PRIMARY BUD-AXIS NECROSIS OF GRAPEVINES .1. NATURAL INCIDENCE AND CORRELATION WITH VIGOR

The incidence of primary bud-axis necrosis (PBN) was studied from 1980 to 1985 in Australian vineyards of varying vigour. Fifteen cultivars of Vitis vinifera L. were initially examined for the presence of PBN a nd, because Shiraz (syn. Syrah) proved to have the highest incidence, subsequent work emphasized this cultivar. Compound buds at nodes 2 to 9 from the base of the shoot (node 9 being the most distal node) were scored for the presence of PBN. PBN was found to be a significant caus e of unfruitfulness in the Australian vineyards examined. Incidence wa s higher in seeded compared with seedless cultivars. Shiraz had the hi ghest incidence but not as great as for other cultivars reported in Is rael, Japan, Chile and USA. PBN incidence was highest in the basal nod es of thick shoots, especially if the node bore a lateral shoot. Buds with PBN produced more shoots but fewer bunches. Thinning of shoots te n days before and after flowering increased both shoot vigour and PBN incidence. This association was attributed to the greater vigour per s e and not to any change in canopy light environment. PBN-caused loss o f primary shoots is concluded to be a major cause of unfruitfulness in basal nodes of grapevines.