Control of Phytophthora brown rot of lemons by pre- and postharvest applications of potassium phosphite

Citrus Brown Rot (BR), caused by Phytophthora spp., provokes important economical losses mainly in periods of high rainfall. The management of this disease in Florida and Brazilian citrus areas, main orange growers worldwide, includes chemical control using phosphite salts. In Argentina, the world leader in lemon production, these compounds are registered only as fertilizers. In this work, the effect of potassium phosphite on different Phytophthora sp. cellular structures and the conditions to control lemon BR by it application at pre and post-harvest stages were evaluated. Phosphite inhibited in vitro the mycelial growth, the sporangia production, and the motility and germination of zoospores of a local isolate of Phytophthora citrophthora. In postharvest applications on artificially inoculated lemons, the phosphites exerted a moderate curative activity, reducing BR incidences similar to 25% in respect to controls. When this salt was applied a week before inoculation, BR incidences were 50-60% lower than those of controls, denoting a significant preventive activity. The application of phosphite with fungicides in commercial packingline prevented BR disease in fruit inoculated at 96 h post-treatment. In pre-harvest, two phosphite applications reduced incidences similar to 40-60% in lemons harvested and inoculated up to 75 d after treatment. Our data confer valuable technical information towards the use of phosphite salts against lemon BR, contributing to the pre- and postharvest management strategies of this disease.