top grafting or top working, the viticultural operation of changing the fruiting vine variety of a mature vineyard by inserting a bud of the selected variety in each vine, but retaining the established root system. An array of approaches is available: cleft grafting, notch grafting, chip budding
, or t-budding, usually applied high on the original trunk just below the head. If the operation is done well, only one season’s crop is lost. The main risk is that of systemic disease spread from the original planting, especially virus diseases to which different varieties and rootstocks have different tolerances.