First Report of Leaf Spot of Lettuce (Lactuca sativa L.) Caused by Phoma tropica in Italy

Lettuce (Lactuca sativa L.) is widely grown in Italy, with the production for the preparation of ready-to-eat salads becoming increasingly important. During the spring of 2011, a previously unknown leaf spot was observed on L. sativa plants, cv Rubia, grown in several plastic tunnels in Lumbardy (northern Italy), 20 to 25 days after sowing. Thirty to forty per cent of leaves of the plants growing in the part of the tunnel with the highest relative humidity were affected. Leaves of infected plants showed extensive, irregular, dark brown, necrotic lesions with a chlorotic halo. Lesions initially ranged from 0.5 to 3 mm, then eventually coalesced, reaching 2 to 3 cm, showing a well-defined, dark brown border. Affected leaves senesced and withered. The crown was not affected by the disease. Diseased tissue was excised, immersed in a solution containing 1% sodium hypochlorite for 60 s, rinsed in water, then cultured on potato dextrose agar (PDA), amended with 25 mg/l of streptomycin sulphate. After 5 days, a fungus developed, producing a greenish grey mycelium with a white border when incubated under 12 h/day of fluorescent light at 21 to 23°C. In order to favor the production of conidia, the fungus was transferred on malt extract agar (MA) and maintained under 12 h/day of fluorescent light at 22°C. After 15 days, black pycnidia, 175 to 225 μm, developed, with hyaline, elliptical, unicellular conidia, measuring 3.21 to 6.7 × 1.08 to 3.2 (average 5.5 × 1.9) μm. On the basis of these morphological characteristics, the fungal causal agent of the disease could be related to the genus Phoma (2). The internal transcribed spacer (ITS) region of rDNA of the isolate PHT30 was amplified using the primers ITS1/ITS4 and sequenced. BLAST analysis (1) of the 466-bp segment showed a 99% similarity with the sequence of Phoma tropica (GenBank Accession No. JF923820.1). The nucleotide sequence has been assigned the GenBank Accession No. JQ954396. Pathogenicity tests were performed by spraying healthy 20-day-old lettuce plants, cv Rubia, with a spore suspension (1 × 10⁵ conidia/ml) prepared from 14-day-old colonies of the strain PHT30 grown on MA cultures. Plants inoculated with water alone served as controls. Ten plants per isolate were used. Plants were covered with plastic bags for 5 days after inoculation and maintained in a growth chamber at 20°C and 80% relative humidity. The first foliar lesions, similar to those occurring on the naturally infected plants, developed on leaves 12 days after inoculation. Control plants remained healthy. The pathogen was consistently reisolated from leaf lesions. The pathogenicity test was completed twice. To our knowledge, this is the first report of the presence of P. tropica on lettuce in Italy as well as worldwide. In the United States, the presence of P. exigua was reported in 2006 (3). The economic importance of the disease at present is limited, probably also because symptoms can be confused with those caused by Botrytis cinerea. However, P. tropica could become a more significant problem because of the importance of the crop.References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) G. H. Boerema. Trans. Br. Mycol. Soc. 67:289, 1976. (3) S. Y. Koike. Plant Dis. 90:1268, 2006.