Scotch broom: a new host of Phytophthora megasperma in Italy

Scotch broom (Cytisus scoparius L.) is a pioneer shrub widely distributed in the Mediterranean region and utilised for ecological restoration of degraded lands. In spring 2002, a severe decline and mortality of scotch broom seedlings and adult plants was observed in a nursery and in a restored mine in Italy. Symptoms were characterised by yellowing and wilting of leaves extending to the main stem, and death (Fig.1). Affected plants showed a severe reduction of the root system and necrosis of tap and feeder roots.

A Phytophthora sp. was isolated from infected tissues and soil removed from the root zone of symptomatic plants. Isolations were made by baiting with rhododendron leaves and plating on PARBhy medium. The species isolated was identified as Phytophthora megasperma Drechsler based on morphological and cultural characterisitcs (Erwin & Ribeiro, 1996). Isolates produced nonpapillate ovoid sporangia and were homothallic with smooth oogonia and irregularly spherical antheridia. Colonies showed optimum growth at 25°C and complete inhibition at 32°C. Identification of the isolates was confirmed by comparing the RLFP patterns of the ITS region of rDNA with those of known Phytophthora species using PhytID, an online database for identification of plant pathogenic Phytophthora species which includes, among with the RFLP pattern for P. megasperma, those of P. sojae Kauf & Gerd, P. medicaginis Hansen & Maxwell and P. trifolii Hansen & Maxwell, species known to be pathogenic to legumes (http://www.phytid.org/).

Pathogenicity tests were conducted by soil infestation on 3 month-old scotch broom seedlings using 2 isolates of P. megasperma. Sporulation was induced by 24 h flooding of the soil. One week after inoculation, symptoms were produced identical to those described on naturally infected plants. Controls remained healthy. P. megasperma was reisolated from the soil and roots of inoculated plants. To our knowledge this is the first report of P. megasperma as a root rot pathogen of scotch broom.