Coulure

right|thumb|Young grapes are most susceptible to coulure right after flowering when they need vital carbohydrate resources in order to develop fully. 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. In English the word shatter is sometimes used. Coulure is triggered by periods of cold, cloudy, rainy weather or very high out-of-season temperatures. The condition is most often manifested in the spring. It also occurs in vines that have little sugar content in their tissue. Flowers stay closed and are not fertilized. Thus the vines are not pollinated as the grape fails to develop and falls off. Coulure can also cause irregular bunches of grapes which are less compact than normal. These bunches are more sensitive to developing various grape diseases. The yield of a vine with coulure will decrease substantially. Grape varieties with high proclivity to coulure are Grenache, Malbec, Merlot, and Muscat Ottonel. Other causes of coulure may be vineyard conditions and practices, pruning too early or too severely, excessively fertile soils or overuse of fertilizers, and improper selection of rootstocks or clones.

During the flowering part of the growing season (May–June in the Northern Hemisphere, November–December in the Southern Hemisphere), grapevines often need dry conditions with sufficient sunlight and ambient air temperature around for pollination to go smoothly. Less ideal conditions, particularly wet, rainy weather, increases the odds that a higher than normal numbers of flowers go unpollinated and coulure to occur.