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(pH 6.9) at the University of Kentucky (UK) Research and Education Center in Princeton, Ky. Trees were fertilized with 28.0 kg /ha of N broadcast distribution of granular fertilizer (34-0-0) in early March. Trees at the Princeton planting were not irrigated. The graft union of trees was about 15 cm at both the Frankfort and Princeton sites.
At both locations, the number of clusters and number of fruit per cluster on each tree were counted in late July each year following the normal fruit drop period. Fruit were harvested on Mondays, Wednesdays, and Fridays during the harvest season (mid August to late September). Average fruit weight was based on the weights of 10 or more fruit per tree. Trunk diameters were measured at 30 cm from the soil surface annually in March. For the Frankfort planting, growing degree days (GDDs) were calculated using a base temperature of 10circC (University of Kentucky Agricultural Weather Center calculator, http://wwwagwx.ca.uky.edu/calculators.html). Temperature data were obtained from the University of Kentucky Agricultural Weather Center's monthly climate summary (http://wwwagwx.ca.uky.edu/cgi-public/climsum2.ehtml), with temperatures recorded at their Lexington station, approximately 25 miles from Frankfort.
For each site, data for trunk cross-sectional area [TCA (cm2)], number of fruit per tree, fruit weight (g), cumulative yield (total kg), cumulative yield efficiency [CYE, (total kg yield/ cm2 TCA)], average number of fruit per cluster, GDDs required for ripening, harvest peak (month/day), harvest duration (days), and biennial bearing index (BBI) were subjected to GLM analysis of variance and Least Significant Difference (LSD) means separation, using the statistical program Costat (CoHort Software, Monterey, Calif.). Treatment means were separated based on a significance level of P < 0.05. The BBI was calculated on the basis of the Pearce and Debusek-Urbank formula (14):
BBI = 1/(n -1) X \ |(a 2- a 1)|/(a 2+ a 1)+|(a 3..;.- a 2)|/(a 3+ a 2)…+|(a (n)- a (n -1))|/(a (n)+ a (n -1))\
where n = number of years, and a1, a2, …, a(n-1), an = yield of corresponding years. The BBI is a measure of a cultivar's tendency to produce alternating high and low yields; ranges are from 0 to 1, with 0 = no alternation and 1 = complete alternation. Biennial bearing index was calculated based on three years of data from each site.
Frankfort, KY trial. At the Frankfort planting, 95% of the trees survived (Table 1). Most of the trees that died were lost during the first summer after planting. ‘Wilson’ and ‘Taylor’, and the advanced selection 2-10, had the poorest survival rate (75%). All other cultivars and advanced selections had survival rates of 88% or higher (Table 1).
Vigor and yield of trees were evaluated annually from 2002 to 2006. In 2003, a frost event destroyed almost the entire pawpaw crop; therefore, only vigor and fruiting characteristics from 2004, 2005, and 2006 are reported in Table 2, but yield for each year is presented (Table 3). The variables TCA, number of fruit per tree, fruit weight, cumulative yield, cumulative yield efficiency, number of fruit per cluster, GDDs required for ripening, harvest peak, harvest duration (days), and biennial bearing index all varied significantly among cultivars and advanced selections (P < 0.001) 7, 8, and 9 years after planting.
Based on TCA recorded in 2006, most clones displayed excellent vigor, ranging from 32.8 to 78.6 cm², especially ‘Wabash’, ‘Wilson’, 7-90, and 10-35, although some clones lacked vigor (e.g., ‘Wells’, ‘Middletown’, and 3-21; Table 2). ‘PA-Golden’, ‘Wilson’, and 10-35 averaged more than 100 fruit per tree, whereas ‘Susquehanna’ and 5-5 averaged fewer than 40 fruit per tree. Variability in number of fruit per tree was high (LSD=31). Fruit weight also varied greatly among cultivars and advanced selections, with some clones having an average fruit weight of 170 g or more per fruit (“Overleese”, 8-20, “NC-1”, “Susquehanna”, “Wabash”, 5-5, and “Potomac”), and some under 100 g (“Middletown”, “Wilson”, and “Rappahannock”).