Epidemiology of apple scab (Venturia inaequalis (Cke.) Wint.) 3. The supply of ascospores

The prevalence of Venturia inaequalis ascospores in orchards was compared both in terms of the number of spores per volume of air (dose) and the number produced per area of dead leaf (productivity). The two parameters often gave divergent estimates, probably because dose depends on total leaf per unit area of ground whereas productivity does not. Differences in the amount of dead leaf surviving until bud‐burst in different orchards or years were enough to explain the anomalies and suggested that because earthworms often removed over 90% of the fallen leaves by spring, they exercised an important control of ascospore number.

Assessments of fruit surface scabbed, an accepted way of judging scab control, were not related to ascospore productivity the following spring, but estimates on the percentage of all leaves infected at leaf‐fall were. Other natural variables, like the dates when leaves were formed or fell, seemed of minor importance, especially when compared with effects of earthworms or chemicals. Applying dinitro‐ortho‐cresol to dead leaves in spring decreased the number of ascospores liberated by at least 90% and there is evidence of a similar effect from broadcast ammonium sulphate. The spring and summer spray programmes were also important. Measurements in several orchards during an 8‐year period showed that dose and productivity both declined to less than a hundredth of the highest levels, but in a single wet year productivity in sprayed orchards increased several times and by 40‐fold on unsprayed trees. Ways to prevent high ascospore doses occurring are discussed, together with possible causes of past severe attacks of apple scab at Wisbech.