ASSOCIATION OF A BACTERIUM AND NOT A PHYTOPLASMA WITH PAPAYA BUNCHY TOP DISEASE

Papaya bunchy top (PET), a major disease of papaya (Carica papaya L.) in the American tropics, was thought to be caused by a phytoplasma. Ho wever, 95 papaya plants with symptoms of PET from 12 countries through out 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 epifluorescenc e microscopy revealed the presence of fluorescing materials associated with PET 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 PET-affected plants f rom Puerto Rico and Costa Rica, but not in a healthy plant from Florid a. The bacteria were rod-shaped, measuring 0.25 to 0.35 mu m in width and 0.8 to 1.6 mu m in length. Their cell wall ultrastructure resemble d that of Gram-negative bacteria, except that a peptidoglycan layer wa s not evident in the periplasmic space. The bacteria appeared to colon ize laticifers. The bacteria were consistently observed by light micro scopy in expressed sap from fresh papaya petioles of plants with PET s ymptoms, but not in similar preparations from healthy plants. All atte mpts to isolate the bacteria in axenic culture were unsuccessful. If t he bacterium causes PET, it would be the first example of a leafhopper -transmitted, laticifer-inhabiting, plant pathogenic bacterium.