Ultrastructural Characterization and Field Performance of Papaya Regrowth Selections Recovered From Bacterial Crown Rot Infection For Disease Tolerance Breeding

Bacterial crown rot (BCR) is an important disease of papaya (Carica papaya Linn.) in Southeast Asia caused by the infection of a necrotrophic, gram-negative, facultative aerobic bacterium (Erwinia mallotivora Goto). It is characterized by necrotic leaf spots and water-soaked petiole or stem lesions that proceed to the soft rotting of the crown. After BCR infection, regrowth or resumption of growth exhibited by growing of side shoots below the rotted portion of the plant occurred. This regrowth from crown rot infection were examined under the scanning electron microscope to ascertain the presence of the bacterium. E. mallotivora or any rod-shaped bacteria were not detected in the epidermal/ cuticular tissues and tracheids of the regrowth plant including the healthy control, but they were present in the external tissues of the infected plant. Open wound entry, epidermal layer degradation and copious exopolysaccharide coating the bacterial cells are some mechanisms by which E. mallotivora induce crown rot infection. Building on the regrowth genotypes free of the bacteria, and further developing experimental F 1 hybrids that were entered into a field trial resulted to the identification of candidate genotypes for further breeding. Eighteen papaya genotypes consisting of 13 experimental hybrids and 5 inbred lines derived from BCRtolerant growth selections had good plant and fruit morphological characteristics. Among these BCR tolerant regrowth genotypes identified, 5648x336 and 413x5648 possessed good horticultural traits and considered candidates for further breeding for BCR tolerance and variety release.