Grape variety used as the underground portion of a grafted vine onto which the scion is grafted. Bred to provide pest resistance and/or adaptability to unfavorable soil or environmental conditions.

Tertiary bud

The smallest of the three buds in an overwintering compound grape bud; generally more cold hardy than either primary or secondary buds but the least fruitful; usually does not develop unless the primary and secondary buds have been killed.