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Mycoplasmalike Organism Associated with Pecan Bunch Disease. Carl E. Seliskar, Principal Plant Pathologist, Northeastern Forest Experiment Station, Forest Service, USDA, Delaware, Ohio; Glenn E. KenKnight(2), and Carol E. Bourne(3). (2)Research Plant Pathologist, Pecan Laboratory, ARS, USDA, Shreveport, La.; (3)Biological Laboratory Technician, Northeastern Forest Experiment Station, Forest Service, USDA, Delaware, Ohio. Phytopathology 64:1269-1272. Accepted for publication 8 March 1974. DOI: 10.1094/Phyto-64-1269.

Pecan bunch is a widespread and serious disorder occurring throughout the commercial range of southern pecan, Carya illinoensis. It was formerly presumed to be virus induced, but the consistent association of a mycoplasmalike organism (MLO) with diseased and not with healthy trees strongly points to MLO etiology. Unlike some other mycoplasma diseases, pecan bunch is not systemic throughout the tree. The MLO are found in phloem sieve elements and companion cells of stems and leaves of affected trees. They are most abundant in severely broomed material and are rarely observed in nonsymptomatic tissues. They are pleomorphic, vary in size from 80 to 800 nm, contain ribosomal bodies and fibrillar material typical of DNA, and are delimited by a tripartite unit membrane. Spherical and oval forms are most common but filamentous forms can be seen near sieve plates.