STUDIES ON THE CONTROL OF ZUCCHINI YELLOW MOSAIC-VIRUS IN COURGETTES BY MILD STRAIN PROTECTION

Mild-strain cross protection was used in field trials at Wellesbourne and in Jersey in attempts to protect courgette plants against severe strains of zucchini yellow mosaic virus. Test plants were sap inoculated with the mild strain and then challenged by an aphid-transmitted severe strain after different periods. Approximately 14 days of incubation were required following mild-strain inoculation to provide protection against subsequent infection by severe strains. No protection occurred if severe strains were introduced 2 days (48 h) after mild-strain inoculation and protection was intermediate if the severe-strain challenge occurred after 7-8 days. Some breakdown in protection occurred in the later stages of the trial at Wellesboume, but not in Jersey. This loss of protection may be associated with high inoculum pressures.