Association of a Bacterium and Not a Phytoplasma with Papaya Bunchy Top Disease

Abstract

Papaya bunchy top (PBT), a major disease of papaya (Carica papaya L.) in the American tropics, was thought to be caused by a phytoplasma. However, 95 papaya plants with symptoms of PBT from 12 countries throughout the American tropics were assayed by polymerase chain reaction for the presence of 16S rRNA genes of phytoplasmas, but none were found. Examination of transverse sections of petiole tissue by epifluorescence microscopy revealed the presence of fluorescing materials associated with PBT on the periphery of the phloem, between the phloem and xylem, and sometimes extending along the phloem rays as far as the cortex. Bacteria were detected within the same region by transmission electron microscopy and were found consistently in three PBT-affected plants from Puerto Rico and Costa Rica, but not in a healthy plant from Florida. The bacteria were rod-shaped, measuring 0.25 to 0.35 μm in width and 0.8 to 1.6 μm in length. Their cell wall ultrastructure resembled that of Gram-negative bacteria, except that a peptidoglycan layer was not evident in the periplasmic space. The bacteria appeared to colonize laticifers. The bacteria were consistently observed by light microscopy in expressed sap from fresh papaya petioles of plants with PBT symptoms, but not in similar preparations from healthy plants. All attempts to isolate the bacteria in axenic culture were unsuccessful. If the bacterium causes PBT, it would be the first example of a leafhopper-transmitted, laticifer-inhabiting, plant pathogenic bacterium.