Indonesia is the largest potato producer in Southeast Asia. Late blight (Phythophtora infestans) is one of the most important diseases especially in the rainy season. Therefore, farmers frequently spray fungicides to prevent infection. Forty farmers in West Java were trained in implementing good agricultural practices. To assess the impact of the training, late blight control strategies at the trainees’ fields were recorded. The baseline registration in the dry season of 2013 revealed that on average 16.6 times fungicides were applied. The average spray volume was 919 L ha-1, whereas the recommended spray volume ranges from 400 to 600 L ha-1 depending on the crop stage. Many farmers added an adjuvant to the spray mixture to increase the rainfastness of the fungicides. The fungicide costs ranged from 11.4 to 14.7% of the total production costs. The second registration, wet season of 2013/2014, showed a similar number of applications as in the dry season but the fungicide input was clearly higher due to a higher spray volume of 1,052 L ha-1 and more fungicides per application which caused higher fungicide costs ranging from 4 to 35% of the total production costs. Parallel and supportive to the trainings, three late blight demos were implemented in wet seasons. In the first demo (2013/2014), more recently registered fungicides controlled late blight similarly compared to the fungicide mixture often used by farmers, but they were 10-15% more expensive. In the second (2014/2015) and third demo (2015/2016), again the more recently registered fungicides did not perform better and were more expensive. A 20% reduction in spray volume had a similar effect on late blight control as the farmers’ practice using high volumes. At reduced spray volumes, the use of an adjuvant did increase late blight control. It is estimated that the spray volume can be reduced with 10-20% without loss of efficacy and thus save costs.