Incidence of Phyllosticta Fungal Spot and Cracking Prevalence in the Fruit of Sixteen Pawpaw (Asimina triloba) Varieties

A leaf and fruit fungal spot consisting of a complex of Mycocentrospora asiminae, Rhopaloconidium asiminae Ellis and Morg, and Phyllosticta asiminae Ellis and Kellerm. has been observed in pawpaw. Symptoms include tan spots with dark brown borders on leaves, and dark brown to black superficial spots on the fruit epidermis. Leaves displaying these symptoms were collected from plantings at the Kentucky State University Research Farm and positively identified as Phyllosticta leaf spot by the University of Kentucky Plant Diagnostic Lab. Pawpaw fruit occasionally exhibit cracking, and field observations suggest cracking may vary by variety. It was hypothesized that there may be a correlation between incidence of Phyllosticta fruit spot on pawpaw and fruit cracking, due to epidermal damage by the fungus. The objective of this study was to determine if Phyllosticta fruit spot and cracking incidence varied by cultivar, and if there was any correlation between fruit spot occurrence and fruit cracking. Ripe fruit were harvested three times per week from a mature pawpaw planting consisting of sixteen pawpaw cultivars and advanced selections (G4-25, G5-23, G6-120, G9-111, Haz-1, Haz-1.5, Haz-2, Hi 1-4, Hi 4-1, Hi 7-5, 7-90, KSU-Atwood, NC-1, Mitchell, Sunflower, and Wabash). Percent coverage by Phyllosticta fruit spot was visually estimated, and fruit cracking was evaluated for each fruit harvested.  Fruit weight, Phyllosticta coverage, and cracking all varied significantly by variety. A positive correlation was found between cracking and Phyllosticta coverage as well as fruit weight and cracking. Hi 1-4 had the largest fruit size, highest incidence of Phyllosticta, and highest prevalence of cracking. Hi 7-5 exhibited a large fruit size, low Phyllosticta coverage, and no fruit cracking. Other factors could also be related to fruit cracking, such as firmness and thickness of the epidermis. Hi 7-5 shows potential for a new cultivar release that produces large high quality fruit with no fruit cracking and low incidence of Phyllosticta. Future studies are needed to determine the relationship between Phyllosticta fungal fruit spot and fruit cracking in pawpaw, and to determine varietal resistance to this fungus and possible control measures.