Tomato is the primary vegetable exported from Mexico to the United States. In June 2007, stem rots were observed in tomato cv. Imperial plants growing in a greenhouse in the Culiacan Valley. Disease symptoms included stem rot with a mycelial growth with a grayish blue sporulation. The disease was observed to be affecting 1% of the tomato plants growing in the greenhouse, and has been observed sporadically in greenhouses during subsequent agricultural cycles in other tomato-growing areas in Mexico. Affected stems showed initial symptoms of a dark brown necrotic area surrounding only the cut regions of pruned leaves and stems. As the infection continued on the tissues, the spot grew and then became covered with a grayish blue sporulation. Severe stem rot led to death of the plants. Rotted stems of tomato plants were collected and samples of the infected tissues were plated onto potato dextrose agar (PDA) to isolate the fungus. The preliminary identification of the pathogen was Penicillium oxalicum Currie & Thom on the basis of morphological criteria with ellipsoidal conidia approximately 4 × 3 μm borne in columns, conidiophores mostly biverticillate, and ampulliform phialides (2). The identification was confirmed by sequencing internal transcribed spacer 1 (ITS1), 5.8S, and ITS2 regions of the ribosomal DNA (GenBank Accession No. HM452308). The isolate was deposited in the Coleccion Nacional Microbiana y de Cultivos Celulares CINVESTAV-IPN, Mexico. Koch's postulates were fulfilled by reproducing stem rots on healthy inoculated tomato stems. Pathogenicity testing involved fungal growth on PDA for 5 days, after which a 4-mm disk of actively growing mycelia was transferred to wounds (4 × 4 mm) made with a scalpel in stems of 6-week-old tomato cv. Imperial plants. Inoculated plants were covered with plastic bags to maintain a high relative humidity for 24 h and were maintained in a greenhouse at 25 ± 2°C. Seven days after inoculation, all of the inoculated stems showed rot symptoms similar to those observed in the greenhouse. Stems that were inoculated only with an agar disk did not show any symptoms of the disease. The pathogen was reisolated from inoculated plants but not from noninoculated plants. Artificial inoculation was performed twice. Although P. oxalicum has been previously reported as a causal agent of cucumber (1) and tomato stem rots in Japan (3), to our knowledge, this is the first report of P. oxalicum causing stem rot in tomato plants in Mexico.
References: (1) T. M. O'Neill et al. Plant Pathol. 40:78, 1991. (2) J. I. Pitt. The Genus Penicillium and Its Teleomorphic States Eupenicillium and Talaromyces. Academic Press, London, UK, 1979. (3) S. Umemoto et al. J. Gen. Plant Pathol. 75:399, 2009.
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