First Report of Leaf Spot of Lettuce (Lactuca sativa) Caused by Fusarium equiseti in Italy.

During the spring and summer of 2014, leaf lettuce (Lactuca sativa) plants cv. Elisa grown under greenhouse conditions in Veneto (northern Italy) showed symptoms of a previously unknown foliar disease. Symptoms were observed on 20-day-old plants and consisted of small, circular, gray-brown leaf spots (1 to 3 mm in diameter), sometimes progressing to elliptical. Necrotic lesions were cracked in the center and showed a well-defined border that frequently was surrounded by a yellow halo. Approximately 10 ha were affected by this problem with 10 to 30% of plants showing symptoms on 15 to 35% of the leaves. An orange-brown colony with characteristics of Fusarium spp. was isolated from infected tissues on potato dextrose agar (PDA). The isolates were purified, subcultured on PDA, and single-spore cultures were obtained. On PDA, they produced orange-brown colonies and purple pigments in the agar. On carnation leaf agar (CLA), the isolates produced hyaline macroconidia with dorsiventral curvature and 5 to 7 septa, measuring 31.7 to 45.5 × 3.7 to 6.1 (average 37.5 × 4.5) µm. Chlamydospores developed after 10 days incubation and measured 8.2 to 14.7 (average 11.2) µm. Microconidia were not observed. Such characteristics are typical of the genus Fusarium (Leslie and Summerell 2006). Amplification of the elongation factor 1 alpha gene (EF1α) with primers EF1/EF2 (O’Donnell et al. 1998) yielded a 685-bp amplicon (GenBank Accession No. KT149290). BLASTn analysis (Altschud et al. 1997) of the sequence obtained a 99% homology with F. incarnatum (GenBank Accession No. JF270175.1), included in the F. equiseti-F. incarnatum species complex (FIESC). However, further phylogenetic analysis based on EF1α and IGS regions showed a clear homology with F. equiseti and not with F. incarnatum. To confirm pathogenicity, 20-day-old leaf lettuce plants cv. Elisa were transplanted in 2-liter pots, filled with a steamed peat: perlite: sand (60:20:20 vol/vol) substrate and maintained in a growth chamber at 25 ± 1°C. Five pots per treatment were used, with each pot containing two plants. The artificial inoculation was carried out by spraying leaves with a spore suspension (1 × 106 macroconidia/ml) prepared from 15-day-old PDA cultures of the pathogen. Control plants were inoculated with distilled water. Plants were kept covered with plastic bags for 5 days. Leaf spots similar to those that developed on the original plants, developed 7 days after the inoculation with a disease incidence from 40 to 50% affected leaf. All the noninoculated plants remained healthy. A fungus morphologically identified as F. equiseti was consistently isolated from all the symptomatic plants. The pathogenicity test was conducted twice with the same results. This is the first report of F. equiseti on L. sativa in Italy as well as worldwide. The same pathogen recently has been observed on rocket in northern Italy (Garibaldi et al. 2011). Currently, this disease is spreading to several farms in northern Italy. © 2016 The American Phytopathological Society.