EFFECTS OF ENVIRONMENTAL FACTORS ON BIOLOGICAL CHARACTERISTICS OF LITCHI DOWNY BLIGHT (PERONOPHYTHORA LITCHII)

Litchi downy blight, caused by Peronophythora litchi, decreases litchi production and fruit quality. The fungus infects litchi fruit through oospores developing into sporangia and zoospores being released by the sporangia in the litchi orchard. Our study investigated the biological characteristics of zoospore and oospore germination. If the germination temperature of sporangia was equal to or below their culture temperature, the sporangia germinated by releasing zoospores. No zoospores were released when P. litchii was kept at 28°C or higher. When P. litchii was cultured at 26°C on 20% litchi flesh agar, the optimum temperatures for zoospore release were 12-20°C and the optimum pH was 5-7. The optimum temperatures for zoospore germination were 20-24°C and the pH was 7-9. Release of zoospores by sporangia and germination of zoospores relied on the availability of free water. The oospores of P. litchi could be produced in a juice medium of litchi pericarp, leaf or tree bark. The highest germinating rate of oospores was 49.68% which was achieved under optimal conditions at 28°C, pH 5 and 12 h alternating illumination. Accordingly, rainfall, night and morning dew and moderate temperatures in the litchi orchard are all conducive to zoospore infection.