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Occurrence of Fusarium Wilt Caused by Fusarium oxysporum f. sp. spinaciae on Spinach (Spinacia oleracea) in Southern Italy

Fusarium oxysporum f. sp. spinaciae (Sherb.) W.C. Snyder & H.N. Hansen is the causal agent of Fusarium wilt of spinach (Spinacia oleracea L.), a serious disease of spinach worldwide (1). In September 2011, several plants of an unknown spinach variety grown for the production of packaged ready-to-eat salads (4th range) in a greenhouse in southern Italy (Pontecagnano, Salerno) showed yellowing of older leaves, reduced root development, blackening of the vascular tissues, and wilting of 60-day-old plants; necrotic lesions at the taproot base were occasionally present. Yield losses up to 70% were observed. Small portions of symptomatic tissues from the basal vascular stem were disinfected with sodium hypochlorite (1% active Cl₂), rinsed with sterile water, and then placed on potato dextrose agar (PDA) amended with neomycin (50 ppm), chloramphenicol (50 ppm), and streptomycin (100 ppm). Pink to white colonies with a fluffy aerial mycelium rapidly developed; pale orange sporodochia containing thin walled macroconidia, mostly 3-septate, short to medium length, with a curved apical cell and a notched or foot-shaped basal cell were present. Oval to reniform, 0-septate microconidia were formed on typical single short monophyalides or abundantly on false heads. Chlamydospores were formed singly or in pairs in 1-month-old plates. On the basis of these morphological characters, the fungus was identified as F. oxysporum. In order to confirm the diagnosis, DNA from a single spore culture of an isolate (11-113PANT3) was extracted, amplified by PCR using primers EF 1H and EF 2T (2) corresponding to a segment of the transcription elongation factor 1α (EF-1α) gene, and the PCR product sequenced at GenChron (Rome, Italy). A homology search in GenBank using the BLASTn algorithm showed 100% identity of the obtained sequence with several sequences of formae speciales of F. oxysporum, including the NRRL26871, corresponding to F. oxysporum f. sp. spinaciae. For the pathogenicity tests, 15-day-old spinach seedlings of the variety Spargo F1 were inoculated at the cotyledon stage by dipping the roots for 5 min in a monosporic conidial suspension of the isolate 11-113PANT3 at a concentration of 1 × 10⁶ CFU ml⁻¹. Twenty plantlets in two replicates were inoculated and incubated in a growth chamber at 26°C. The same number of uninoculated plants were used as controls. Yellowing of leaves and vascular blackening was observed in all inoculated plants that wilted and died within 10 days. The colonies re-isolated from all the inoculated plants were identified morphologically as F. oxysporum, thus fulfilling Koch's postulates. Experiments were repeated twice. Based on host source and sequence similarity, the fungus was identified as F. oxysporum f. sp. spinaciae. To our knowledge, this is the first report of this species on spinach in Italy. Since this first report, Fusarium wilting of spinach has been frequently observed in other greenhouses, and it is becoming of concern for Italian salad mix producers. Continuous cropping and high-input cultivation systems are among the possible factors favoring the spread of the fungus. The management of the disease could be achieved through the adoption of crop rotation, the use of partially resistant cultivars, and by spinach seed treatments.

References: (1) J. C. Correll et al. Plant Dis. 78:653, 1994.(2) K. O'Donnell et al. Proc. Nat. Acad. Sci. 95:2044, 1998.