Effect of Timing of Topping to Reduce Tree Height on Subsequent Year Vigor of Early-Season ‘Arctic Star’ Nectarine

Production costs could be substantially reduced if the height of fruit trees could be lowered enough to eliminate the need for, or reliance upon, ladders in the orchard. Recent research demonstrates that high yields can be obtained on shorter trees with appropriate management techniques, but vigor control can be a problem. In 2004, selected rows of ‘Arctic Star’ nectarine trees growing in the Dinuba, CA area were topped in July, September and November. In July and September the plots chosen for topping were divided into two sub-treatments. One sub-treatment was topped to 3 m (down from approximately 5.5 m) and the other was topped at 3.7 m. In November, each of the 3.7 m July and September sub-treatments were retopped to 3 m along with a block that was previously not topped. One treatment was also left untopped, and received normal dormant pruning during which height was reduced to ~4 m. All trees were hand pruned during dormancy to select fruiting wood, and pruning weights were recorded. Trunk and rootstock samples of wood tissue were taken from four trees of each treatment by using a 12 mm hole saw to extract a plug of wood from about 20 cm above the graft union and 10 cm below the graft union on December 1, 2004. Topping increased light penetration, and the July severe topping treatment stimulated regrowth that resulted in delayed bloom and fruit maturity in the following year. Contrary to our hypothesis, stored carbohydrates in the roots were greatest in the early topping treatments and lowest in the late topping events. Topping in September resulted in the lowest amount of summer pruning necessary in the subsequent year. INTRODUCTION Over half of the annual production costs for peaches, nectarines and plums in California involve hand labor for pruning, thinning and harvest which is done from ladders because of tall trees (Day et al., 2004; DeJong et al., 1999). It is widely recognized (Day et al., 2005) that production costs could be substantially lowered if the height of trees could be contained to reduce the need for ladders in the orchard. Our recent research demonstrates that high yields can be obtained on shorter trees with appropriate management techniques (Day and Johnson, 2003). One technique that growers use extensively to uniformly reduce tree height is topping. Mechanical tree topping has been practiced for more than 40 years and there are many theories about optimal timing of topping operations. Physiological reasoning predicts that the earlier one tops after the primary period of shoot growth (approximately June 30), the greater the loss in carbohydrate storage for the subsequent year and this should result in less vigorous re-growth in the subsequent year. However, there have been no systematic studies to test this theory. MATERIALS AND METHODS In June 2004 a fairly uniform, excessively vigorous 7-year-old block of early maturing, ‘Arctic Star’ nectarine trees growing in northern Tulare County (California) was selected for the study. The orchard was managed for optimum growth by the growers Proc. IX IS on Orchard Systems Ed.: T.L. Robinson Acta Hort. 903, ISHS 2011 712 and received normal amounts of irrigation and fertilizer throughout the experiment. Beginning in July, selected rows of trees were topped in July, September and November. In July and September the plots chosen for topping were divided into two sub-treatments. One sub-treatment was topped to ~3 m (down from approximately 5.5 m) and the other was topped at ~3.7 m. In November, each of the 3.7 m July and September sub-treatments were re-topped to 3 m feet along with a treatment block that was previously not topped. One treatment was also left untopped and received normal dormant pruning and was reduced to a height of ~4 m in the winter. Data were collected from four 4-tree experimental units within each treatment. The weight of biomass removed by the topping treatments was assessed by weighing the pruned material after topping. All trees were hand pruned to select fruiting wood during the winter and pruning weights were recorded. Shortly after fruit harvest in June, 2005, all treatments were summer pruned (most of the major water sprout growth was removed) and pruning weights were recorded. Trunk and rootstock samples of wood tissue were taken from four trees of each treatment by using a 12 mm hole saw to extract a plug of wood from about 20 cm above the graft union and 10 cm below the graft union on December 1, 2004. The xylem tissue that was extracted was dried, ground and sent to the UC DANR tissue analysis laboratory for total glucose (digested starch and soluble glucose) analysis to determine if there were any topping treatment effects on carbohydrate storage in the tree major storage organs. To assess the effects of the topping treatments on orchard light interception, measurements of percent ambient sunlight penetrating the orchard canopy and reaching the orchard floor were taken with a light ceptometer on June 29, 2004 during mid-day, shortly after the first topping treatments. Since the September topping treatments were just a repeat of treatment done in July these were not repeated subsequent to those treatments. Readings were taken on four sets of four trees within each treatment. Average photosynthetically active radiation (PAR) was computed, and also expressed as percentage readings taken in full sun. Since the goal of this research was to assess the effects of different topping treatments on tree regrowth the subsequent year and not on crop yield effects detailed yield data by treatment were not collected. However the grower kept rough track of the amount of fruit that were harvested in each treatment by counting the number of 425 kg bins produced by each treatment in 2005. RESULTS AND DISCUSSION Both topping heights improved light penetration into and through the tree when measured in late June (Table 1). Topping at ~3 m removed an average of more than 40 kg of wood biomass per tree in both July and November (Table 2) and probably also in September; however the brush weight data were not available after the topping in September because the prunings were shredded before the data could be collected. When the trees were topped to ~3 m in one operation, a little more than half of the prunings were in the form of brush and the remainder was in the form of major scaffold pieces. Topping the trees at 3.7 m in one operation and following that with a second operation to cut the trees down to ~3 m resulted in removing about the same total amount of total biomass in two operations, but in the first topping the majority of the prunings were comprised of brush (water sprouts) while scaffold wood was primarily removed in the second topping operation. As expected, the topping treatments dramatically reduced the amount of prunings that were removed in the dormant season compared to the non-topped treatment (Table 2). However, the total prunings (combined topping and dormant) from the topping treatments was significantly more than the non-topped treatment regardless of when the topping was done. This was partly due to greater height of the dormant pruned trees (4 m) vs the topped trees (3 m). There was a tendency for the trees in the July topping treatments to have more dormant pruning weight than the other topping treatments and, although these differences were not statistical, they may reflect the fact that there was a