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currently being screened to identify seedling rootstocks that could result in improved pawpaw establishment rates and precocity.

Micropropagation of pawpaw has not been successful. In establishment studies, seedling nodal explants of pawpaw responded more favorably than apical sections for establishment (Finneseth 1997; Geneve et al. 2003). When nodal explants were treated with a range of benzyladenine (BA) concentrations (0, 5, 10, or 15 μM) on MS medium, fewer than 0.5 shoots developed per explant (Finneseth 1997). However, when seedling explants were treated with 1.0 μM thidiazuron (TDZ) plus 10 μM BA, all cultures produced over 1.0 shoot per culture. Explant establishment from seedling, mature, or rejuvenated sources was attempted using Murashige and Skoog (MS) medium (Murashige and Skoog 1962) supplemented with 10 μM BA plus 0.1 μM TDZ (Finneseth et al. 2000). A small number of mature explants survived and produced a limited number of shoot buds after 7 months in culture. However, these never stabilized and no mature cultures survived for more than 12 months in culture. Bellini et al. (2003) has reported difficulty in disinfecting pawpaw explants collected in the field, with cultures being lost to contamination by fungal and bacterial organisms.

Geneve et al. (2003) reported one pawpaw accession (A10-11) developed from rejuvenated explant sources showed continued growth and shoot production during subculturing. It has been maintained in culture for over 3 years. The ability for single stem explants from 3-year-old cultures of A10-11 to support shoot multiplication was investigated using 9.8 μM IBA plus 5.4 μM NAA in combination with BA (0 to 20 μM). Initial explants elongated but did not form additional shoots after 8 weeks in culture (J. Egilla, unpubl.). These were subcultured to the same medium and after 9 weeks cultures treated with 15 or 20 μM BA had the greatest number of shoots per culture and 15 μM BA had the most vigorous shoot growth. These data indicate that cultures of pawpaw can retain morphogenetic potential for a considerable time in culture. Preliminary experiments to root microcuttings of pawpaw have met with little success (S. T. Kester and R. L. Geneve, unpubl.). In these experiments, microshoots were developed from pawpaw cultures maintained for over 2 years. The original explants were shoots developed on root pieces from the A10-11 understock. Initial treatments placing explants on one-half strength MS salts medium containing IBA (0.49 to 4.9 μM) resulted in a small percentage (3.0%) of microcuttings developing one to two roots per cutting at the 4.9 μM IBA level. However, these rooted shoots did not thrive during the acclimatization stage and failed to develop further.

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Poor establishment rates have been a problem in pawpaw orchards. Establishment and survival of trees in the PRVT has varied between 10% to 95% survival rates (Merwin et al. 2003; Pomper et al. 2003d; Postman et al. 2003). Generally, trees that are 45 to 90 cm at planting have a lower mortality rate than smaller trees. Tree establishment and survival was enhanced when trees were grown in copper treated 8 L containers (K. W. Pomper, unpubl. data).

Present recommendations for pawpaw plantings are 2.4 m within rows and 3.7 to 4.6 m between rows. During the first year of establishment, pawpaw trees show little growth and can have an early leaf drop the first fall. For the first two years, top growth is slow as the root system establishes itself, but thereafter it accelerates substantially given proper fertility and soil moisture. Row orientation should be north-south if possible.

Shading of pawpaw in the field the first year is recommended and can be accomplished by installing translucent double-walled polyethylene "tree-tubes" around each tree, securing them with bamboo stakes (Layne 1996). However, trees taller than 45 cm at planting do not require shading. During warm summer temperatures (>35°C), the tubes should be removed from the trees, otherwise foliage within tubes can became heat-stressed and desiccated. In New York, open mesh black plastic trunk guards provided adequate shade and protection for newly planted paw-paw trees, whereas translucent plastic tree-tubes caused heat stress and scorching of the young trees (Merwin et al. 2003).

Weed control is important to improve establishment, but there are no herbicides currently labeled for use on pawpaw. Mulching around trees with wood chip mulch (15 cm depth) or with straw can be an effective method of weed control for at least one year and possibly two. However, this is a labor-intensive method, and may be cost prohibitive. Weed mats or landscape fabric can control weeds for up to 3 years and assist in water conservation. Irrigation during establishment improves tree survival rates, but irrigation requirements of pawpaw have not been determined. Trickle irrigation with emitters that provided 3.8 L/hr with 2 emitters/tree for a total of about 945 L/tree per year has given good results.

Fertilization requirements have not been determined yet for bearing pawpaw trees. Trees fertilized with water-soluble fertilizer (20N-8.6P-16.6K) plus soluble trace elements once in May, June, and July during active growth in Kentucky have achieved 30 to 45 cm of shoot extension