New Shoots From Stubs
While the meetings and education sessions at events like Great Lakes Expo are valuable and informative, the posters sharing research from graduate students shouldn’t be ignored. They provide bite-sized nuggets of larger projects worth looking into.
The title on this one is a bit unwieldy, but the research it covers could be invaluable to those with orchards. Check out the highlights from “The Number, Length and Quality of New Apple Replacement Shoots After Renewal Pruning Depends on the Stub Length and Cultivar” from Michigan State University’s T.C. Einhorn and M. Elsysy below.
The research team recognized that the establishment of high-density apple orchards is expensive and to make it worthwhile, growers must prioritize limb management (initiation, placement/selection, vigor control and renewal) to optimize canopy volume, light interception and fruiting potential. The goal is to develop an even, relatively narrow fruiting canopy of “weak” limbs to fit in tight spacing. Overly vigorous limbs are less yield efficient and produce canopy shade, which isn’t great for fruit set, color development, fruit growth, pest pressure, etc. Therefore, the annual renewal of a portion of limbs is required.
The main objective of renewing fruiting limbs is to generate horizontal, relatively weak “replacement” limbs with high fruiting potential, according to the researchers. This renewal process can be achieved with a specialized heading cut, dubbed a “Dutch” or “bevel” cut. The angled cuts expose more surface area on the underside of the remaining stub to facilitate a flat replacement limb from a latent bud.
Recently, apple growers have started leaving very long stubs (termed “four-finger cuts,” about four inches long) when renewing limbs. The Michigan State team doesn’t believe that a one-size-fits-all recommendation should be applied to all cultivars based on the inherent physiology differences among them, often associated with their vigor.
That’s why they designed an experiment to test the effect of stub length on the number and quality of renewal limbs for three major cultivars of contrasting vigor – Honeycrisp (low), Gala (moderate) and Fuji (high). The study took place at the MSU Clarksville Research Center in early spring (before bud swell) in 2019 and 2020 in an orchard with tree spacing of three feet by 11 feet. The orchard grew all three cultivars on G41 rootstock, trained to tall spindles.
The researchers tested four renewal limb treatments (one-, two-, three- and four-inch stubs). Similar trees were selected based on canopy and trunk size, with new trees chosen in 2020. The trees generally had 15 to 20 limbs. The largest limbs were prioritized for removal, and each tree had several limbs renewed.
The results weren’t collected until autumn of both years, but they included stub diameter, the number of renewal shoots per stub, the total and average length of all shoots per stub and the shoot orientation of the stub.
The team found that the total length of the sum of new shoots from a stub was significantly and positively affected by stub length, but it varied from cultivar to cultivar – Fuji did better than Gala, which did better than Honeycrisp. The orientation of renewal shoots (their position around the stub) was not consistently affected by treatments, however, irrespective of cultivar or year.
With Honeycrisp, stubs need to be at least two inches long. A one-inch stub only returns a shoot 50% of the time. The renewal success for a single shoot increased to 95% with a three-inch stub but produces an extremely weak second shoot on 50% of the stubs.
With Fuji, a two-inch stub produced a 12- to 16-inch shoot and a six-inch facilitating selection of an optimum shoot depending on the system. A three- to four-inch stub doesn’t affect shoot length but produces yet another shoot requiring elimination.
And with Gala, a one- to two-inch stub is ideal but in 2019 one-third of one-inch stubs were blank.
According to the researchers, the percentage of stubs returning one or more shoots increased with increasing stub length – but long stubs did not always result in higher quality shoots. They also noted that when multiple shoots were returned on a stub, the longest shoot length was not affected by competition from other shoots. This means having more shoots does not reduce the vigor of the longest shoot.
Depending on the desired length of a renewed piece of fruiting wood, the second or third longest shoot on a stub (depending on the cultivar) could be selected during dormant pruning, they concluded.
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