Root and Basal Stem Rot of Scotch Broom Caused by Phytophthora citricola and P. drechsleri in Italy

Abstract

Scotch broom (Cytisus scoparius (L.) Link, Fabaceae), an evergreen shrub native to Europe, is cultivated as a garden plant. In 2003 and 2004, potted plants with symptoms of leaf chlorosis, defoliation, and eventual wilt and associated with root and collar rot were observed in ornamental nurseries in Sicily. As much as 10% of plants were affected in a single nursery. Two species of Phytophthora were consistently isolated alone or together from the same pot with the selective medium of Masago et al. (2). Pure cultures were obtained by single-hypha transfers and the species were identified as P. citricola Sawada (approximately 40% of isolations) and P. drechsleri Tucker (60% of isolations) on the basis of morphological, cultural characters, and electrophoretic phenotype. The isolates of P. drechsleri grew between 10 and 37°C (optimum 27°C) on potato dextrose agar (PDA). The sporangia produced on V8 juice agar (V8A) were ellipsoid to obpyriform, nonpapillate, persistent with internal proliferation, and often forming in a sympodium. Sizes varied, 30 to 60 × 20 to 40 μm (length/width ratio between 1.4 and 2.2). The hyphal swellings were produced in aqueous culture. All isolates were A1 mating type and formed plerotic oospores (mean diameter (ф) 25 μm) with amphigynous antheridia when paired with the A2 reference isolates of P. cryptogea on V8A plus β-sitosterol. The aryl-esterase and malate dehydrogenase isozymes of scotch broom isolates on polyacrylamide slab gels (1) were identical to those of the authentic isolate CBS 292.35 of P. drechsleri and differed from reference cultures of other nonpapillate species. The cardinal temperatures of P. citricola isolates on PDA ranged from 2 to 30°C (optimum 25°C). In liquid culture, the isolates produced irregular-shaped, obovoid to obpyriform sporangia 20 to 70 × 21 to 44 μm that were noncaducous, semipapillate or with inconspicuous papilla, often with two apices. The isolates were homothallic and produced oospores (mean ф 22 μm) with paragynous antheridia. The electrophoretic phenotype of these isolates was identical to the phenotype of P. citricola reference isolates and very different from that of the reference isolates of other semipapillate species. The pathogenicity tests of the representative isolates of P. drechsleri (IMI 391710) and P. citricola (IMI 391715) were carried out in a screenhouse. Twenty 3-month-old scotch broom seedlings were transplanted into pots (12 cm ф) filled with soil infested with the inoculum produced on a mixture of vermiculite and autoclaved oat seeds. The plants were maintained at 20 to 28°C and watered to field capacity once a week. After 30 to 40 days, all inoculated plants showed symptoms of wilting and root rot. The 20 control plants transplanted into pots containing noninfested soil remained healthy. P. citricola and P. drechsleri were reisolated from infected tissues. To our knowledge, this is the first report of P. citricola and P. drechsleri on scotch broom. A root rot of scotch broom caused by P. megasperma has been reported in central Italy (3).