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Unreliability of Latex-Flow Test for Diagnosis of Bunchy-Top of Papaya Caused by a Mycoplasmalike Organism. R. R. Webb, College of the Virgin Islands, St. Croix, U.S. Virgin Islands 00850. M. J. Davis, IFAS, Ft. Lauderdale, FL 33314. Plant Dis. 71:192. Accepted for publication 23 September 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0192C.

The diagnosis of 250 papaya plants (Carica papaya L.) showing symptoms of bunchy-top (stunted growth, shortened internodes, interveinal chlorosis, and marginal leaf scorching) using a latex-flow method was conducted on eight islands in the eastern Caribbean. Observed foliar symptoms were at variance with a previously reported diagnostic technique distinguishing bunchy-top from viral disorders based on the failure of affected plant parts to exude latex upon wounding (1). Observation of all 250 plants with foliar symptoms of bunchy-top showed no appreciable reduction in latex flow after wounding compared with a similar number of symptomless trees. Electron microscopy of petiole and leaf vein tissue from plants with foliar symptoms and a flow of latex upon wounding confirmed the presence of a mycoplasmal ike organism in the phloem of 10 of 12 samples. No MLO were found in similar tissues from two healthy appearing trees. A recent survey of papaya diseases in Florida (2) indicated the absence of bunchy-top based on latex flow after wounding. In view of the above observations, this type of diagnosis would appear to be unreliable.

References: (1) J. Adsuar. J. Agric. Univ. P.R. 56(4)397,1972. (2) S.-H. Wan and R. A. Conover. Plant Dis. 67:353, 1983.