On grapevines, spontaneous mutations occassionally give rise to shoots having unique characteristics. Pomologists generally refer to such shoots as "sports". If a sport has desirable characteristics, such as higher yields, or better fruit quality, than the parent plant, the sport may be collected and propagated. Sports that are very different from the parent plant may be considered a new cultivar, or if the differences are less striking, they may be considered a new "clone" of the old variety. Here is an example of a potentially desirable sport of Flame Seedless which had berries that were variegated and uniquely shaped.
Not all sports are desirable, however. An example of an undesirable sport is a mutation that gives rise to "Bull vines" and "Witches' brooms". This mutation is most often observed on old Thompson Seedless or Muscat of Alexandria vines. It starts with a single mutated shoot, but the abnormal growth is vigorous and with time will eventually crowd out normal shoots. The internodes are abnomally short, the nodes are crowded with buds, and the shoots are unfruitfull. Therefore, this sport should be removed as soon as it is noticed, and propagation wood should never be taken from affected vines.
Winter is a good time to look for Bull vines and Witches Brooms, and rogue them from your vineyard.
