 2 days, respectively, the error is considerably reduced. It was studied whether a local weather station could be used to adjust the forecast of temperature and RH and thereby achieve a better forecast at field level. The temperature forecast was improved, but the forecast of RH was neither better nor worse.

The results and experience from the project were used for further development of the decision support system Blight Management (BM). In BM, the risk of potato late blight attacks and the appropriate control time are assessed and the fungicide dosage is adjusted according to the risk of infection and cultivar resistance in the respective field. The various BM versions were tested in six field trials in 2005-2006. The results showed that the decision support system BM could reduce the treatment frequency only on few occasions, especially at low infection pressure. However, there were too few trials to carry out a proper validation of BM. The data are thus mainly suited to test whether the models are operational in the field. Parts of the system were implemented in 2007 in the existing decision support system in www.planteInfo.dk.

The results of the project indicate that the climate-based decision support model (BM) is not yet safe enough or designed to be used as a pure decision model that points out a certain date for the application of fungicides. However, BM in its present form can be considered as a tool for growers or advisers which in a combination with other information and experience can help the decision process concerning the choice of dosage and fungicide depending on the risk of dispersal and infection by the late blight fungus.

Experience form practical advice concerning late blight control both at home and abroad shows that there is a large demand among potato growers and advisers for a reliable forecast tool to determine “late blight weather” in order to be able to reduce the treatment frequency and improve the economic results of potato growing. There are therefore great perspectives for extending and using a reliable decision support system to reduce the still increasing consumption of pesticides but also as an aid to fix the time for application of fungicides with different modes of action.

The results and experience from this project can be used in the task of developing decision support for other fungi depending on humidity in especially fruit and vegetables.