Concurrent Spore Release and Infection of Lettuce byBremia lactucaeDuring Mornings with Prolonged Leaf Wetness

Abstract

In a previous field study in California, infection of lettuce by Bremia lactucae, the downy mildew pathogen, occurred mainly on days on which leaves dried late in the morning. This observation led to the hypothesis that spore release and infection take place concurrently during mornings with prolonged leaf wetness. To test this hypothesis, spore dispersal experiments were carried out during 13 nights and mornings for which prolonged leaf wetness was expected. At sunset, diseased spore-source plants (infected with a metalaxyl-insensitive isolate of B. latucae) and healthy spore-trap plants (grown in metalaxyl-amended nutrient 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 ended later in the day, the trap plants were moved to a growth chamber and incubated in conditions favorable for colonization by B. lactucae, but not for infection. They were inspected for disease 10 to 14 days after exposure. Trap plants developed signs and symptoms of downy mildew after experiments from five nights, all of which had leaf wetness persisting for at least 3 h after sunrise. Infection on these plants must have taken place during the morning of the exposure period, concurrently with spore release from source plants. Since a metalaxyl-insensitive 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 could be excluded