A Guide to Grapevine Abnormalities in South Africa - P.G. GOUSSARD

Bunch stem necrosis Characteristic abnormalities associated with this complex physiological phenomenon manifest as small, dark spots of dead cells that usually develop on the narrowest parts of pedicels (during advanced berry development), whereafter they expand to develop a continuous/girdling band or ring – the consequence being that individual berries are eventually subjected to wilting, shrivelling and drying out. In the case of table grapes especially a similar situation is very characteristic, the difference being that the formation of the band is often restricted to positions where lateral branches develop out of the rachis – whereafter it expands to contribute to the browning and drying out of the affected branch in its entirety. Obviously the same cycle is followed, namely wilting, shrivelling and drying out of berries on such branches (Photos 442 & 443). Growth arrestment phenomenon Although this phenomenon (GAP) is mainly associated with temporary shoot growth/ arrestment in the period between flowering and bud burst (when flower clusters are choked off at a very early stage), comprehensive research investigations revealed that several and even divergent abnormality phenomena may occur, either individually or in combination. Such situations complicate concrete identification procedures, to the extent that it is considered absolutely essential to obtain expert advice at all times. From the spectrum of abnormality phenomena, the characteristic trait of GAP is a delay/arrestment in the elongating tempo of shoots that emanate from successful bud burst, to be followed by active growth later in the season, either without or with smaller bunches only. Such situations therefore indicate that inflorescences, although initiated, may have been choked/ damaged in dormant buds already (even before bud swell). In cases where flower clusters are affected at a later stage – but before calyptra drop – superficial black lesions appear where pedicels are later choked. Usually reduced shoot growth and shiny, dark green leaves may be observed in affected grapevines (Photo 444).

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