Rootstock and Interstem Effects on Pome Fruit Trees - 2006
Joseph Masabni and Dwight Wolfe, Department of Horticulture

Introduction
Apple is the principal tree fruit grown in Kentucky because of generally favorable weather and other growing conditions. Still, the hot and humid summers and heavy clay soils make apple production more difficult in Kentucky than in neighboring apple-producing regions with more favorable conditions. The hot and humid summers are also a factor in high disease and insect pressure in Kentucky orchards.

In spite of these challenges, productive orchards are high per-acre income enterprises, suitable for rolling hills and upland soils. Furthermore, orchards in these sites have less soil erosion potential. Unfortunately, Kentucky imports more apples than it produces.

Identification of improved rootstocks and cultivars is fundamental for advancing the Kentucky apple industry. For this reason, Kentucky cooperates with 39 other states and three Canadian provinces in the Cooperative Regional NC-140 Project entitled, "Rootstocks and Interstem Effects on Pome Fruit."

The NC-140 trials are critical to Kentucky growers, allowing them to gain access to and test new rootstocks from around the world. The detailed and objective evaluations allow growers to select the most appropriate rootstocks for Kentucky.

The 1999 apple rootstock trial was designed to compare the adaptability of the slender-spindle and the French vertical-axe systems in orchards on Kentucky soils. In addition, the semi-dwarf rootstocks in this trial evaluate the rootstocks’ abilities to support trees without a trellis. The 2002 apple rootstock trial provides information on performance differences among newly-released rootstock clones. The 2003 apple rootstock trial evaluates the adaptability of some new rootstocks to Kentucky climates and soils. The 2003 apple rootstock physiology trial primarily evaluates the relationship between different environments (sites), crop loads, and fruit size.

The NC-140 orchard trials are demonstration plots for visiting fruit growers, extension personnel, and researchers. The data collected from these trials will help establish base-line production and economic records for the various orchard system/rootstock combinations that can be used later by Kentucky apple growers.

Materials and Methods
Grafts of known cultivars on various rootstocks were produced by nurseries and distributed to cooperators for each planting. The University of Kentucky has three NC-140 rootstock plantings at the UK Research and Education Center (UKREC) at Princeton:

Eight of the dwarfing and three of the semi-dwarfing rootstocks have not been tested previously at UKREC.

Orchard floor management consists of a 6.5 ft herbicide strip with mowed sod alleyways. Trees are fertilized and sprayed with pesticides according to local recommendations (1, 2). Yield and trunk circumference measurements are recorded for all of the rootstock trials. Tree height and canopy spread (the average of the within row and across row tree widths) are recorded at the end of the 5^th and final (usually the 10^th) seasons of each trial. Fruit size is calculated as the average weight (oz) of 50 fruits.

Results and Discussion
The winter of 2006 was generally mild. Temperatures in the 60’s and 70’s in January were followed by a cold snap where temperatures dropped to 5ºF on February 19. Rainfall was near normal for most months, but September was one of the wettest and coolest on record.