CONCURRENT SPORE RELEASE AND INFECTION OF LETTUCE BY BREMIA-LACTUCAE DURING MORNINGS WITH PROLONGED LEAF WETNESS

In a previous field study in California, infection of lettuce by Bremi a lactucae, the downy mildew pathogen, occurred mainly on days on whic h leaves dried late in the morning. This observation led to the hy pot hesis that spore release and infection take place concurrently during mornings with prolonged leaf wetness. To test this hypothesis, spore d ispersal experiments were carried out during 13 nights and mornings fo r which prolonged leaf wetness was expected. At sunset, diseased spore -source plants (infected with a metalaxyl-insensitive isolate of B. la ctucae) and healthy spore-trap plants (grown in metalaxyl-amended nutr ient solution) were placed next to each other outdoors. Lesions on the source plants sporulated at night, and spore release (measured with a volumetric spore sampler) began at sunrise. After leaf wetness had en ded later in the day, the trap plants were moved to a growth chamber a nd incubated in conditions favorable for colonization by B. lactucae, but not for infection. They were inspected for disease 10 to 14 days a fter exposure. Trap plants developed signs and symptoms of downy milde w after experiments from five nights, all of which had leaf wetness pe rsisting for at least 3 h after sunrise. Infection on these plants mus t have taken place during the morning of the exposure period, concurre ntly with spore release from source plants. Since a metalaxyl-insensit ive isolate of B. lactucae was used in all experiments and trap plants were grown in metalaxyl-amended nutrient solution, the possibility of infection by inoculum occurring naturally at the experimental site co uld be excluded.