Effects of fertilizers and plant strengtheners on the susceptibility of tomatoes to single and mixed isolates of Phytophthora infestans

Three organic fertilizers, Horn meal, i.e., BioFeed Basis and Bio-ILSA, and three plant strengtheners, i.e., Alfalfa extract, PEN and QUALITY were compared to chemical fertilizer (NPK) application and the synthetic inducer BABA (DL-3-amino-butyric acid) for reactions of leaf disks of six different tomato cultivars to three isolates of Phytophthora infestans . Late blight severity was significantly reduced on plants fertilized with Bio-ILSA and BioFeed Basis compared to plants fertilized with Horn meal and chemical fertilizer. There were no interactions between fertilizers and isolates or between fertilizers and cultivars. All plant strengtheners significantly reduced susceptibility of all tomato cultivars. However, effects were isolate and tomato cultivar specific. The reductions in area under the disease progress curve compared to the water control for the different tomato cultivars and isolates were 23–78%, 21–77%, 17–66%, and 37–100% for Alfalfa extract, PEN, QUALITY and BABA, respectively. Similar, but somewhat smaller reductions were observed for sporulation capacity. The plant strengtheners were more effective in controlling disease on plants challenged with isolate mixtures than on plants challenged with single isolates. BABA performed significantly better than the plant strengtheners in 65% (i.e., 34 out of 54) of the tested cultivar-isolate combinations, when single isolates were used. When two isolates were mixed, the percentage was reduced to 45% (25 out of 54 cases) and to 33%, (6 out of 18 cases) with a mixture of three isolates.