Coulure

Coulure (pronounced coo-LYUR) is a viticultural hazard that is the result of metabolic reactions to weather conditions that causes a failure of grapes to develop after flowering.

Less ideal conditions, particularly wet, rainy weather, increases the odds that a higher than normal numbers of flowers go unpollinated and coulure to occur.

[2] Coulure is a distinct phenomena unrelated to another viticultural hazard, millerandage, where the flowers are pollinated but the resulting berries develop without seeds and remain small.

As carbohydrate levels drop, soon after flowering the stems connected to the berries shrivel as the small grapes (0.2 inches/5 millimeters in diameter) eventually fall off.

But when the situation is exacerbated by certain weather conditions and disruption to photosynthesis, coulure can have a more severe impact on yields that may negatively affect a region's grape supply and thus influence pricing.

Warm temperatures can also exacerbate coulure in some grape varieties by promoting cellular respiration and excessive shoot growth that further competes with the berries for the resources derived from carbohydrates.

Young grapes are most susceptible to coulure right after flowering when they need vital carbohydrate resources in order to develop fully.
Grapevines need a balance of sufficient leaf cover in order to produce photosynthesis but not shade the grapes too much from sunlight.
Most likely a clear example of coulure.
Malbec can be highly susceptible to coulure but newer clones are being produced with less sensitivity.