The Response of `Montmorency' Tart Cherry to Renewal Pruning Strategies in a High Density System
Tart cherry (Prunus cerasus) production in the U.S. is based on low density plantings designed to accommodate large trunk shake mechanical harvesters. Availability of canopy shake harvesters adapted to smaller trees could facilitate transitioning to high density (HD) precocious systems, based on continuous fruiting walls that are highly efficient at light capture. HD systems would require specialized pruning techniques to maintain long term productivity while maximizing the efficiency of a limited labor supply. Experimental HD orchards with multiple rootstocks, training systems and tree densities were used for investigating renewal pruning strategies. Pruning cuts of predetermined lengths ranging from 0 cm to 25 cm were made on branches of differing size (0.6 cm to 4.7 cm diameter) and renewal growth was monitored for shoot number and length. The minimum stub length for generating at least one renewal shoot was approximately 10 cm. However, this differed somewhat with rootstock and diameter of the cut branch, where the critical length was shorter for larger diameter branches and on the more vigorous rootstock. Results provide guidelines on pruning of HD plantings to renew fruiting wood and maintain productivity. In many temperate fruit tree species, renewal pruning is used to replace large, rigid branches with smaller, flexible branches with healthy, young spurs and fruiting shoots. In apples and peaches, renewal pruning often involves leaving short, 2 cm long bevel cut stubs, referred to as Dutch or stub cuts. From this short stub, a bud will break, typically on the underside of the stub. The resulting shoot will grow with a wide crotch angle and at a flatter angle, effectively replacing or renewing the cut branch (Robinson, 2003) Renewal pruning is an important practice aiding in improvement of light penetration and airflow in the canopy and helps to maintain the health of the tree (Cain, 1972) It has been well documented in multiple fruit crops, that dry matter production, fruit size, color, soluble solids concentration and total fruit yield are directly related to the amount of sunlight intercepted in the tree (Campbell and Marini,1992; Flore and Layne, 1999; Palmer, 1997; Wunsche and Lakso, 2000) As light filters through the canopy, a gradient of light interception is established that contributes to both whole tree light interception and light microenvironment around the fruiting spur. By selectively renewing the largest branches in the canopy, light penetration may be increased. This increased light in the canopy promotes tree health, increased flower bud formation, and the development of quality fruit. Renewal pruning for improved light distribution can increase air circulation and improve spray penetration through the canopy (Sutton and Unrath, 1984; Ferree and Hall, 1980) Tart cherry is susceptible to a host of fungal infections, such as powdery mildew (Podosphaera clandestine) as well as arthropod pests that thrive in dense canopies where air circulation is poor. Selective renewal pruning can improve air circulation, decrease canopy humidity, decrease the prevalence of disease, and at the same time may make crop protectant applications more effective by in54 Journal of the american Pomological Society creasing distribution uniformity through the canopy. Hedging is being used to efficiently prune and reduce labor costs in the orchard, but this practice can create a denser outer canopy in the trees (Nugent, 2002) Renewal pruning may be needed in conjunction with hedging to maintain appropriate canopy density. It has been suggested that one fifth of the largest branches in tart cherry trees be renewed annually to keep wood small, flexible, and fruitful (Crandall,1979) Maintaining flexible fruiting wood may also be important for over the row harvesters that remove fruit from the plant by shaking the canopy instead of the tree trunk. These canopy shake harvesters are commonly used in grapes, raspberries, blueberries, olives, and some nut crops. Fruiting in tart cherries occurs primarily on spurs on two year old and older wood, but in certain circumstances, may also occur at the base of one year old wood, which can result in blind wood after the first fruiting year. In contrast, vegetative buds formed on oneyear old wood result in spurs that will fruit for several years (Perry et al. 1998) Any management practice that promotes more precocious fruiting, potentially risks excessive flower bud formation on one year old wood, further contributing to the amount of blind wood in the canopy. This can be managed with the application of gibberellic acid (Anderson et al. 1996) which suppresses flower bud formation in favor of vegetative bud formation, resulting in less blind wood formation in the canopy. However, renewal pruning is also required to replace blind wood with spur bearing branches by removing unfruitful wood and encouraging new potentially fruitful growth in its place. Although apples and peaches respond best to cutting branches to at least 2 cm stubs (Robinson, 2003) the stub length for tart cherry branch renewal is a topic that has not been well researched. Preliminary results indicate that cherries respond to short renewal cuts differently than apples and peaches, where cherries appear to need longer stub lengths to regrow fruiting wood. Long stub lengths >15 cm from the base) in sweet cherries have been found to increase the number of flower buds and lateral shoots (Guimond et al. 1998) Nugent (2002) recommended 10 to 15 cm long stubs to promote renewal growth in 'Montmorency' tart cherry, but did not cite data from which this recommendation was based. The objective of this research was to find the minimum stub length to generate at least one new shoot on 'Montmorency' tart cherry in a high density orchard, and to determine whether this critical length was affected by rootstock vigor. Our hypothesis was that tart cherry requires 10 cm long stubs to generate a renewal shoot. Materials and Methods Renewal pruning strategies were applied to a high density (HD) tart cherry orchard planted in 2010 at the Utah Agricultural Experiment Station research farm in Kaysville, Utah (41deg01'16degN latitude, 1328 m elevation, 165 freeze free days) The orchard consists of combinations of 3 rootstocks and 3 training systems with 'Montmorency' tart cherry as the scion cultivar. Rootstocks included the dwarfing Gisela(r) 3 (Gi.3) and Gisela(r) 5 (Gi.5) and the commercial standard 'Mahaleb' Tree training included a single leader, a double leader, and a quad leader, with leaders oriented in line with the row to facilitate machine harvest. Annual dormant pruning was based on a columnarized system with renewal cuts made back to the leaders in a 3 4 year cycle, patterned after protocols for tall spindle apples (Robinson et al.,2006) Briefly, annual dormant pruning involved renewal cuts to 2 to 3 of the largest branches of each tree. The targeted result was 1, 2 or 4 permanent leaders with weaker fruiting lateral shoots that are frequently replaced. Row orientation was approximately northeast to southwest corresponding to the slope of the field to improve air drainage. Each rootstock training system combination was replicated in ~9 m long plots. Orchard soil