Cabernet Franc (variety)
Cabernet Sauvignon (variety)
Carignan (Variety)
Carmenère (Variety)
Cienna (variety)
Coulure is a carbohydrate deficiency in plant tissues triggered by cold, cloudy, rainy weather in spring or very high out-of-season temperatures resulting causing grapes to fail to develop after flowering. Grapevines need enough leaf cover to produce photosynthesis but not shade the grapes too much from sunlight. Cloudy or wet weather reduces photosynthesis during the flowering cycle and limited sunshine brings lower sugar levels, encouraging the vine to conserve sugars that would otherwise be channelled into developing berries.
Warm temperatures can also encourage coulure in some varieties by encouraging shoot growth that competes with the berries for carbohydrates. In the same way very fertile vineyard soils, overly vigorous rootstock and severe pruning that limits the leaf surface and reduces photosynthesis can also bring coulure into the vineyard.
Coulure can also cause irregular bunches of grapes which are more sensitive to disease. Varieties that can have seriouus issues with coulure include Grenache, Malbec and Merlot.
