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Phytophthora capsici: A Soilborne Pathogen Dangerous on Grafted Tomato (Solanum lycopersicum × S. hirsutum) in Italy

During an extensive survey carried out in Piedmont (northern Italy) aimed at identifying the emerging soilborne diseases affecting tomato in commercial fields where alternatives to methyl bromide have been implemented in response to national and international regulations, sudden collapse of tomato plants, cv. Tomahawk, grafted on cv. Beaufort, were repeatedly observed in a commercial plastic tunnel operation. Affected plants suddenly collapsed 60 days after transplant during the month of May 2010. Symptoms included chlorosis, stunting, and severe root and crown rot, leading to sudden collapse of approximately 25% of the plants within 60 days of transplant. Symptomatic tissues from the root and collar of infected plants were surface disinfested for 1 min in a 1% NaOCl solution, rinsed for 5 min in water, and submerged in selective medium based on corn meal agar. A Phytophthora-like organism (2) with characteristic coenocytic hyphae was consistently isolated and transferred to V8 agar. The sporangia were spherical to ovoid, papillate, and 40 to 77 × 23 to 34 (average 55.1 × 30.3) μm. Oospores were globose and 22.2 to 30.8 μm. The internal transcribed spacer (ITS) region of rDNA of a single isolate was amplified using the primers ITS1/ITS4 and sequenced. BLAST analysis (1) of the 750-bp segment showed a 100% homology with the sequence of Phytophthora capsici JN382543.1. The nucleotide sequence has been assigned the GenBank Accession No. JX090306. Pathogenicity tests were performed on healthy 30-day-old tomato plants cv. Beaufort by using one strain of P. capsici grown for 15 days at 22 to 25°C on a mixture of 2:1 wheat/hemp kernels, and then 1 g per L of the inoculum was mixed into a substrate based on peat blonde/peat black (15:85 v/v). Two plants were transplanted into 3-L pots, with five replicates. Ten non-inoculated plants represented the control treatment; the trial was repeated once. All plants were kept in a greenhouse at temperatures ranging from 22 to 25°C. Inoculated plants became chlorotic 7 days after inoculation and root and crown rot developed 30 days after inoculation. Control plants remained symptomless. P. capsici consistently was reisolated from inoculated plants. In Italy, the presence of P. nicotianae on hybrids of Solanum lycopersicum × S. hirsutum is known (3), while, to the best of our knowledge, this is the first report of P. capsici on the hybrid S. lycopersicum × S. hirsutum in Italy. The economic importance of the disease can increase due to the expanding use of grafted tomato plants.

References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997 (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. APS Press, St Paul, MN, 1996. (3) A. Garibaldi and M. L. Gullino. Acta Hortic. 833:35, 2010. (4) H. M. Masago et al. Phytopathology 67:425, 1977.