2015 San Joaquin Valley Bud Fruitfulness Survey

George Zhuang, UCCE Fresno County

Grape yield per vine is the result of three factors: 1) the number of clusters per vine, 2) the number of berries per cluster, and 3) berry weight. Bud fruitfulness and the number of buds left after winter pruning determine the potential number of clusters per vine, whereas the number of berries per cluster is determined during the period of fruit set, when environmental factors such as temperature and precipitation, and overly vigorous shoot growth may reduce fruit set. Final berry size is affected by many factors, including genetics, water and nutrient availability, crop load, rootstock, and, in table grapes, girdling and the use of certain plant growth regulators. The number of clusters per vine, berries per cluster, and berry weight have the dominating effect on the yield of all grapes, but fruit quality has a major effect on marketable yield of table grapes and soluble solids (sugar) content strongly affects yield and quality of raisins.

F ig. 1. Primary and secondary bud anatomy
(image sourced from Dr. Luis Sanchez)

For grapevines, the potential number of clusters per node is determined the year before, as cluster primordia are initiated on preformed shoots that develop on the buds of greengrowing shoots that can be left as spurs or canes in the following season.

Good light exposure and optimal (77 °F) temperature increase the number of cluster primordia. Water deficit stress can reduce bud fruitfulness starting from current bloom. Excessive or deficit nutrient supply (especially nitrogen) might also limit the inflorescence initiation.

Grape has a compound bud and one dormant compound bud includes a primary bud and two secondary buds (Fig. 1). In this survey, the potential bud fruitfulness per node included cluster primordia from the primary and secondary buds.

Bud Dissection to Assess Fruitfulness

In order to track the potential fruitfulness of grape vines, I conducted a field survey at the beginning of 2015 for different varieties using bud dissection.

Fig. 2. Selma Pete 2015 buds on nodes from 0 to 5 were not as fruitful as buds on nodes 6 t o 13 in Parlier, CA

For Selma Pete, buds on nodes from 0 to 5 were not as fruitful as buds on nodes 6 to 13 (Fig. 2). Less than one cluster (0.2 to 0.7) primordia per node was found in each of the first five nodes, whereas nearly 2 cluster primordia per node were found in more apical nodes (6 to 13) on the cane.

The complete data and information about bud fruitfulness can be accessed at http://ucanr.edu/sites/viticulture-fresno/

Fig. 3. Bud wood collected from the field

What’s Next?

The usefulness of bud fruitfulness data in-creases with each year if the data is repeatedly collected from the same vineyard. For example, comparing next year’s data with the data from this year will provide some evidence to suggest yields might be similar, higher, or lower, and this information can inform pruning decisions since the data can be collected at any time after harvest. However, actual yield per vine can vary due to any changes of one of three yield compo-nents. Therefore, I will continue to collect this data each year, as a service to SJV growers.

I would like to acknowledge the technical support from Dr. Matthew Fidelibus, Dept. of V&E, UC Davis and the SJV grape growers that allowed me to use their vineyards