No. 9 — May 20, 2019

With Bloom Approaching, Remember Petiole Sampling

close-up of florets on plant about to flower

Grape growers that have vineyards just coming into fruit production often have questions about vineyard nutrient management. The questions often have to do with the when and why they should monitor the nutrient status of the vines. Tissue analyses or petiole sampling is normally undertaken for two reasons; routine nutrient status evaluation and to diagnose an observed suspected nutrient deficiency.

The routine assessment of the vines nutritional status should be done at a specific phenological stage. These phenological stages are full flowering and veraison. Full flowering can be defined as when 70% of the caps have been shed, whereas veraison is the onset of rapid fruit maturation, where visually a distinct color change is occurring. This color change is obviously more distinct in red varietals than white varietals. In white varietals, the color change often is from a dull green to a lighter transparent yellow.

The goal of routine nutrient assessment is to gauge the response of the vines to nutrient additions or determine the vine’s nutrient needs. The overall objective is to maintain the nutrient balance to produce high quality grapes by applying only nutrients that are needed, thereby saving money and also protecting the environment.

Petioles are collected from different locations on the shoot depending on the sampling period — full flowering or veraison. At full flowering you select leaves that are opposite of the flower clusters. Whereas at veraison, the 6th or 7th leaf from the shoot tip, which is the most recently fully expanded mature leaf is selected. Avoid collecting leaves from laterals. Once a leaf is collected, break the petiole (leaf stem) away from the leaf blade and discard the leaf blade. You will need to collect 60 to 100 petioles for a sample. These petioles should come from 25 to 50 different vines, but be of the same variety, rootstock, and soil type. If your vineyard has variable soil types, divide your vineyard into blocks and petiole sample each block.

In the case of trying to diagnose a suspected nutrient imbalance in a vine(s), you may only be able to select leaf samples from one or a few vines. Select leaves that are showing the suspected deficiency, but do not collect petioles from leaves that are insect, disease, or mechanically damaged. In addition to this sample, also collect petioles from the same variety that is not showing the deficiency. Send in both samples to be analyzed. You will then have results to compare from a healthy vine(s) and those vine(s) showing a deficiency.

Petioles should be stored in a paper bag to allow the petioles to dry. Petiole samples can be dropped off at University of Missouri County Extension Centers and will be analyzed by the University of Missouri Soil Testing and Plant Diagnostic Services.

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