Site description and experimental design
The experiment was conducted using ‘Honeycrisp‘ grafted onto 9 different
rootstocks at Washington State University (WSU) Sunrise Research Orchard
in Rock Island, WA (47°18’35.6”N 120°03’59.5”W). The soil is a shallow
sandy loam soil with good drainage. The rootstocks in this experiment
were: Budagovsky- 9 (B.9), CG.4292, CG.5257, M.9-T337, Geneva 210
(G.210), Geneva 814 (G.814), Geneva 87 (G.87), Geneva 890 (G.890), and
Geneva 969 (G.969). The dwarfing capacity of these rootstocks ranged
from dwarfing (B.9) to semi-vigorous (G.890). The experiment was planted
in 2017 and arranged in a completely randomized design (N = 3) with
three to eight trees per replication. The orchard was oriented north to
south and rows were spaced 3.6 m apart and 0.9 m between trees which
were trained to a spindle system and irrigated using at approximately
110% replacement of estimated evapotranspiration during the growing
season using a combination of drip and micro-sprinklers. Trees were
managed and pruned according to commercial tree fruit management
practices. In 2018 and 2019, flowers were completely removed from the
trees to eliminate any cropping during early growth. Soil volumetric
water content and soil temperature were measured using an
ECH2O 5TM soil moisture and temperature probe (Meter
Group, Pullman, WA, USA) (Table 1). Three soil moisture sensors were
placed in different locations in the herbicide strip equidistant between
two trees at 20 cm depth. Relative humidity and air temperature were
measured with a VP-3 sensor placed in the middle row 1.8 m above ground.
Each soil probe, air temperature, and relative humidity sensors were
interfaced with an EM50G data logger (Decagon Devices, Pullman, WA, USA)
and data were logged every 15 minutes from April to September. Solar
radiation and wind speed were obtained through the AgWeatherNet weather
stations located immediately adjacent to the WSU-Sunrise Orchard
(https://weather.wsu.edu/).