Michigan State University horticulturist Dr. Greg Lang visited Stefano Musacchi in Italy in 2008, and subsequently invited him to speak at the International Fruit Tree Association annual conference in Grand Rapids, Michigan, in March. Musacchi joined the IFTA tour of Lang’s work with fruiting walls of sweet cherries growing in high tunnels at the MSU Southwest Research Station near Benton Harbor. Three weeks later, Lang returned to Italy to follow up on Musacchi’s work there at the University of Bologna.
“It’s a way to get a fruiting wall with half the number of trees,” Lang said of Musacchi’s bi-axis work. “It’s a way of diffusing vigor to keep trees smaller.” In Europe, vigor control is a major issue on stone fruits and pears, and trunk girdling and root pruning are routinely used to constrain tree size.
Reducing cost by reducing tree numbers is also important, Lang agrees.
Lang has been developing and testing pruning and training concepts for sweet cherry fruiting wall systems for more than ten years. He originated the idea of precision bud selection to form a narrow wall of multiple upright leaders arising from a split horizontal trunk or bilateral cordon as in grapes. Washington State University horticulturist Dr. Matt Whiting took this further to create the unilateral cordon-based Upright Fruiting Offshoot (UFO) system, in which a cherry nursery tree is planted at an angle and the trunk oriented horizontally, with up to 12 shoots chosen to grow upright from that trunk. This also reduces the number of rootstocks required and diffuses vigor, and it is conducive to renewal pruning as two or three of the largest upright shoots are removed and re-grown each year.
In Musacchi’s, Lang’s, and Whiting’s work with cherries, Gisela rootstocks are being used to promote early fruiting as well as to keep tree size down.
What Lang really likes about Musacchi’s work is the pruning concept, in which only the trunk is permanent—whether one or two or more—and no lateral branches develop into scaffold limbs. “All shoots are cut very short,” he said. Musacchi and Lang refer to the one-trunk version as a “super slender axe” or “unibaum.”
In his tunnels, Lang is testing Musacchi’s pruning concept to maintain the narrow fruiting wall systems, no matter how many trunks the tree has.
Speaking at the IFTA conference, Musacchi said, “In traditional volume or hedgewall orchards, and even in the new hybrid systems, most of the cropping occurs on permanent wood, like short shoots with spurs, that is at least two years old and more or less severely pruned to renew bearing spurs.
“By contrast, cropping in a high-density system with vertical-axis trees should take place as far as possible on basal buds of year-old shoots.”
To get this, trees need a fair number of cropping laterals (“feathers” or future branches) that are uniformly arrayed along the central leader and have, as far as is possible, uniform vigor.
“These fruiting feathers are cut back every year so as to induce spring renewal via short winter pruning that leaves two to three vegetative buds, with length slightly decreasing from tree base to apex,” Musacchi said.
For high-density central-leader systems in which fruiting primarily occurs on spurs formed on permanent or renewable lateral shoots, Lang has used a young tree development technique of rubbing off two or three buds (at budswell) on the previous season’s trunk growth, then leaving one, and repeating the process from apex to base to form a whorl pattern of future fruiting laterals around the trunk that are positioned to optimize light interception and reduce shading. He does not currently recommend this technique for Musacchi’s ultrahigh-density super-slender-axe system, since the annual short pruning technique is adequate for managing optimal light interception and distribution. In orchards where vigor of the new laterals is excessive, Musacchi recommends late-summer pruning.
“Such a planting needs cultivars combining a pronounced aptitude for good feathering with flower buds at the basal parts of shoots,” Musacchi said. “Girdling and notching can help to induce proper feathering, and prepruning in summer or autumn can improve and even accelerate bud maturity.”
All this pruning requires careful attention to prevention of diseases, especially the organisms causing brown rot and bacterial canker, the latter of which can enter the tree through wounds. For orchards in humid areas in Italy, or where spring rainfall is heavy or hail damage occurs, brown rot may require six or seven fungicide applications for control, he said.
For Lang, growing sweet cherries in high tunnels where trees are protected from both excessive rainfall and the wounding effects of wind has allowed him to reduce fungicide applications.