The effectiveness of contact fungicides mancozeb in controlling potato leaf blight disease (Phytophthora infestans (Mont) de Barry) in Karo District in the wet month and in the laboratory

The increase of potato harvest area in North Sumatra was not accompanied by the production is caused by Phytophthora infestans, especially in the wet months. The most control method used in farming is using synthetic fungicides with mancozeb ingredients. To achieve efficiency, the effectiveness of management, and safety products, accuracy is needed in the use of concentration and application intervals. This research was conducted from December 2018 to December 2019 at the Laboratory of Plant Diseases, Faculty of Agriculture, Universitas Sumatera Utara and in an experimental field in Berastagi, Karo Regency. Laboratory experiment used a completely randomized design with fungicides concentrations 160 ppm, 240 ppm, 320 ppm and without fungicides. The field experiment was arranged in a Factorial Randomized Block Design with 1st factor was: concentrations of contact fungicide mancozeb 2 gr fungicide/L, 3 gr fungicide/L, 4 gr fungicide/L and without fungicides. The 2nd factor was: fungicide application interval 4 days, 8 days and 12 days respectively. The fungicide concentration could inhibit the growth of P. infestans in vitro. Consequently, the fungicides concentration and application intervals as well as their interactions were not effective in decreasing the severity of potato leaf blight in the field experiment during the wet months.