SHOOT GROWTH-RATE AND DENSITY AFFECT BUD NECROSIS OF RIESLING GRAPEVINES

Examination of 'Riesling' grape (Vitis vinifera L,) in Virginia sugges ted that a high incidence of bud necrosis (BN) in some vineyards was a ssociated with canopy shade and rapid shoot growth. BN appeared to ori ginate as an abortion and dehydration of the primary, and occasionally secondary, buds of the developing dormant bud. BN frequency was lowes t among the basal four nodes of a given shoot or cane, and increased i n frequency through node 20. Experiments were conducted in 1991 and 19 92 to evaluate the specific involvement of shoot growth rate and canop y shade on 'Riesling' BN. Shoot growth rate (SGR), measured in a 17-da y period around bloom, had a significant, positive relationship with B N in one of two vineyards. BN was positively associated with cane diam eter and average internode length, Applying the growth retardant paclo butrazol significantly reduced SGR and;BN incidence up to 80% among no des 6 to 15 in two separate vineyards. Artificial shade (64% or 92% re duction in photosynthetic photon flux), suspended over vine canopies i n the 3-week period before veraison, did not affect BN. Shoots of cano pies that had been thinned before bloom to 10 shoots/m of canopy expre ssed slightly lower BN levels than shoots sampled from canopies that h ad been thinned to 20 shoots per meter. 'Riesling' BN appeared more in fluenced by shoot vigor than shade under Virginia growing conditions. Chemical name used: a-(1,1-dimethyl-ethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).