Papaya Bunchy Top Disease

Papaya bunchy top disease was first identified in 1931 in Puerto Rico. For decades the identity of its pathogen was highly contested because researchers could not isolate it, so Koch's postulates could not be fulfilled, and it was variously suspected to be caused by a virus, a mycoplasma-like organism or a phytoplasma before each possibility was disproven; diagnosis in the meantime relied only on a list of commonly associated symptoms. Through sequencing and microscopy, scientists identified the actual pathogen in 1996 as a bacterium in the genus Rickettsia, described as rod-shaped, small, gram-negative and laticifer-inhibiting. Other Rickettsia species cause diseases such as typhus and spotted fever in animals and phony disease of peach and almond leaf scorch in other plants. Papaya bunchy top disease occurs throughout the American tropics and has been economically significant for its impact on fruit production, though little information exists on its current economic toll.