First Report of Brown Rot and Wilt of Fennel Caused by Phytophthora megasperma in Italy

Abstract

Fennel (Foeniculum vulgare Mill. var. azoricum (Mill.) Thell.) in the Apiaceae family is native to southern Europe and southwestern Asia. It is an economically important crop in Italy that produces approximately 85% of all fennel worldwide. The main producing regions are Apulia, Campania, Latium, and Calabria. During the late winter of 2004 in the Crotone Province of the Calabria Region, following heavy rains, patches of fennel plants with symptoms of brown, soft rot of the bulb-like structure formed by the thickened leaf bases, development of yellow leaves, stunting, and wilting of the entire plant were observed in fields. A homothallic Phytophthora sp. was isolated consistently from the brownish tissues of the stout stems and leaf bases of symptomatic plants using a selective medium (3). Pure cultures were obtained by single hyphal tip transfers. On potato dextrose agar (PDA), the diameter of oospores varied from 28 to 42 μm (mean = 36.3 ± 0.4). Antheridia were primarily paragynous. Sporangia were not produced on solid media but were formed in sterile soil extract solution. They were nonpapillate, noncaducous, ovoid and obpyriform (25 to 45 × 35 to 60 μm), and internally proliferating. Optimum and maximum temperatures for radial growth of the colonies on PDA were 25 and 30°C, respectively. At 25°C, radial growth rate was approximately 6 mm per day. On the basis of morphological and cultural characteristics, the isolates were identified as Phytophthora megasperma Drechsler. Electrophoretic patterns of mycelial proteins and four isozymes (acid and alkaline phosphatase, esterase, and malate dehydrogenase) on polyacrylamide gels of the fennel isolates were identical to those of reference isolates of P. megasperma of the BHR (broad host range) group included in P. gonapodyides-P. megasperma Clade 6 (1,3), but distinct from those of the isolates of other nonpapillate species included in Waterhouse's taxonomic group VI. Internal transcribed spacer (ITS) regions of rDNA sequences (2) confirmed that fennel isolates belonged to P. megasperma BHR group. Pathogenicity of a fennel isolate from Calabria (IMI 391711) was confirmed by pouring a zoospore suspension at 2 × 104 zoospores per ml on the soil of 10 3-month-old potted fennel plants. The soil of the inoculated and 10 control seedlings was flooded for 24 h. After 10 days, stems and leaf bases of the seedlings showed a brown rot. Chlorosis and wilting of all seedlings developed after 20 days. Controls inoculated with water did not develop any symptoms. The pathogen was reisolated from typical brown rot and tests were repeated with similar results. To our knowledge, this is the first report of P. megasperma causing disease on fennel. References: (1) S. O. Cacciola et al. For. Snow. Landsc. Res. 76:387, 2001. (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (3) H. Masago et al. Phytopathology, 67:425, 1977.