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First Report of Fusarium oxysporum on Sweet Pepper Seedlings in Almería, Spain

In March of 2013, new symptoms were observed in more than seven million nursery-grown sweet pepper (Capsicum annuum) plants in El Ejido, Almería (southern Spain). Symptoms included wilting without yellowing of leaves and stunting of plants. Plant crowns exhibited necrosis that advanced through the main root along with slight root rot. Xylem was not affected above or below the crown. Symptoms were thought to be caused by the well-known pepper pathogen Phytophthora capsici. However, sporodochia of Fusarium oxysporum were observed on plant crowns. Symptomatic seedlings (n = 200) were sampled and analyzed. Tissue from roots and epidermal crowns were plated on PDA, PARP, and Komada media, as well as stem discs on PDA and Komada. No Phytophthora sp. were observed and F. oxyporum was exclusively isolated from all 200 samples, from roots and crowns, but not from xylem. Pathogenicity of 60 of these F. oxysporum isolates was studied by inoculation onto sweet pepper plants (cv. del Piquillo) at the 2-true-leaf stage. Twelve plants per isolate, grown on autoclaved vermiculite, were inoculated by drenching with 20 ml of a conidial suspension (1 × 10⁵ CFU/ml) of each isolate per plant. Each suspension was obtained by blending one PDA petri dish fully covered with one isolate. Non-inoculated plants served as control. Plants were maintained for 30 days in a growth chamber with a 14-h photoperiod (1.6 ×·10⁴ lux) and temperatures at 23 to 26°C. The assay was conducted twice. Symptoms described above were reproduced on crown and roots of the inoculated plants with no symptoms in stem discs. No symptoms were observed on controls after 48 days. Host specificity was tested for 13 isolates to tomato (Solanum lycopersicum) cv. San Pedro, eggplant (S. melongena) cv. Alegria, cucumber (Cucumis sativus) cv. Marketmore, watermelon (Citrullus lanatus) cv. Sugar Baby, and Chinese cabbage (Brassica campestris subsp. condensa) cv. Kasumi (4). These plants were inoculated as previously described for pathogenicity tests (12 plants per species, repeated twice). None of the plants exhibited the characteristic symptoms after 60 days. Five isolates of F. oxysporum f. sp. radicis-cucumerinum and four isolates of F. o. f. sp radicis-lycopersici were also inoculated without any symptoms in any of the inoculated sweet pepper plants. Morphological identity of all isolates corresponded to F. oxysporum. The fungi were identified following the morphological keys and methodology provided by (1) and (2). Three isolates from the 60 tested were selected for molecular identification. Molecular identification was performed by sequencing partial TEF-1α gene (3). Subsequent database searches by BLASTn indicated that the resulting sequence of 659-bp had 100% identity with the corresponding gene sequence of F. oxysporum. The sequences were identical for the three isolates and were deposited on the EMBL Sequence Database (HG916993, HG916994, and HG916995). Results suggest that the pathogenic ability of the isolates varies from a vascular Fusarium wilt. F. oxysporum f. sp. capsici is a reported pathogen to sweet pepper (5), but the symptoms we have found are closer to those manifested by the formae speciales that causes root and crown rot of other plants. Consistent with the convention stablished for similar diseases we propose the name F. oxysporum f. sp. radicis-capsici f. sp. nov.

References: (1) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell, Ames, IA, 2006. (2) P. E. Nelson et al. Fusarium species. An Ilustrated Manual for Identification. The Penn St. University Press, 1983. (3) K. O'Donnell et al. Proc. Nat. Acad. Sci. 95:2044, 1998.(4) L. M. Oelke and P. W. Bosland. Capsicum Eggplant Newsl. 20:86, 2001. (5) V. C. Rivelli. M.S. Thesis. Dep. Plant Pathol. and Crop Phys. Louisiana State Univ., Baton Rouge, 1989.