Fusarium rot is considered a minor disease of citrus fruits. Several Fusarium species have been associated with fruit decay, most commonly F. lateritium Nees, F. moniliforme J. Sheld., F. oxysporum Schltdl., and F. solani (Mart.) Sacc. (2,3). In the winters of 2007, 2009, 2010, and 2011, lemon [Citrus limon (L.) Burm. f.] fruit with white mycelium covering the peduncle were submitted to the Phytopathology Lab at the Estación Experimental Agroindustrial Obispo Colombres. All fruit samples from Tucumán, Argentina, were stored in boxes kept in packinghouse for more than 1 month. In 2007 only, light to dark brown flavedo around the peduncle was observed in less than 1% of the sample fruit received. No internal breakdown was visible. No change in rind color was observed in the samples received in remaining years. Abundant Fusarium sp. conidia were observed on the mycelium. Colonies with white to violet fluffy aerial mycelium developed on potato dextrose agar (PDA) and produced abundant ovoid or oblong microconidia (1.9 to 3.6 × 4.8 to 10.8 μm), usually unicellular, borne in false heads on short monophialides, and loculated slightly falcate macroconidia were mostly three to five septate (2.4 to 4.8 × 19.2 to 31.2 μm). Unbranched and branched-monophialidic conidiophores were observed. Simple or paired chlamydospores developed on synthetic nutrient agar (1 g KH2PO4, 1 g KNO3, 0.5 g MgSO4.7H2O, 0.5 g KCl, 0.2 g sucrose, and 20 g agar/liter distilled water). On the basis of morphological and cultural criteria, 22 isolates were identified as F. oxysporum (4) designated as D1 to D22. Morfological identification was confirmed by PCR (1) using genomic DNA extracted from the mycelium of pure culture, and an amplified product of 70 bp, specific for the species F. oxysporum, was obtained. The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS4/ITS5 and secuenced. BLAST analysis of the 600 bp segment showed a 100% indentity with F. oxysporum, strains CCF 4362 and 1166 (GenBank Accession Nos. HE974454 and FR731133, respectively). Pathogenicity tests were conducted twice by inoculating 10 surface-disinfected wounded lemon fruit. A rind disc (5 mm in diameter and 1 mm deep) near the stem end was removed and a 5-mm-diameter agar disc of D2 isolate (grown at 25°C for 5 days on PDA) was attached to the wound replacing the rind disc. The inoculation site was covered with moistened cotton wool and the fruit were wrapped in plastic bags to prevent the inoculum from drying out. Ten control fruit were inoculated with uncultured PDA plugs (5 mm in diameter). All fruit were maintained in a growth chamber at 25°C under humid conditions. After 5 to 6 days, all inoculated fruit showed white aerial mycelium, initially on the inoculation site and then on the peduncle, similar to that observed on naturally infected fruit. After 20 days, two fruit developed stem end dry rot and showed peduncle fall but no internal breakdown was visible. Control fruit developed any symptom as described above. F. oxysporum was consistently reisolated from infected tissues, completing Koch's postulates. To our knowledge, this is the first report of Fusarium rot caused by F. oxysporum on lemon in Tucumán, Argentina.
References: (1) V. Edel et al. Mycol. Res. 104:518, 2000. (2) H. S. Fawcett. Citrus Diseases and Their Control, 1936. (3) A. Z. Joffe and M. Schiffmann-Nadel. Fruits 27:117, 1972. (4) P. E. Nelson et al. Fusarium species: An Illustrated Manual for Identification, 1983.