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The genus Grevillea (family Proteaceae) comprises over 300 species and is a popular and widely cultivated group of Australian plants. In the last 3 years, numerous potted grevilleas with symptoms of decline associated with a rot of feeder roots were found in ornamental nurseries in Sicily. Aboveground symptoms were reduced growth, yellowing of foliage, wilt, dieback, and death of the entire plant. The disease was observed on many commercial cultivars and was especially severe on G. alpina (mountain grevillea), G. juniperina (juniper-leaf grevillea), G. lavandulacea (lavender grevillea), and G. rosmarinifolia (rosemary grevillea) as well as the hybrid cultivars Clearview David (G. lavandulacea × rosmarinifolia) and Poorinda Rondeau (G. baueri × lavandulacea), while G. lanigera (woolly grevillea) cv. Mount Tamboritha and G. thelemanniana subsp. obtusifolia appeared resistant. A species of Phytophthora was consistently isolated from rotted roots of symptomatic plants using a selective medium (4), and pure cultures were obtained by single-hypha transfers. The species was identified as P. palmivora (E.I. Butler) E.I. Butler on the basis of morphological and cultural characters. On solid media, all isolates produced elliptical to ovoid, papillate sporangia with a mean length/width ratio of 1.8. Sporangia were caducous with a short pedicel (5 μm) and a conspicuous basal plug. All isolates were heterothallic (mating type A1) and produced oogonia and oospores only when paired with A2 mating type reference isolates of P. nicotianae and P. palmivora. Antheridia were amphyginous. Identification was confirmed by electrophoresis of mycelial proteins in polyacrylamide slab gels (1). The electrophoretic patterns of total soluble proteins and six isozymes (alkaline phosphatase, esterase, fumarase, NAD-glucose dehydrogenase, malate dehydrogenase, and superoxide dismutase) of isolates from grevillea were identical to those of a reference isolate of P. palmivora from Coronilla valentina subsp. glauca (2) but distinct from those of reference strains of eight other papillate species of Phytophthora included in Waterhouse's taxonomic group VI. Koch's postulates were fulfilled using 6-month-old rosemary grevillea plants that were transplanted into pots filled with soil that was artificially infested with chlamydospores (50 per gram of soil) produced in submerged cultures (3) by grevillea isolate IMI 390579. Plants were maintained in a glasshouse at 20 to 28°C and watered to field capacity once a week. One month after transplanting, infected plants showed decline symptoms similar to those of naturally infected plants. Control plants grown in pots containing noninfested soil remained healthy. P. palmivora was reisolated from roots of symptomatic plants. It appears that P. palmivora has become a widespread root pathogen in commercial ornamental nurseries in Italy (2). References: (1) S. O. Cacciola et al. EPPO Bull. 20:47, 1990.D. (2) S. O. Cacciola et al. Plant Dis. 86:327, 2002. (3) J. Y. Kadooka and W. H. Ko. Phytopathology 63:559, 1973. (4) H. Masago et al. Phytopathology 67:425, 1977. |
