Epidemiology of apple scab (Venturia inaequalis (Cke) Wint.) 1. Frequency of airborne spores in orchards

The number of ascospores caught by automatic volumetric spore traps differed widely in six orchards where the air was sampled in some or all of the years 1955 to 1959. To compare amounts of inoculum in different orchards the total catch in each was expressed as the ‘relative ascospore dose’. The reason for the much smaller doses in the later years is uncertain. The results confirmed earlier work on the phenology of ascospore liberation and stressed the practical need to decrease their number as much as possible.

Because conidia have previously been trapped almost only on wet and windy days, they have been considered to be rarely air‐dispersed. Our apparatus, which efficiently traps dry conidia but catches water‐borne ones only when in very small droplets, caught airborne conidia in all orchards when they operated during summer. Traps among heavily infected, unsprayed trees caught most conidia around noon on warm dry days and the highest concentration measured exceeded 1300 per m.3of air.