Joseph D. Postman,1 Kim E. Hummer,2 and Kirk W. Pomper3
ADDITIONAL INDEX WORDS. Asimina triloba, disease, stem canker, bacteria, phytoplasma, fungi, blue stain
SUMMARY. 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.
The pawpaw tree produces the largest edible fruit native to the United States (Darrow, 1975) and has great potential as a new high value crop (Layne, 1996; Pomper et al., 1999). Pawpaws are hardy to USDA growing zone 5 [−26.1 °C (−15 °F)] and grow wild in the mesic hardwood forests of 26 states in the eastern United States, ranging from northern Florida to southern Ontario (Canada) and as far west as eastern Nebraska (Kral 1960). In 1995, the PPF, embarked on a joint venture with the United States Department of Agriculture-Agricultural Research Service (USDAARS) National Clonal Germplasm Repository in Corvallis, Ore., to test commercially available pawpaw varieties and PPF advanced selections in Oregon, a region outside the native range of the pawpaw (Layne, 1996; Pomper et al., 1999).
A PRVT was established at the NCGR, Corvallis, Ore. in autumn 1995. This orchard was a replicated planting of 10 commercially available pawpaw varieties and 18 PPF advanced selections, with 8 replicate trees of each variety or selection, grafted onto seedling rootstocks and planted in a randomized block design.
Named varieties in the trial included: ‘Middletown’ (selected in Ohio), ‘Mitchell’ (Illinois), NC-1 (Ontario, Canada), ‘Overleese’ (Indiana), ‘PA-Golden #1’ (New York), ‘Sunflower’ (Kansas), ‘Taylor’ (Michigan), ‘Taytwo’ (Michigan), ‘Wells’ (Indiana), and ‘Wilson’ (Kentucky).
1Plant pathologist and curator, USDA National Clonal Germplasm Repository, 33447 Peoria Road, Corvallis, OR 97333.
2Research leader and curator, USDA National Clonal Germplasm Repository, 33447 Peoria Road, Corvallis, OR 97333.
3Principal investigator of horticulture and curator, USDA National Clonal Germplasm Repository for Asimina species., Atwood Research Facility, Kentucky State University, Frankfort, KY 40601-2355.