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First Report of Phytophthora palmivora on Coronilla valentina subsp. glauca in Italy

The genus Coronilla L. (family Fabaceae), which includes several species native to central and southern Europe, such as C. varia L. (axseed or crown-vetch), C. emerus (scorpion senna), and C. valentina L., is used in Italy as a landscape shrub or potted ornamental plant. During the summer of 2001, 80% of approximately 10,000 1-year-old plants of C. valentina subsp. glauca (L.) Batt. used to landscape an industrial area in the Caltanissetta Province (Sicily) showed symptoms of dieback associated with basal stem and root rot. Plants had been transplanted from pots in April and watered using a trickle irrigation system. A species of Phytophthora was isolated consistently from rotted roots and basal stems using BNPRAH selective medium (3). Pure cultures of this fungus were obtained by single-hypha transfers. Ten isolates, each originating from a single plant, were identified as P. palmivora (Butler) Butler on the basis of morphological and cultural characters as described by Erwin and Ribeiro (1). On solid media, including potato dextrose agar, cornmeal agar, and V8-juice agar, all the isolates produced elliptical to ovoid, papillate sporangia with a mean length/breadth ratio of 1.8. Sporangia were caducous with a short pedicel (mean pedicel length = 5 µm) and a conspicuous basal plug. Mating type was determined on V8 agar in dual culture with mating type A₁ and A₂ of reference isolates of P. nicotianae and P. palmivora. All isolates were heterothallic and produced oogonia and oospores only with reference isolates of the A₂ mating type. Antheridia were amphigynous. Electrophoresis of mycelial proteins on polyacrylamide slab gel confirmed that all isolates were pure cultures and belonged to the same species. Koch's postulates were fulfilled using 6-month-old C. valentina subsp. glauca plants that were transplanted into pots filled with soil artificially inoculated with chlamydospores (50 chlamydospores per gram of soil) produced in submerged axenic cultures (2). The plants were maintained in a glasshouse at temperatures ranging from 18 to 28°C, and the pots were watered to field capacity once a week. One month after transplanting, 70% of plants showed dieback symptoms, while control plants, which were grown in pots containing noninoculated soil, remained healthy. The pathogen was reisolated from roots and basal stems of symptomatic plants. These results demonstrate that P. palmivora is the causal agent of dieback of C. valentina subsp. glauca plants. High temperatures in summer and waterlogging of soil due to excess irrigation water could have enhanced disease development. To our knowledge, this is the first report of P. palmivora on a species of Coronilla. P. palmivora is an exotic pathogen, but it is becoming widespread in Italy, where it has been reported from various regions on different hosts, including cyclamen, English ivy, palms, Pittosporum, and olive.

References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society. St Paul, MN, 1996. (2) J. Y Kadooka and W. H. Ko. Phytopathology 63:559, 1973. (3) H. Masago et al. Phytopathology 67:425, 1977.