THE EFFECT ON YIELD IN COURGETTE AND MARROW OF THE MILD STRAIN OF ZUCCHINI YELLOW MOSAIC-VIRUS USED FOR CROSS-PROTECTION

The effects on yield in courgette and marrow (Cucurbita pepo) crops re sulting from inoculation with the mild strain of zucchini yellow mosai c virus (ZYMV:WK), have been determined in polythene-house trials and in three years of outdoor, commercial field trials. In polythene-house trials ZYMV:WK inoculated plants were up to 10 days later in flowerin g than uninoculated plants and their cumulative yields were between 5% and 26% less than uninoculated plants depending on the cultivar. In m ost field trials cumulative yields from inoculated plants were between 4% and 38% less than uninoculated plants depending on the site and cu ltivar, but in one trial the yield was 7% higher from inoculated plant s. In all experiments, courgette and marrow fruits harvested from ZYMV :WK inoculated plants were symptomless and indistinguishable from frui t harvested from uninoculated plants. The mild leaf symptoms induced b y ZYMV:WK infection did not intensify to severe leaf symptoms and wher e there were natural outbreaks of severe ZYMV infection, fruits from i noculated plants remained symptomless whilst those from uninoculated p lants were severely affected and unmarketable.