Late blight, caused by Phytophthora infestans is commonly thought to be the factor most limiting yield in organic potato production. However, because there is no fully effective fungicide available to control late blight, there are virtually no yield loss data available for organic farming conditions.
In large-scale experiments covering 2-6 ha from 2000-2002, late blight assessments were conducted throughout the season in small sections throughout the field. The same sections were harvested, resulting in between 400 and 700 data points per experiment and year. In a second set of experiments, from 2002-2004, the interactive effects of N-availability in the soil, climatic conditions and late blight were studied in the presence and absence of copper fungicides for the mid-early main-crop potato variety Nicola. Again, late blight and yield assessments were conducted within defined sections in the field resulting in about 100 data points per experiment. In 2005 and 2006, new copper products with minimal copper contents (157g Cu/ha and applica-tion) and optimised applications using the model Bio-PhytoPRE were integrated.
Depending on year and variety, between 0 and 40% of the variation in yield could be explained through late blight severity. Copper fungicides in most cases did slow down epidemics somewhat adding an average of 3 days to the growth duration. However, only 26% of the variation in yield could be attributed to disease reductions. A multi-variate model including disease reduction, growth duration and temperature sum, and soil mineral N contents for the years 2002-2004 (FINCKH et al., 2006) could explain 61% of the observed variation in yield. However, the model failed when N-supply was extremely high.
In 2005-2006, without the forecasting model, copper had no significant effect on dis-ease in plots with low nutrient availability while minimised applications combined with the forecasting model resulted in more reliable disease reductions even under low nutrient conditions. A reduction of the current maximally allowed Copper inputs from 3 kg to 1.5 kg per ha and year should thus be considered. Overall yield gains through copper applications were only 10% on average. The economic usefulness of copper applications needs to be scrutinised before recommending its use. The implications of the results on the management of organic potatoes will be discussed
