During a survey in May 2011, tomato (Lycopersicon esculentum Mill.) plants were observed exhibiting wilt symptoms on the vegetable farm of the University of the Punjab Lahore (31°32′59″ N, 74°20′37″ E), Pakistan. Plants were pale yellow and stunted. Diseased plants were scattered in the field either singly or in groups of two or three. Lower leaves turned yellow following necrosis and shedding from the plants. Roots of diseased plants were dark brown in comparison to the healthy ones. Vascular browning was observed when infected stems were split longitudinally and sectioned transversely. Necrotic vascular strands were also observed in infected stems. With a stereoscope, white hyphae were noticed in the vascular region of infected plants. Stained cross sections of stems of wilted plants showed fungal hypha. A diseased plant sample was taken to the First Fungal Culture Bank of Pakistan (FCBP) for identification of the pathogen. Conidia appeared on potato dextrose agar after incubation in diurnal light, and were unicellular, smooth, thin-walled, and oval in shape (3 to 6 × 1 to 3 μm), forming aggregations on the tip of conidiophores. With the help of diagnostic keys and available literature, the isolated fungus was identified as Acremonium strictum W. Gams. The fungus formed whitish flat colonies with deposition of gum granules on its surface. Hyphae and phialides were hyaline and non-pigmented. Phialides with expanded bottoms were elongated measuring 25 to 40 μm and solitary in position at right angles to the filament. To further confirm identification, amplification of the ITS1 and ITS2 regions was performed with universal primers ITS1 and ITS4 (4). The ITS sequence obtained was submitted to NCBI as accession JQ916901. The culture was deposited in FCBP (1099), Institute of Agricultural Sciences, University of the Punjab, Pakistan. A pathogenicity test of the isolated fungus was performed by inoculating 15 healthy tomato plants cv. Rio Grande in three replicates. The spore suspension (2 × 105 conidia/ml of sterile distilled water) was prepared from 12-day-old cultures of the fungus grown from a single spore on PDA in continuous light. Control plants were treated with sterile distilled water. Tomato plants inoculated with the fungus started showing wilting symptoms within 2 weeks, while the control remained healthy. A. strictum was re-isolated from inoculated wilted plants, thus satisfying Koch's postulates. Previously, A. strictum was a reported cause of wilt in many plants including Chrysanthemum maximum (2), watermelon (1), and Gladiolous grandiflorus (3). To our knowledge, wilt in tomatoes has been reported only from Fusarium oxysporum and Pseudomonas solanacearum from Pakistan. To our knowledge, this is the first record of Acremonium wilt in this economically important crop.
References: (1) B. D. Bruton et al. Plant Dis. 79:754, 1995. (2) A. R. Chase. Calif. Agric. 32(10):21, 1978. (3) E. Gonzalez-Perez. J. Plant Pathol. 90:586, 2008. (4) T. J. White et al. Pages 315-322 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990.
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