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  • Author or Editor: Kirk W. Pomper x
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Growth of pawpaw (Asimina triloba) seedlings in containers was examined in a factorial greenhouse experiment with four treatment levels of the slow-release fertilizer, Osmocote 14-14-14 (14N- 6.1P-11.6K), incorporated in Pro-Mix BX potting substrate at 0, 0.13, 0.26 or 0.81 kg·m-3 (0, 0.22, 0.44, or 1.37 lb/yard3) and three treatment levels of liquid-feed fertilizer of Peters 20-20-20 (20N-8.7P-16.6K) water-soluble fertilizer at 0, 250, or 500 mg·L-1 (ppm). When plants were harvested 18 weeks after sowing, seedlings subjected to the highest rate of Osmocote 14-14-14 at 0.81 kg·m-3 and liquid-feed at 500 mg·L-1 had the greatest total biomass, about 3-fold greater than nonfertilized plants. In a separate greenhouse experiment, growth of seedlings was examined with Osmocote 14-14-14 as the sole fertilizer source at six treatment levels of: 0, 0.81, 2.22, 4.43, 8.86, or 17.7 kg·m-3 (0, 1.37, 3.74, 7.47, 14.9, or 29.9 lb/yard3). Early seedling growth was hastened in the 2.22 kg·m-3 treatment rate, but delayed in 17.7 kg·m-3 treatment rate, when compared to nonfertilized control plants. When seedlings were harvested 17 weeks after sowing, plants had the greatest shoot, root, and total dry weight with Osmocote 14-14-14 at a rate of 2.22 kg·m-3. Root:shoot ratio decreased from about 1.5 without Osmocote 14-14-14, to about 0.65 at rates of 2.22 kg·m-3 or greater. Based on the results of this study, the slow-release fertilizer, Osmocote 14-14-14, can be used effectively as a sole fertilizer source when incorporated into potting substrate at a rate of 2.22 kg·m-3 or at a reduced rate of 0.81 kg·m-3 when supplemented with weekly applications of liquid-feed fertilizer at a rate of 500 mg·L-1 of Peters 20-20-20, to enhance production of container-grown pawpaw seedlings.

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A pawpaw regional variety trial (PRVT) was established at Cornell University, Ithaca, N.Y. in Apr. 1999 consisting of 28 commercially available pawpaw (Asimina triloba) varieties or advanced selections from the PawPaw Foundation (PPF; Frankfort, Ky.). Eight replicate trees of each selection, grafted onto seedling rootstocks, were planted in a randomized block design. The first two winters at the test planting site were unusually mild for the Finger Lakes region, with the lowest recorded temperatures above -16 °C (3.2 °F). Despite these mild winters, there was extensive winter mortality of some pawpaw varieties. Survival rates were >75% for 11 varieties, and were <40% for five other varieties. Poor establishment of grafted clonal pawpaws and insufficient pollination or fertilization of established pawpaws were important limitations of successful commercialization of this new fruit crop under conditions typical of upstate New York. Open mesh black plastic trunk guards provided adequate shade and protection for newly planted pawpaws, whereas translucent plastic tree-tubes caused heat stress and scorching of the young trees.

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Pawpaw (Asimina triloba) is a native North American tree that has potential as a new fruit crop or for use in landscapes, but until recently, little information has been available to nurseries on containerized production of this species. Pawpaw seedlings develop a strong taproot with a fragile root system, which can be easily damaged upon digging; therefore, most nurseries propagate trees in containers. Pawpaw seed requires stratification for optimal germination and seed is sensitive to desiccation. The seed also cannot tolerate freezing temperatures [<-15 °C (5.0 °F)]. A well-aerated potting substrate with a high sphagnum peat moss component (>75% by volume), cation exchange capacity, and water holding capacity can be used effectively in container production. Tall containers should be used to accommodate the developing taproot of seedlings. The slow-release fertilizer Osmocote 14-14-14 (14N-6.1P-11.6K) incorporated into Pro-Mix BX potting substrate can be used effectively as the sole fertilizer source at a treatment rate of 2.22 kg·m-3 (3.742 lb/yard3) in containerized pawpaw production. It can also be used at a lower rate of 0.81 kg·m-3 (1.365 lb/yard3) when supplemented with weekly applications of 500 mg·L-1 (ppm) of Peters 20-20-20 (20N-8.78P-16.6K) liquid-feed fertilizer. Bottom heating [32 °C (89.6 °F)] of container-grown pawpaw seedlings results in greater lateral and total root dry weight than in seedlings grown at ambient temperature [24 °C (75.2 °F)], which could increase the rate of establishment of seedlings in the field. Bottom heating of container-grown pawpaw seedlings could decrease both the time to produce a saleable plant and the cost of heating greenhouses. Growth of containerized pawpaw seedlings is enhanced by low to moderate shading with polypropylene shade fabric (28% or 51%) outdoors and low shading (33%) in the greenhouse, in a manner typical of that reported for other shade-preferring plants. Low to moderate shading of pawpaw seedlings grown outdoors greatly increases leaf number, total leaf area, and total plant dry weight compared to nonshaded seedlings, suggesting that commercial nurseries can improve production of containerized pawpaw seedlings using a shading regime outdoors.

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To facilitate the growth of a commercial pawpaw (Asimina triloba) industry, several problems with harvest and postharvest handling of fruit need to be resolved. Pawpaw fruit ripening is characterized by an increase in soluble solids content, fl esh softening, increased volatile production, and a loss of green color intensity. Within 3 days after harvest, ethylene and respiratory climacteric peaks are clearly evident. Softening of fruit is due to the action of at least four enzymes, with the softening proceeding from the surface to the interior tissue. Fruit on a single tree can ripen over a 2-week period, creating labor problems. When immature fruit is harvested it does not ripen, even if treated with ethephon at 1000 mg·L-1 (ppm), but the use of commercially available growth regulators both pre- and postharvest warrants further study. Fruit soften very rapidly at room temperature after harvest and have a 2-to 4-day shelf life. However, we have stored pawpaw fruit for 1 month at 4 °C (39.2 °F) with little change in fruit firmness and fruit apparently continue normal ripening upon removal to ambient temperature. The optimum temperature and duration for holding fruit will need to be determined. Further extension in pawpaw storage life may be feasible with controlled or modified atmosphere storage. Although there are a number of practical problems with pawpaw harvest and postharvest storage that need to be addressed, we hope to develop recommendations for harvest and handling of fruit in the near future.

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A pawpaw (Asimina triloba) regional variety trial (PRVT) was established at the U.S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository (NCGR), Corvallis, Ore., in Fall 1995. This orchard was a replicated planting of 28 commercially available varieties or advanced selections from the PawPaw Foundation (PPF; Frankfort, Ky.), with eight replicate trees of each selection grafted onto seedling rootstocks and planted in a randomized block design. Two years after planting, 32 trees had either failed to establish or had died after an initial healthy start. By July 1999, 25% of grafted trees had died due to a vascular wilt-like disease, and 2 years later mortality exceeded 50%. Grafted selections with the lowest symptom severity include 1-7-2, 2-54, 7-90, 8-58, 9-58, `Mitchell', `PA-Golden #1', `Taylor' and `Wilson'. Seedling guard trees were unaffected until July 2000, when six guard trees of 76 died and 10 more were declining. By July 2001, 14 guard trees were dead. No fungi were consistently isolated from declining trees. A number of bacteria were isolated from infected trees, but no specific pathogen has been confirmed as the causal agent. Polymerase chain reaction (PCR) tests for phytoplasmas and for the bacterium Xylella fastidiosa were also negative. Research is ongoing to determine if a bacterial pathogen was the cause of the pawpaw decline in the Oregon PRVT.

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Marketed as a fresh fruit, the pawpaw (Asimina triloba) has a short shelf life, only 2-3 days at room temperature and up to 3 weeks with refrigeration. There is commercial processing potential for pawpaw pulp in juices, ice cream, yogurt, baked goods, and other products. Consumer acceptability of such products needs to be investigated. One hundred and five attendees of the 2nd Annual Pawpaw Field Day at Kentucky State University (KSU), Frankfort, Ky., participated in a tasting of pawpaw products; 56% of tasters were male; 76% were over 40 years of age; 72% of tasters had eaten pawpaw previously. Each item was rated on a scale from 1 = liked it extremely to 7 = disliked it extremely. Pawpaw ice cream was the best-received item (55% of tasters liked it extremely), followed by pawpaw cake with lemon icing, liked extremely by 45%. The pawpaw/grape juice drink was liked extremely by 31% of participants. Three alternative recipes for pawpaw butter were presented; the plain pawpaw butter was liked extremely by 26% of tasters; pawpaw butter prepared with lemon and grape juice was liked extremely by 11%, while the version prepared with orange and lemon was liked extremely by only 8%. Two versions of pawpaw custard were presented. The custard prepared from ripe, mild-fl avored fruit was liked extremely by 42% of tasters, while the custard prepared from mixed under-ripe, over-ripe and bruised fruit was liked extremely by only 16%. Ratings by persons unfamiliar with pawpaw fl avor were significantly lower (P < 0.05) only for the two pawpaw custards; tasters age 40 years or younger gave significantly higher ratings for pawpaw ice cream (P < 0.05) and significantly lower ratings for both pawpaw custards (select, P < 0.05 and mix, P < 0.01) and the pawpaw/grape juice drink (P < 0.05).

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Beginning in 1993, 12 institutions and individuals and The PawPaw Foundation (PPF) embarked on a joint venture to evaluate commercially-available, named pawpaw (Asimina triloba) varieties and PPF's advanced selections within and outside of the pawpaw's native range. Each Pawpaw Regional Variety Trial (PRVT) planting, consists of about 300 trees, with five to eight replications (blocks) of 28 grafted scion varieties per block in a randomized complete block design (10 named varieties and 18 clones selected in the PPF orchards at the University of Maryland Experiment Stations at Queenstown and Keedysville, Md.). Variables being examined in the trial include climatic effect, culture, pests, growth, fl owering, yield, and fruit characteristics. In 1995, PRVT plantings were established in Kentucky (Princeton, Ky.), Louisiana, North Carolina, Oregon, and South Carolina. In 1998, a second planting was established in Kentucky (Frankfort, Ky.). In 1999, PRVT plantings were established in Indiana, Iowa, Michigan, Maryland, Nebraska, New York, and Ohio. In the Frankfort planting, 95% of the trees have survived. Based on height and trunk diameter measurements taken from 1998 to 2001, most selections displayed good vigor. The variety PA-Golden had the best early fruit production as evidenced by the fact that five of eight trees had fruit in 2001. In the Princeton, planting, only 54% of the trees have survived. The selections `Sunfl ower', `PA-Golden', `NC-1', `Wilson', 1-23, 8-20, and 9-58 showed the best fruit production and survival rates (>63%) in 2001. Based on limited data collected so far in the Kentucky trials, `PA-Golden' and `Sunfl ower' have performed well in the two locations and other varieties and PPF selections show promise.

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Pawpaw (Asimina triloba) is an under-exploited small tree with commercial potential as a fruit crop, ornamental tree, and source of secondary products with insecticidal and medicinal properties. It is most often propagated from seeds that are recalcitrant and must be stored moist at a chilling temperature. Seeds display combinational (morphophysiological) dormancy. Endogenous, physiological dormancy is broken by about 100 days of chilling stratification followed by a period of warm moist conditions where the small embryo develops prior to seedling emergence about 45 days after the warm period begins. Pawpaw cultivars with superior fruit characteristics are propagated by grafting onto seedling understocks. The most common practice is chip budding. Other methods of clonal propagation have proven problematic. Pawpaw can be propagated from cuttings, but only in very young seedling stock plants. Micropropagation from mature sources is not yet possible, but shoot proliferation has been accomplished from seedling explants and explants rejuvenated by induction of shoots from root cuttings of mature plants. However, rooting of microcuttings and subsequent acclimatization has not been successful.

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