Table 3. First-year whip dieback (FYWD), third-year terminal dieback (TYTD) from cold injury, and fourth-year year bloom characteristics for seven grafted pawpaw varieties and one nongrafted seedling strain planted in 1998.

Values are means of eight trees/variety in 1998; in 1999, 2000, and 2001, for 'Sunflower' and 'Overleese' trees N=7; for seedling trees N=4.

yPercent of total tips on tree.

xMeans separated by least significant difference (LSD) values at P = 0.05 significance level; 1.0 cm = 0.39 inches.

wSignificance of variety main effect after analysis of variance.

buds will likely break evenly along the stem, but those furthest from the cut will be the most vigorous. At the end of 2000, average shoot lengths were not recorded, but total shoot numbers ranged from 13.4 ('Taylor'), to 23.6 ('Prolific') (Table 2).

Loss of first season whip growth (FYWD) to cold damage the winter after planting ranged from 20.9 to 76.5% (Table 3). Temperature data during the coldest months (Dec. 1998-Feb. 1999) indicate a period mean minimum temperature of -1.7 circ C (28.94 circ F) and a period minimum of -16.4 circ C (2.48 circ F) (Midwest Climate Information System, Belleville SIU Research Center, Belleville, Ill.). Pawpaw is known to be a hardy species, so winter injury was unexpected. The variety ‘Overleese’ lost significantly more top growth than ‘Taylor’ and ‘Rebeccas Gold’. When evaluated as a percent of total growth ‘Overleese’ lost more top growth than all other entries. Both ‘Sunflower’ and ‘Wells’ lost a higher percentage than ‘Taylor’ which had the least percent loss among the named varieties. One tree of ‘Overleese’ and ‘Sunflower’ died following the winter of 1998-99, probably due to cold damage. During Winter 1999-2000, no damage was noted in any variety. However, following the third winter (2000-01), dieback of tips (TYTD) was noted but was not influenced by variety (Table 3). For the last two winters, the period minimum temperature from December to February was -15.8 circ C (3.56 circ F) and -16.4 circ C, respectively. These data indicate that relative hardiness and/or fall acclimation differences may be present in the varieties tested. Because grow tubes were left on during the first winter, it is suspected that the shoot loss was

related to acclimation and hardening characteristics, as trees grew later into the fall with tube protection. Fluctuations in fall temperatures may also have been responsible for damage observed. Temperature data for the months of September to November do indicate wide variations. Period maximum/minimum temperatures for the fall of 1998, 1999 and 2000 ranged respectively from 35.6 to -4.4 °C (96.08 to 24.08 °F), 35.6 to -8.9 °C (96.08 to 15.98 °F), and 34.4 to -10 °C (93.92 to 14.0 °F). This period also included 8, 8 and 12-d of subfreezing [<0 °C (32.0 °F)] minimum temperatures for the respective years.

Due to great variability, significant differences among varieties for the number of flower buds/tree was not observed (Table 3). Seedling trees developed no buds at all and remained in a juvenile stage. Most flower buds did not develop fully to anthesis but instead aborted on the tree. This could have been due to cold damage during the winter 2000–01, or other unknown physiological reasons. The percentage of flower buds that developed into open, viable flowers in Spring 2001 were significantly greater in ‘Wells’ than all other named varieties except ‘Rebeccas Gold’. Interestingly, ‘Overleese’ had the highest average number of buds per tree (5.6), but only 0.7% developed into flowers, where as ‘Wells’ with a moderate number of buds (3.6), had a very high percentage of opened flowers (63.8%). No fruit set in any variety with open flowers. This is not surprising because pawpaws have been shown to be strongly protogynous, self-incompatible and require cross pollination by native insects, either fly or beetle species, which may or may not have been present on this site (Willson and Schemske, 1980).

There were no differences in survival but significant differences in growth and development among the seven grafted varieties and the non-grafted seedling strain planted in this trial. The non-grafted seedlings consistently out-paced the grafted varieties in growth parameters such as height and TCSA, but it did not initiate flower buds and instead remained juvenile in character in the time frame evaluated. Second year bud break and subsequent early limb development characteristics were similar for all varieties. Tree height and shoot growth characteristics differed, but differences varied from year to year. Trends for growth patterns did become evident during the three seasons the trees were evaluated, but they were not always significant. Among named varieties, ‘Taylor’, ‘Davis’, ‘Overleese’ and ‘Rebeccas Gold’ were least vigorous, where as ‘Prolific’ and ‘Sunflower’ were most vigorous. Differences in overall growth could be related to rootstock/scion compatibility or simply greater vigor of these varieties. For all entries, lack of apical dominance was exhibited when buds breaking at the top of tipped stems consistently grew less than buds originating from lower locations. This resulted in an early canopy structure resembling a candleabra. These observations may provide insight into early pruning responses as management techniques for this crop are developed. Precocity varied among all varieties in the spring of the fourth year. Not all flower buds developed into an open flower. The percentage of buds opening did not correlate well with initial bud number. Successful early anthesis thus appears to be a variety trait.

Shoot hardiness may be strongly related to ability of the tree to accli-

HorTechnology · July–September 2003 13(3)