VIEW ARTICLE

Etiology

Leuconostoc mesenteroides subsp. mesenteroides, The First Report of a Coccoid Bacterium Causing a Plant Disease. K. E. Conn, Department of Plant Pathology, University of California, Davis 95616, Present address: Petoseed Co., Inc., Woodland Research Station, Woodland, CA 95695; J. M. Ogawa, B. T. Manji, and J. E. Adaskaveg. Department of Plant Pathology, University of California, Davis 95616. Phytopathology 85:593-599. Accepted for publication 24 January 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-593.

A coccoid bacterium, Leuconostoc mesenteroides subsp. mesenteroides, was identified for the first time as the causal agent of a postharvest decay of fresh-market tomato fruit in California and Mexico. Symptoms included a sour odor and watery texture of firm and intact fruit. Our original strain (LMM 89-1) and an additional 10 strains isolated from Leuconostoc-decayed or fungal-decayed fruit were identified as the heterofermentative, lactic acid–producing bacterium L. m. subsp. mesenteroides. These strains were identical to the type strain, ATCC 8293. Bacterial strains were identified as members of the genus Leuconostoc primarily on the basis of Gram-positive stain, coccoid morphology, lack of catalase, inability to hydrolyze arginine, and production of CO₂ from glucose; strain LMM 89-1 was identified as L. mesenteroides based, in part, on a fatty acid methyl ester analysis. The strains were identified to the subspecies level based on production of acid from l-arabinose and dextran production on sucrose-amended media. Pathogenicity of strain LMM 89-1 was demonstrated on mature-green tomato fruit by fulfilling Koch's postulates. Symptom development was discernible within 24 h of inoculation as small, slightly sunken, firm, water-soaked lesions. Sequential sampling and culturing over 5 days indicated a logarithmic increase of bacterial populations in fruit tissues. The average doubling time over these periods was 3.6 h for fruit incubated at 20 C and 2.9 h for fruit incubated at 33 C. Using aqueous cell suspensions, strain LMM 89-1 was found to be more virulent (sensu aggressive) to tomato fruit than was the type strain. Strain LMM 89-1 was pathogenic to wound-inoculated fruit of pepper but not tomato leaflets, tomato stems, potato tubers, eggplant fruit, or other nonsolanaceous crops tested. In subsequent samplings of tomato fruit from packinghouses in California and Mexico, L. m. subsp. mesenteroides was isolated from 17 of 21 (81%) symptomatic fruit, as well as from 309 of 387 (80%) fungal-decayed fruit. From asymptomatic fruit, the bacterium was isolated from surface sections of tomato fruit but not from internal mesocarp tissue.