2Rating scale: 0 = no growth (dead), 5 = most growth.

Trees were planted in an orchard site that should be well suited for this species. The glacial till soil was a gravelly silt loam in texture with pH ranging from 5.6 to 6.2. The site sloped about 8% toward the southwest and was near the deepest part of Cayuga Lake, which remains unfrozen in winter and creates a strong lake effect that moderates mid-winter cold temperatures. Trickle irrigation was installed at planting, and weed control in the tree row was provided with May and July applications of glyphosate and paraquat postemergence herbicides.

In accordance with protocols for the PRVT, shortly after planting we installed translucent double-walled polyethylene tree-tubes around each pawpaw, securing them with bamboo stakes. During a week of unusually warm days in July 1999 when afternoon temperatures exceeded 36circC (96.8circF), most of the foliage within tubes became heat-stressed and dessicated. Tubes were removed promptly, and the young trees then grew replace-

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Table 1. Average vigor of shoot growth and percent survival of 28 clonally propagated pawpaw varieties in an upstate New York planting during 1999–2001.

Fig. 1. View of one block (center of picture) in a Lansing, N.Y., pawpaw regional variety trial during its third year of growth in late June 2001. Each tree represents one replicate for each of the 28 varieties. The adjacent row (upper right hand side of picture) of much larger seedling pawpaws was planted 2 years earlier, and has established very successfully with excellent tree growth and negligible winter mortality. Open black plastic mesh cylinders were placed around each grafted tree (left-center of figure) after heat stress damaged foliage within the initially used translucent plastic tree-tubes.

ment leaves. We subsequently replaced the tree tubes with 45-cm-high (17.7-inch) black polyethylene open mesh cylinders (OESCO Inc., Conway, Mass.) often used to protect young fruit trees from meadow voles (Microtus spp.) in the northeastern U.S. These open mesh cylinders provide a semi-shaded environment similar to the natural understory conditions where pawpaw seedlings and root suckers thrive, and we recommend that others evaluate them as an alternative to the more expensive tree-tubes for promoting establishment of newly planted trees.

Variety survival rates

The first two winters at our test planting site were unusually mild for the Finger Lakes region, with the lowest recorded temperatures above -16 circ C. Despite these mild winters, there was extensive winter mortality of some paw-paw varieties. Cumulative tree survival rates were > 75% for 11 varieties, and were < 40% for five other varieties (Table 1). We also rated tree-growth vigor, which differed substantially among varieties (P = 0.02) and was positively correlated with survival rates (Table 1). We attributed the poor survival of some varieties in our test planting to mid-summer heat stress and damaged foliage (as described above), to intrinsic varietal differences, or to potential incompatibilities among scions and seedling root stocks. We did not observe the putative fungal blight that has been reported at the source nursery and at some of the other PRVT sites around the U.S. (Postman et al., 2003).

seedling rootstocks. We did not observe the putative fungal blight that has been reported at the source nursery and at some of the other PRVT sites around the U.S. (Postman et al., 2003).

By coincidence, our test planting was adjacent to a row of 50 seedling pawpaws planted in Apr. 1998 and maintained similarly to the PRVT except that open mesh guards were used instead of tree tubes. The growth and survival of these 50 seedlings has been excellent in comparison with the PRVT planting (right vs. left hand sides of Fig. 1, respectively), demonstrating that site conditions were not the cause of the high mortality and low vigor observed in many of the grafted pawpaws.

The main problem observed in our seedling pawpaw planting has been very poor fertilization or viability of flowers. These seedling trees (and a few of our grafted varieties) have flowered profusely during recent springs, but each year only a few flowers have produced viable fruit. We speculate that lack of suitable pollen vectors is the cause of this poor fruit set because observations during bloom revealed little or no insect activity in the pawpaw flowers. Insect pollinator activity (primarily hymenopterans and dipterans) in adjacent pome and stone-fruit plantings has been abundant during bloom every year, so there has been no lack of potential pollen vectors.

We conclude that poor establishment of grafted clonal pawpaws, and insufficient pollination or fertilization of established pawpaws may be important limitations of the successful commercialization of this new fruit crop under conditions typical of upstate New York.

Literature cited

Darrow, G.M. 1975. Minor temperate fruits, p. 276–277. In: J. Janick and J.N. Moore (eds.). Advances in fruit breeding. Purdue Univ. Press, West Lafayette, Ind.

Kral, R. 1960. A revision of Asimina and Deeringothamnus (Annonaceae). Brittonia 12: 233–278.

Layne, D.R. 1996. The pawpaw [Asimina triloba (L.) Dunal.]: A new fruit crop for Kentucky and the United States. HortScience 31:777–784.

Pomper, K.W., D.R. Layne, and R.N. Peterson. 1999. The pawpaw regional variety trial, p. 353–357. In J. Janick (ed.). Perspectives on new crops and new uses. ASHS Press, Alex., Va.

Postman, J. D., K. E. Hummer, and K. W. Pomper. 2003. Vascular decline in the Oregon pawpaw regional variety trial. HortTechnology 13(3):418–420.

HorTechnology · July–September 2003 13(3)