During the 2011 to 2012 crop season, a severe leaf spot disease of cucumber (Cucumis sativus) cv. Cadiz was noticed on crops in some greenhouses in the Goudouras area, Lasithi, Crete, Greece. Symptoms appeared in late winter, mainly on the leaves of the middle and upper part of the plants. Initially, small necrotic pinpoint lesions with white centers, surrounded by chlorotic halos, 1 to 3 mm in diameter, appeared on the upper leaf surfaces, and these progressively enlarged to spots that could coalesce to form nearly circular lesions up to 2 cm or more in diameter. Stemphylium-like fructifications appeared on necrotic tissue of older lesions. Severely affected leaves became chlorotic and died. No other part of the plant was affected. Small tissue pieces from the edges of lesions were surface disinfected in 0.5% NaClO for 5 min, rinsed in sterile distilled water, plated on acidified potato dextrose agar and incubated at 22 ± 0.5°C with a 12-h photoperiod. Stemphylium sp. was consistently isolated from diseased samples. Colonies showed a typical septate mycelium with the young hyphae subhyaline and gradually became greyish green to dark brown with age. Conidiophores were subhyaline to light brown, 3- to 10-septate, up to 200 μm in length, and 4 to 7 μm in width, with apical cell slightly to distinctly swollen, bearing a single spore at the apex. Conidia were muriform, mostly oblong to ovoid, but occasionally nearly globose, subhyline to variant shades of brown, mostly constricted at the median septum, 22.6 ± 6.22 (11.9 to 36.9) μm in length, and 15.1 ± 2.85 (8.3 to 22.6) μm in width, with 1 to 8 transverse and 0 to 5 longitudinal septa. DNA from a representative single-spore isolate was extracted and the internal transcribed spacer region (ITS) of ribosomal DNA (rDNA) was amplified using the universal primers ITS5 and ITS4. The PCR product was sequenced and deposited in GenBank (Accession No. JX481911). On the basis of morphological characteristics (3) and a BLAST search with 100% identity to the published ITS sequence of a S. solani isolate in GenBank (EF0767501), the fungus was identified as S. solani. Pathogenicity tests were performed by spraying a conidial suspension (105 conidia ml–1) on healthy cucumber (cv. Knossos), melon (C. melo, cv. Galia), watermelon (Citrullus lanatus cv. Crimson sweet), pumpkin (Cucurbita pepo, cv. Rigas), and sponge gourd (Luffa aegyptiaca, local variety) plants, at the 5-true-leaf stage. Disease symptoms appeared on cucumber and melon only, which were similar to those observed under natural infection conditions on cucumber. S. solani was consistently reisolated from artificially infected cucumber and melon tissues, thus confirming Koch's postulates. The pathogenicity test was repeated with similar results. In 1918, a report of a Stemphylium leaf spot of cucumber in Indiana and Ohio was attributed to Stemphylium cucurbitacearum Osner (4), but that pathogen has since been reclassified as Leandria momordicae Rangel (2). That disease was later reported from Florida (1) and net spot was suggested as a common name for that disease. For the disease reported here, we suggest the name Stemphylium leaf spot. This is the first report of a disease of cucumber caused by a species of Stemphylium.
References: (1) C. H. Blazquez. Plant Dis. 67:534, 1983. (2) P. Holliday. Page 243 in: A Dictionary of Plant Pathology. Cambridge University Press, Cambridge, UK, 1998. (3) B. S. Kim et al. Plant Pathol. J. 15:348, 1999. (4) G. A. Osner. J. Agric. Res. 13:295, 1918.