Effect of timing of application of the biological control agent Microsphaeropsis ochracea on the production and ejection pattern of ascospores by Venturia inaequalis

Field and in vitro trials were conducted to establish the influence of the biological control agent Microsphaeropsis ochracea on the ejection pattern of ascospores by Venturia inaequalis and on apple scab development, and to establish the best timing of application. The ejection pattern of ascospores was similar on leaves sprayed with M. ochracea and on untreated leaves. Fall application of M. ochracea combined with a delayed-fungicide program was evaluated in orchards with intermediate and high scab risk. For both orchards, it was possible to delay the first three and two infection periods in 1998 and 1999, respectively, without causing significant increase or unacceptable leaf and fruit scab incidence. To evaluate the best timing of application, sterile leaf disks were inoculated with V. inaequalis and then with M. ochracea 0, 2, 4, 6, 8, 10, 12, 14, and 16 weeks later. After incubation under optimal conditions for pseudothecia development, the number of ascospores was counted. Similarly, M. ochracea was sprayed on scabbed leaves on seven occasions from August to November 1999 and 2000. Leaves were overwintered on the orchard floor and ascospore production was evaluated the following spring. Ascospore production was reduced by 97 to 100% on leaf disks inoculated with M. ochracea less than 6 weeks after inoculation with V. inaequalis, but ascospore production increased with increasing period of time when M. ochracea was applied 8 to 16 weeks after the inoculation with V. inaequalis. In the orchard, the greatest reduction in production of ascospores (94 to 96% in 2000 and 99% in 2001) occurred on leaves sprayed with M. ochracea in August. The production of ascospores was reduced by 61 to 84% in 2000 and 93% in 2001 on leaves sprayed with M. ochracea in September, reduced by 64 to 86% in 2000 and 74 to 89% in 2001 on leaves sprayed in October, and reduced by 54 and 67% in 2000 and 2001, respectively, on leaves sprayed in November. It was concluded that M. ochracea should be applied in August or September and that ascospore maturation models and delayed-fungicide program could be used in orchards treated with this biological control agent.