G. Stanley Howell, VESTA Viticulture Coordinator, Professor and Viticulturist Emeritus, Michigan State University
This information is based on known relative productivity of Concord and Niagara primary and secondary buds and was written in 2002, a year with similar conditions to 2012.
The protocol we have used for 30 years is simple. We record the number of nodes per vine retained at pruning, and we keep a column in the data book dedicated to Blind/Frost for each treatment vine. If conditions for the winter or spring have created circumstances which could make an assessment informative, we count each treatment vine twice, as:
For Vitis labruscana cultivars like Concord or Niagara, experience suggests that a primary bud has the capacity to produce 100% of a crop at that node, then the secondary bud for Concord and Niagara at that node would be assessed at about 35%. If both are dead, the most that can be hoped for is a tertiary shoot that can produce leaf area, trap sunlight, and create the potential for next year’s crop. That is why I believe that it is premature to write-off the 2002 crop. It may indeed be a washout, but it is too early to tell. One reason for that potential crop is based on last season. The low 2001 crop due to last year’s poor fruit-set conditions very likely raised the productivity potential for secondary shoots in 2002 due to improved initiation and differentiation of cluster and floret primordia. At this stage this is a speculation, but it does underline the basis for my belief that it is too early to write-off the 2002 Concord and Niagara crops.
In the next few weeks I believe it important that while we not encourage undue optimism about the 2002 crops on Concord and Niagara, and that we also avoid a defeatist “gloom and doom” position until we have better information.
How could we estimate a potential crop in a specific vineyard? This will be a key question as individual growers will likely see portions of their vineyards which have insufficient crop to justify the expense of harvest while other parts have economic potential. For our research purposes, a representative number of nodes (preferably not less than 250) for a given treatment, record the number of Blind nodes (nodes with no shoot), number of Live Primaries, and number of Live Secondaries and express each as a percentage of the total number of nodes counted. It might look like the following:
Nodes counted: 753
Blind nodes: 85
Live primaries: 96
Live secondaries: 572
Based on this there would be:
11.3% blind nodes
12.7% live primaries
76% live secondaries
Given what we said earlier about the relative productivity of primary and secondary buds, 13% of the nodes on the vine were at 100% of a full crop and 76% of the nodes on the vine are at 35% of a full crop (if not more). If we estimate that the low crop in 2001 raised shoot fruitfulness and growers retained more buds than “normal” given the low cash-flow in 2001 (both valid suggestions in my experience) then yields of 9 tons/acre would have been common had we not suffered the frost losses.
Given an estimated 9-ton crop, then one would expect the potential of 1.14 tons/acre from primaries that survived and 2.39 tons/acre based on live secondaries for a 3.5 tons/acre crop. There are several opportunities for this estimate to be erroneous: a) based on the industry levels of buds retained at pruning I have observed (>120/vine) the 9 tons/acre is conservative. Should the potential crop based on such high bud numbers shift the potential pre-frost crop to 12 tons/acre, the potential crop shifts from 3.5 to 4.75 tons/acre; b) There is a tendency for more flowers to set fruit per cluster when the crop is low, and this has been observed across a range of cultivars, including hybrids and vinifera.
While this crop news is not great, neither is it an absolute disaster. As I said, it is too early to tell.
Other Varieties. The big issue for other cultivars is productivity of secondary buds. For most hybrids, we do not have adequate data for relative productivity of secondary shoots, and the issue is further complicated by the potential crop from basal bud shoot growth that produces fruit: this can be substantial depending on cultivar. In 1980, 1981, and 1982 we reported crop from non-count positions of Vidal blanc at 23%, 47%, and 15% respectively (AJEV. 1987. 38:105-112). I lack data, but have observed similar situations in Seyval, Chancellor and de Chaunac. I suspect that others are similarly inclined, but may vary in amount of crop. I have never seen such production in vinifera and it seldom occurs in labruscana.
Potential for Compensation? A final caveat is the potential for compensation, mentioned above. Data suggest that severe pruning will increase fruit set within flower clusters on that vine. Conversely, a high number of nodes retained on unpruned vines are characterized by reduced number of berries per cluster. This suggests the potential of more berries per cluster on clusters of surviving primary and secondary shoots. Further, data also suggest that the mean berry weight on the clusters will increase. Again, this suggests a possibility for more potential yield.
Take Home Message. Never write off a crop until a careful assessment of vineyard status for live buds at count nodes is done, and an estimate is made based on previous crop history, that gives weight to fruitful shoots from non-count positions. Concord and Niagara growers in Michigan wrote off the 2002 crop, did not make necessary sprays as a cost saving approach, and found in mid-July that there was indeed a crop there, about 65% of normal, but economically viable. But, it was a loss since the protective sprays necessary to influence insects (berry moth in particular) and several fungal diseases resulted in harvested loads rejected by the processor.
The situation in 2012 looks even more grim than 2002. Very early warmth has resulted in bud phenology stages commonly analogous to early May, but occurring in late March. The best experience suggests that there will be shoot mortality on vines, but let us respond to that condition with careful assessment, and not fall prey to collective hysteria. The worst case may be true! But let’s have valid answers supporting that situation before we decide to make decisions regarding the coming season’s culture.
Assessing Bud Injury video, Part 1 Cornell University
Assessing Bud Injury video, Part 2, Cornell University
Deacclimation of Grapevines, Brock University, Ontario, Canada
Reviewed by Eric Stafne, Mississippi State University and Tim Martinson, Cornell University