Ann. appl. Biol. zyxwvutsrqp (1996), zyxwvutsr 129:247-259 Printed in Great Britain 247 The effect on yield in courgette and marrow of the mild strain of zucchini yellow mosaic virus used for cross-protection By N J SPENCE, A MEAD, A MILLER, E D SHAW and D G A WALKEY Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, UK (Accepted 25 August 1996) Summary The effects on yield in courgette and marrow (Cucurbita pepo) crops resulting from inoculation with the mild strain of zucchini yellow mosaic 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 flowering than uninoculated plants and their cumulative yields were between 5% and 26% less than uninoculated plants depending on the cultivar. In most field trials cumulative yields from inoculated plants were between zyxwv 4% and 38% less than uninoculated plants depending on the site and cultivar, but in one trial the yield was 7% higher from inoculated plants. In all experiments, courgette and marrow fruits harvested from ZYMV:WK inoculated plants were symptomless and indistinguishable from fruit harvested from uninoculated plants. The mild leaf symptoms induced by ZYMV:WK infection did not intensify to severe leaf symptoms and where there were natural outbreaks of severe ZYMV infection, fruits from inoculated plants remained symptomless whilst those from uninoculated plants were severely affected and unmarketable. Key words: Yield, courgette, marrow, mild strain, ZYMV Introduction Zucchini yellow mosaic virus (ZYMV) (family Potyviridae; genus Potyvirus) has caused severe losses in cucurbit crops throughout the world since the late 1970s (Lisa et al., 1981; Lecoq, Pitrat zyxwvuts & CICment, 1981; Prowidenti, Gonsalves & Humaydan, 1984; Nameth, Dodds, Paulus & Kishaba, 1985; Al-Musa, 1989; Ullman, Cho & German, 1991; Walkey, Lecoq, Collier & Dobson, 1992). In the UK, ZYMV has continued to cause disease outbreaks in courgette and marrow (Cucurbitapepo) crops, with numerous crops being lost in the 1991 and 1992 growing seasons. In 1991, the disease first appeared in the Vale of Evesham area in mid- season (July and August) and caused complete loss of the late-sown courgette crop at five sites and serious losses in a marrow crop at a sixth site. In 1992, the first outbreak of the disease in the Vale of Evesham was observed in mid-August and serious losses in courgette crops occurred at two sites. A small number of ZYMV infected plants were also observed in a marrow crop late in the season (4 September). At present there are no ZYMV-resistant courgette or marrow cultivars available, but control of ZYMV by mild-strain cross-protection has been successfully used in France (Lecoq, Lemaire & Wipf-Scheibel, 1991), Taiwan (Wang, Gonsalves, Prowidenti & Lecoq, 1991) z 0 1996 Association of Applied Biologists