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striped cucumber beetle (Acalymma vittatum F.), melon aphid (Aphis gossypii Glover), Mexican bean beetle (Epilachna varivestis Mulsant), and a free-living nematode [Caenorhabditis elegans (Maupas) Dougherty] (Alkofahi et al. 1989).
A market exists for biomass produced by growers to supply annonaceous acetogenins for the production of pawpaw products such as head lice remover shampoo. Acetogenin concentration varies monthly in pawpaw tree tissues, being highest in the spring and summer months (Johnson et al. 1996). However, pawpaw twigs can be harvested, dried, ground, and then stored for later extraction (Johnson et al. 1996).
Pawpaws have few disease problems. Pawpaw leaves can exhibit leaf spot, principally a complex of Mycocentrospora aiminae, Rhopalocini-dium asiminae Ellis & Morg., and Phyllosticta asiminae Ellis & Kellerm (Farr et al. 1989; Peterson 1991) and some trees in the PRVT planting in Frankfort, Kentucky have exhibited signs of these foliar diseases. The pawpaw peduncle borer (Talponia plummeriana Busck) is a small moth larva, about 5 mm long, that burrows into the fleshy tissues of the flower, causing the flower to wither and drop (Heinrich 1926; MacKay 1959; Peterson 1991). Signs of the pawpaw peduncle borer have been observed in pawpaw orchards in Maryland (R. N. Peterson, pers. commun.), but not in Kentucky. The zebra swallowtail butterfly (Eurytides marcellus), whose larvae feed exclusively on young pawpaw foliage, will damage leaves, but this damage has been negligible at the PRVT plantings. The larvae of the leafroller (Choristoneura parallela Robinson) may damage flowers and leaves (Norman et al. 1992). Deer will not generally eat the leaves or twigs, but they will eat fruit that has dropped on the ground. Male deer may rub trees with their antlers breaking branches. Japanese beetles (Popillia japonica Newman) can damage young leaves on pawpaw trees.
Pawpaw trees usually survive for 20 years or more; however, tree decline may be a problem in some pawpaw orchards. About 1% of trees in the PPF orchard at Queenstown, Maryland die annually from an unidentified decline. Grafted pawpaw trees do not survive as long as seedling trees, suggesting rootstock/scion incompatibility could result in tree decline and death (G. Reighard, Clemson University, pers. commun.). Rootstock produced from seed from the same scion may be more compatible and promote long-term survival of grafted trees.
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Rootstock produced from seed from the same scion may be more compatible and promote long-term survival of grafted trees.
Vascular wilt-like symptoms have been observed in the spring after pawpaw trees leafed out in orchards in Oregon (Postman et al. 2003), North Carolina (M. Parker, North Carolina State University, pers. commun.), and Maryland (N. Peterson, pers. commun.). As transpiration demand increased with warmer and drier weather, severely affected trees collapsed and died. Moderately affected trees became chlorotic with stunted new growth. Blue and black vascular discoloration was observed beneath the bark of declining trees along the lower parts of the main stem, particularly at and above the graft unions. This symptom has been described as "blue-stain." A canker-like bark splitting was also observed near the base of many declining trees, with smaller cankers on upper scaffold branches. Several species of fungi in the genera Ceratocystis and Leptographium have been associated with "blue-stain" in conifer timber, and a few species are associated with tree diseases. In most cases, spread of these fungi has been attributed to assorted bark beetles or other insects (Sohlheim and Safranyik 1997; Jacobs et al. 2000; Jacobs and Wingfield 2001). Blue discoloration beneath the bark of pawpaw trees seems to be a common response of this host to injury and may be associated with more than one disease or disorder.
A PRVT that was established at the USDA-NCGR in Corvallis, Oregon, in the fall of 1995 has had difficulties with high tree mortality. Postman et al. (2003) reported that two years after planting, 13% of 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%. 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. Despite the vascular wilt-like disease in this plot, healthy, producing pawpaw trees have been growing for more than 20 years at several gardens in Oregon's Willamette Valley (J. Postman, pers. commun.). The demise of the NCGR Corvallis PRVT may help to identify sources of resistance to this as yet unidentified disease. Trees in the PPF orchard at Queenstown, Maryland have exhibited similar disease symptoms to those in Oregon, but as stated previously only about 1% die annually (N. Peterson, pers. commun.).