Coulure: Understanding the Challenging Phenomenon Affecting Grapevines

MONDAY IN THE VINEYARD SERIES

Delving into the Challenges of Coulure in Grapegrowing and Winemaking

You might find this article useful if you are a curious wine enthusiast or if you are currently preparing for your WSET Diploma D1 or WSET 3 examination. In that case, don't forget to check out our FLASHCARDS as well

In the picturesque landscapes of vineyards, where luscious grape clusters hang like jewels on the vine, a curious and often misunderstood phenomenon known as "coulure" can disrupt the dreams of winemakers. Coulure, a term derived from the French word "couler" meaning "to flow," refers to the natural drop or failure of grape flowers to develop into full-fledged fruit. To a certain degree, coulure and fruit drop represent a vine's natural and beneficial response to self-regulate its resources and fruit yield; however, excessive coulure has a profound impact on yield and on the grapevine's health. In this article, we delve into the world of coulure, examining its causes, effects, and significance in the world of winemaking.

What is Coulure?

Coulure is a phenomenon that occurs during the early stages of grape development when flowers fail to convert into fruit due to unsuccessful ovule fertilization. The process begins with flowering, where tiny grape flowers appear on the vines, ready to be fertilized and transform into berries. However, certain factors can interfere with the natural fertilization process, leading to coulure.

Causes of Coulure

Coulure occurs due to a deficiency in carbohydrate levels. Following the flowering stage, as carbohydrate levels decline, the stems attached to the flowers or small berries start to shrink, causing the small grapes to wither.

A Carbohydrate deficiency may stem from reduced rates of photosynthesis or excessively vigorous shoot growth.

• Reduced Photosynthesis: Weather plays a crucial role in the occurrence of coulure. Cold, wet, and windy weather or excessively high temperature and prolonged water stress negatively impact photosynthesis and therefore fewer carbohydrates are produced., consequently resulting in coulure.

• Vigorous Shoot Growth: excessive growth of shoots diverts carbohydrates away from the inflorescence. The use of highly fertile soils, excessive fertilization, and vigorous rootstocks can all contribute to substantial shoot growth, consequently resulting in coulure.

Effects on Grapevines

Coulure can have several consequences for grapevines and subsequent wine production:

• Reduced Yields: Coulure leads to a lower grape yield, resulting in fewer grapes per vine and potentially impacting the overall harvest size with the consequent financial impact.

• Uneven Ripening: When some berries are missing from the clusters due to coulure, the remaining grapes may ripen unevenly, affecting the quality of the wine produced.

• Irregular bunches: When some berries are missing from the clusters due to coulure, the bunches are less compact than normal. These bunches are more sensitive to developing various grape diseases.

Management of Coulure

Coulure cannot be completely avoided; however, a vineyard manager can adopt various precautions to minimize its severity and effects. Addressing the issue requires a multi-faceted approach:

• Vineyard Management: By practicing thoughtful vineyard management techniques, such as balanced pruning, managing canopy density, and optimizing nutrition, winemakers can reduce the likelihood of coulure.

• Hand Harvesting: In severe cases, where coulure significantly impacts yields, winemakers may resort to hand harvesting to select only the ripest and healthiest grapes.

• Varietal Impact: Certain grape varieties, such as Grenache, Malbec, Merlot, Cabernet Sauvignon and Muscat Ottonel, are more susceptible to coulure than others. To address this issue, a grower has the option to cultivate clones of these varieties which are now readily available and less prone to coulure development or avoid completely planting these grape varieties.

• Climate Considerations: While it is challenging to control weather patterns, vineyard location selection can help mitigate the impact of adverse conditions known to trigger coulure.

Coulure challenges winemakers and shapes the final product in the bottle. Understanding the causes and effects of coulure is critical for vineyard owners and winemakers to implement suitable strategies to mitigate its impact. As the climate continues to evolve and viticulture practices adapt, studying and managing coulure becomes even more relevant.

I hope you found this article both useful and insightful. If you enjoyed it, make sure to stay tuned for my next article. Join me next Monday for more fascinating insights!

Have a happy week and happy studying,

Nicky