Phytophthora palmivora a New Pathogen of Lavender in Italy

Root rot caused by Phytophthora nicotianae is considered the most serious disease of lavender in commercial cultivations in Italy. In summer 2001, in the Gela area (Sicily), ≈60% of 34,000 2-year-old landscape shrubs of English lavender (L. angustifolia) grown in a clay loam soil showed symptoms of dieback associated with root rot. Plants had been transplanted from pots in May and watered using a trickle irrigation system. A species of Phytophthora was isolated consistently from roots of symptomatic plants using potato dextrose agar (PDA) containing benomyl, nystatin, pentachloronitrobenzene, rifampicin, ampicillin, and hymexazol. The species was identified as P. palmivora on the basis of morphological and cultural characters. Ten representative single-zoospore isolates were characterized. On agar media, the isolates produced elliptical to ovoid, papillate sporangia, with a mean length/breadth ratio of 1.8. Sporangia, produced on sporangiophores forming simple sympodia (as many as 20 sporangia per sympodium), were caducous with a short pedicel (mean pedicel length = 5 μm) and a conspicuous basal plug. In addition to typical sporangia, all isolates produced sporocysts, i.e., subglobose, nonpapillate sporangia (2). The minimum temperature for mycelium growth on PDA was 10°C, the optimum was 27°C, and the maximum was 35°C. All isolates were A1 mating type. Antheridia were amphyginous. The identification was confirmed by electrophoresis of mycelial proteins on a polyacrylamide slab gel. Electrophoretic banding patterns of total soluble proteins and eight isozymes of the isolates from lavender were identical to those of a reference isolate of P. palmivora from olive (1). Conversely, the electrophoretic phenotype of the isolates from lavender was distinct from those of reference isolates of other species, including P. cactorum, P. capsici, P. citrophthora, P. nicotianae, and P. tropicalis. The pathogenicity of a representative isolate of P. palmivora from lavender was tested in the greenhouse using 6-month-old plants of English lavender, Rosea, a commercial cultivar very susceptible to root rot caused by P. nicotianae (3). Inoculum was produced on a mixture of vermiculite and autoclaved oat seeds (4) and mixed with soil (sand/lime/peat 1:1:1) at a concentration of 4% (vol/vol). Plants were transplanted to pots filled with infested soil. Control plants were grown in pots containing noninfested soil. After transplanting, all pots were flooded for 24 h by plugging the drain hole. Three months after transplanting all plants grown in pots containing infested soil showed extensive root necrosis and dieback symptoms. Control plants remained healthy. P. palmivora was recovered from roots of symptomatic plants. To our knowledge, this is the first report from Italy of P. palmivora on lavender. Root rot caused by P. palmivora may be a potential problem for commercial cultivation of lavender. References: (1) S. O. Cacciola et al. Plant Dis. 84:1153, 2000. (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St Paul, MN, 1996. (3) G. Minuto et al. Inf. Fitopatol. 51:69, 2001. (4) E. Sánchez-Hernández et al. Plant Dis. 85:411, 2001.