In southern Wisconsin most of the grape cultivars have reach the E-L* stage 31 (Berries pea-size) stage. One thing I have noticed in many wine and table grape cultivars clusters with poor fruit set.

Vine nutrition plays a significant role in fruit set, with carbon (C) and nitrogen (N) levels being particularly influential. Imbalances in C status can lead to poor fruit set and inflorescence necrosis. Micronutrient deficiencies, especially of boron (B) and zinc (Zn), can further exacerbate this issue, as these nutrients are vital for early season shoot growth and pollen tube generation, necessary for fertilization.

Weather conditions are another critical factor impacting fruit set. Overcast, cool, and wet weather can reduce fruit set depending on when these conditions occur. Cold and overcast weather before bloom can impair floral development, while cold weather during bloom can delay its progression, resulting in reduced fruit set. Rain during bloom physically inhibits pollination and fertilization by diluting the stigmatic surface that receives pollen. These weather impacts are often related to the vine’s C and N status, particularly if growth is stunted during early stages of the growing season.

Additionally, severe damage to the vine’s canopy from events like early fall frost, winter damage, hail, or defoliation due to herbicides or insect feeding can lead to poor fruit set. Such damage disrupts nutrient storage and carbon assimilation, weakening the vine.

By understanding these factors, growers can take proactive steps to manage vine nutrition and weather impacts, ensuring a balanced vine for optimal fruit set and a successful grape harvest.

This article was posted in Grapes and tagged Amaya Atucha, Grapes, Notes from the Vineyard, Poor Fruit Set.