October, 1999


Managing for possible mixup of Gisela 5 and Gisela 6 rootstocks

By Gregory A. Lang
Washington State University

A recent mix-up in cherry rootstock liner propagation has resulted in the inadvertent substitution of Gisela 6 for Gisela 5 rootstocks, affecting the 1999 delivery (and expected 2000 delivery) of a number of finished tree orders from several nurseries.

This article will chronicle the problem and explore the ramifications for those sweet cherry growers who may be affected (as well as offer suggestions for those yet contemplating the possibility of planting an orchard or test plot with these interesting new rootstocks).

The problem

Until technological advances can make routine plant identification tests quick, inexpensive, and conclusive, the tree fruit industry is destined to have to deal with the occasional occurrence of mixed propagation materials. Fortunately, the Gisela rootstock switch was discovered relatively early, considering that it can be notoriously hard to recognize differences in tree fruit varieties or rootstocks until fruiting commences.

The discovery of this mix-up was led by several alert tree fruit nurseries with good horticultural observational skills. They noticed that liners of Gisela 5 and Gisela 6 rootstocks appeared to show variability in subtle leaf characteristics that were not consistent with their labels.

This led Gisela Inc. and its German partner, the Consortium Deutscher Baumschulen, to undertake state-of-the-art DNA testing on certain production batches of Gisela liners and "mother block" source plants. These tests were conducted in collaboration with Amy Iezzoni of Michigan State University, Darush Struss of Germany, and the International Dwarf Fruit Tree Association.

Together with the detective work of the involved nurseries, they determined that some of the 1997 and 1998 spring deliveries of Gisela 5 rootstock liners were actually Gisela 6. As the liners are budded in the fall and grown for another year before delivery to orchardists, this means that some nursery trees sold as Gisela 5 and delivered to growers for planting in spring 1999 are actually on Gisela 6.

Similarly, some grower orders for trees on Gisela 5 that were budded in 1998 and nursery-grown this summer (1999), to be delivered for spring 2000 planting, are also actually on Gisela 6. This information was subsequently passed on to the licensed Gisela tree fruit nurseries in July, and each affected nursery has subsequently tracked down their affected customers (including tree fruit scientists!) for notification during the past month and a half.

The 1987-88 NC-140 regional rootstock (and other subsequent) research trials have shown that Gisela 5 and Gisela 6 are similar in their abilities to induce precocious bloom, high productivity, and more horizontal lateral branching. Both have displayed good tolerance of infection by the pollen-borne viruses prune dwarf and Prunus necrotic ringspot.

Where they differ is in their level of vigor control. In both western trials (Bing on irrigated orchard sites) and eastern trials (Hedelfingen and other varieties, some sites of which were not irrigated), Gisela 5 has yielded a small tree about 40 to 50% the size of trees on Mazzard, Mahaleb, or Colt rootstock. Gisela 5 is currently the primary rootstock of choice in Germany, where it originated, for most growers planting high-density orchards of small trees.

However, Gisela 6 has tended to yield larger trees across most test sites, even approaching Mazzard in tree size on irrigated, fertile orchard test sites in the Pacific Northwest (yet with earlier and higher production, and a more spreading habit). In Germany, vigor has been 65 to 80% that of trees on Colt. In some eastern North American cherry trials with Hedelfingen (some on marginal soils under non-irrigated conditions), trees on Gisela 6 were as small as 40 to 50% of "standard" tree size.

Subsequent eastern trials have seen more vigor with different varieties and/or better management to avoid the incidence of young orchard stress that may have plagued some of the early NC-140 trials.
Consequently, at least three important universal tenets can be derived from these research trials regarding both Gisela 5 and 6. First is that both rootstocks are likely to respond to stressful conditions by stopping growth before such responses would be observed in trees on Mazzard or Mahaleb.

Second, due to similar levels of precocity and high productivity, trees on both Gisela rootstocks require skillful management of fertility, irrigation, and annual pruning to encourage new leaf area and keep cropping in balance, lest small fruit and de-vigorated trees result. The more naturally vigorous Gisela 6 is generally not as challenging in this regard as is the smaller Gisela 5.

Third, final tree size can clearly be altered by the inherent "soil vigor" of the orchard site, subject to differences in soil depth, texture, water-holding capacity, fertility, etc. Trees on either rootstock are likely to be smaller than published research reports suggest if soil conditions are less optimal. Thus, on marginal sites, trees on Gisela 5 may be too small for good cropping, and conversely, on excellent sites, trees on Gisela 6 may be too large for containing in very high density training systems.

Gisela 5 vs. 6 planting considerations

So, the main concern of most growers affected by the Gisela 5/6 switch is trying to determine whether their planned orchard spacing is inappropriate for the anticipated final tree size, and what to do if this is the case. It is fortunate that only one year's worth of new orchards already in the ground are affected, as growers have the widest range of options available to best manage or modify this situation.

Certainly, the first step is to consider the above information on trial experiences with trees on Gisela 5 and 6 relative to orchard site "soil vigor," whether or not the orchard will be irrigated regularly, and the final tree architecture anticipated for whatever training system is being adopted. Scion variety may even play a role, though little good data exists as yet for varietal differences. A general rule might be that varieties that are highly productive and precocious on standard rootstocks, like Rainier, are probably going to be easier to tame that extra vigor of Gisela 6 in close spacings than would a less productive variety.

Once affected growers have evaluated their inherent site vigor factors, training system plans, and variety traits, some may conclude that they actually might be better off with Gisela 6 rather than Gisela 5. However, there will certainly be others who find themselves having planted at orchard densities likely to require further modification of their management strategies.

For example, a Pacific Northwest grower indicated that his now-Gisela 6 Bing trees are planted at eight by 15 feet (about 360 trees/acre), which was his plan for a high-density orchard of multiple leader trees on Gisela 5. His orchard site is a relatively deep silty loam soil with good fertility, undertree sprinkler irrigation, and the warm, sunny growing conditions of eastern Washington.

Under this situation, it will likely be a challenge throughout the life of the orchard to contain these trees in their currently allotted space. One option, given the young tree age and that more orchard land may be available (perhaps an existing old Red Delicious apple or Lambert cherry block?), would be to transplant every other young tree into a new orchard this winter by tree spade or backhoe. This would result in a modified original orchard that is spaced 16 by 15 feet, certainly a manageable spacing for trees that are going to require more rigorous pruning in any case to balance fruiting capacity with leaf area.

A second option would be to prune every other tree (the "primary trees") as you would if you were developing an orchard for a final 16 x 15 foot spacing. The intervening trees (the "temporary trees") would be pruned to fill their space rapidly to contribute to early yields. But as the primary trees reached the temporary tree space, the temporary trees would be pruned back more severely over time until the primary trees could fully fill a 16-foot spacing and the temporary trees could be removed. This dedication to differential pruning is critical if the eventual removal of intervening trees is not going to disrupt annual yield capacity.

An option that delays (but prolongs) the need for intensive management would be to simply try to prune all of the trees more aggressively as they filled their space. The goal would be to wisely balance annual thinning cuts and heading cuts to intensively regulate tree size and crop load. Summer thinning cuts would help control tree size while dormant thinning/heading cuts would help induce young replacement wood close to each tree's permanent structure.

Dormant or post-harvest heading cuts would help prevent excessive expanses of fruiting spurs that tend to set more fruit than leaves can support for good fruit size. To help facilitate containment at this close spacing, possibly changing the tree architecture over from multiple leader to central leader (targeting somewhat of a spindle architecture) during the second and third growing seasons may delay production by a year but lessen management challenges in the future.

Another intensive management option may be to allow the existing trees to fill their eight by 15 foot space, and begin limiting annual growth via reduced irrigation. Regulated deficit irrigation techniques have been around for nearly two decades, and a new strategy of alternate root zone irrigation under preliminary study at WSU/Prosser may provide another strategy. Of course, deficit irrigation is difficult to achieve in locations with plentiful summer rainfall.

Alternatively, perhaps at the existing trees fill their eight by 15-foot spacing, one or more new growth regulators may provide growers an alternative tool for annual control of mature tree vigor. In very preliminary studies conducted in the Pacific Northwest, the currently experimental growth regulator, Apogee (BASF Corporation) has shown good promise for controlling cherry tree vigor.

What if affected growers have orchard spacings different from the above example? Trees on Gisela 6 spaced 12 feet or more apart in the row, on a moderate to vigorous growing site, are probably readily manageable as is, at least within the level of increased management intensity one should expect with any Gisela rootstock. Trees spaced at 10 feet might yet be manageable on some eastern North American sites with some cultivars.

In the west, trees spaced at 10 feet are likely candidates for any of the strategies outlined above for those at eight feet, unless the soil factors are quite poor for potential vigor. Trees spaced at six feet are likely to be too vigorous for any intensive management strategies, leaving only the options of alternate tree transplanting this winter or alternate tree differential pruning (with complete removal of the "temporary" trees in a couple of years).

As for tree row spacing, growers generally have a lot more flexibility (compared to tree spacing) to adapt tree architecture to the tractor aisles, reserving only enough space for the tractor to pass without damaging branches. The worst case scenario is for rows of trees on Gisela 6 to be spaced as tight as 10 feet, which would necessitate transplanting or removal of every other row.

Thus far, I have yet to hear of any situations with rows spaced this closely. However, we do have a small trellis trial at WSU/Prosser planted at 12 feet row spacings in which the Gisela 5 trees have turned out to be Gisela 6 - giving us the inadvertent research objective of managing the same kind of problem that growers are now facing!

In summary, this mixing of rootstock identities in several batches of Gisela nursery liners appears to be relatively manageable, in part due to early nursery detection, rapid confirmation of the problem, and the diverse research experiences of cherry scientists across the country in coordinated projects like the cherry rootstock NC-140 trials.

In any case, inappropriate spacing is a situation that numerous apple growers have faced, and often managed well, over the years from inadvertently putting together the wrong combination of scion variety, rootstock, and soil type. Such are the growing pains of dealing with a greater choice of orcharding tools.

 

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