Treatment Plan

Poor fruit set can be caused by several factors and may be referred to as “coulure” or “millerandage”.  Coulure refers to poor fruit set, where soon after flowering small berries shatter excessively from the clusters.  Coulure is caused by low carbohydrate levels in vine tissue due to cold, cloudy and wet weather during bloom; from insufficient leaf area and photosynthesis to support the developing berries; and, to excessively vigorous shoot growth during bloom caused by excessively fertile soils, too much nitrogen, overly severe pruning, or overly-vigorous rootstocks.  Millerandage refers to a similar phenomenon in which fruit set is poor, and clusters contain “hens and chicks” with a few normal-size berries and many smaller, seedless berries.  Millerandage can be caused by boron deficiency, and perhaps low carbohydrate levels during bloom.  Additional causes of poor fruit set include zinc deficiency, pre-bloom feeding injury on florets by banded grape bug and lygus bug, and immediate pre-bloom or bloom feeding by rose chafer.  Bloom time petiole sampling for nutrient deficiencies such as zinc and boron are recommended, especially where insect feeding and poor weather during bloom are not suspected as the cause of poor fruit set.