Which statement best explains the cause of coulure?

• A. Imbalance in carbohydrate levels due to low photosynthesis and vigorous shoot growth diverting carbs.
• B. Excess sugar accumulation in berries.
• C. High rainfall during harvest.
• D. Low vine vigor.

Answer: (B)

Coulure happens when there isn’t enough carbohydrate available to support the developing fruit right at flowering. The vine needs a steady supply of sugars from photosynthesis to power fruit set; if photosynthesis is limited and a lot of carbohydrates are diverted into vegetative growth (shoots), there isn’t enough energy to complete the process of set for many flowers. That mismatch in carbohydrate supply and demand leads to flowers failing to develop into berries, producing a sparse crop.

The statement describing a carbohydrate imbalance caused by low photosynthesis and vigorous shoot growth diverting carbs best captures this mechanism. Excess sugar in berries isn’t a trigger for coulure, and weather around harvest or simply low vine vigor don’t directly explain the failure of flowers to set during bloom.