The contributions of potassium phosphite and hormonal pathways to the control of Phytophthora infestans in tomato cv. Micro-Tom

Phytophthora infestans causes the destructive late blight disease in tomato and its control is achieved mainly by fungicides. Resistance inducers such as potassium phosphite (KPhi) represent an environmentally friendly alternative for control. The effect of KPhi on disease severity and on the activity of antioxidant enzymes was investigated in wild-type (WT) Micro-Tom plants and mutant/transgenic genotypes were used to determine the hormonal pathways required for resistance. KPhi presented a systemic action and reduced disease severity up to 86% depending on the mode (foliar spray or irrigation) and time of application (3 or 7 days before inoculation). The compound reduced the concentration of phenolics and altered the activity of antioxidant enzymes, notably of catalase, whose activity was reduced even before pathogen inoculation, indicating a physiological effect. In addition, KPhi was toxic to the pathogen in an in vitro assay. Disease severity on plants that overproduce ethylene, are insensitive to brassinosteroid (BR) or do not accumulate salicylic acid was higher than in the WT, revealing that these hormonal pathways are involved in the reaction to P. infestans. The results indicate a possible mode of action of KPhi through the early accumulation of H2O2 and interplay with the salicylic acid pathway. This is the first report on the involvement of BR against this pathogen and the results point to a positive effect on resistance. This study extends the understanding of the role of KPhi as a disease control agent, confirming it as a good option to be used in the management of late blight.