Grape varieties are notoriously difficult to tell apart, but people like Christophe, who have worked with them all their lives, can do so at a glance. He says you need to look at the leaves and the wood. By wood he means the annual growth above the graft. Below the graft is the rootstock and will look much the same for all the varieties. He has planted examples of 5 varieties along a barn wall in his farmyard, and he explained the differences to us as follows:
CotCot - known as Malbec outside of the Loire Valley. This is the original red grape grown in the Touraine in pre-phylloxera days. Cot leaves have 3 shallow lobes, pointed teeth and an open V shaped petiolar sinus (where the stem arrives). The leaves are rough, with dark veins. The wood is brown.
Chenin blanc.Chenin Blanc - sometimes called Pineau de la Loire in the Touraine, it is one of the most iconic of the Loire Valley varieties, believed to have been brought from Hungary in the 4th century by Saint Martin. The new buds have a pinkish tinge and young leaves are downy and bronzed. Chenin blanc leaves are 3 or 5 shallow lobed with rounded teeth and a lyre shaped just overlapping petiolar sinus. The leaves are smooth, with pinkish veins. The wood is green.
Cabernet franc.Cabernet Franc - the leaves have a tooth, referred to as a 'tongue' (langue) in French, in the lateral sinuses (the deep indentations between lobes). They are large leaves, with fine veins. In the spring the young shoot tips are bronze and the undersides of the leaves downy. There are 5 deep lobes with slightly jagged wavy edges.
Sauvignon blanc.Sauvignon Blanc - the leaves are small and fairly smooth with rounded teeth and 5 fairly deep lobes and a closed lyre shaped petiolar sinus. The wood is green and the leaf veins quite fine and pale green.
Gamay.Gamay - the leaves are 3 shallow lobed, with a V shaped petiolar sinus and rounded teeth. The surface of the leaves is a bit rough, with quite fine green veins. The wood is brown.
These are the main varieties Christophe grows, in the Touraine and now the new Touraine-Chenonceaux appellations. He also grows some Pineau d'Aunis and Grolleau, both used in rosé.
Apologies to Christophe if I've got any of this wrong.