First report of Phytophthora boehmeriae on black wattle in Brazil

Abstract

Black wattle (Acacia mearnsii) is an Australian tree species cultivated in several countries. In Brazil, particularly in Rio Grande do Sul State (RS), approximately 140 000 ha are cultivated for tannin extraction (Dos Santos et al., 2005). The gummosis complex, which has Phytophthora nicotianae as one of the causal agents, is the main disease of black wattle in Brazil and is characterised by lesions at the trunk base with gum exudation (Dos Santos et al., 2005). Different symptoms were observed in plantations at the municipality of Piratini, RS (southern Brazil) and were characterised by dark lesions without gum exudation on trunks of 3- to 4-year-old wattle trees up to 10 m height. Isolates of Phytophthora spp. were obtained from the lesions and deposited in the Brazilian collection of Phytophthora species, under accession numbers CBP 307, 308 and 309. Sporangia of all isolates were ovoid to spherical, papillate and caducous, measuring 35 µm ± 1·42 × 30 µm ± 1·37, with a length/width ratio of 1·16:1, mean depth of papillae of 4·83 µm ± 0·04, and pore exit of 4·69 µm ± 0·04. The isolates were homothallic, forming plerotic oospores with smooth walls and amphigynous antheridia. The ITS sequences obtained for isolates CBP 307 (AY428533), CBP 308 (AY428534), and CBP 309 (AY428535) were identical and most closely matched those of two isolates of P. boehmeriae KACC40173 (AY228076) from Korea and SCRP23 (DQ297406) from China. This and the morphological similarity (Erwin & Ribeiro, 1996) suggest that these isolates are P. boehmeriae. However, seven clear single base pair differences were noted between the Brazilian and other P. boehmeriae isolates. This, combined with isozyme variation (Oudemans & Coffey, 1991) suggests that further studies are needed to confirm the taxonomic status of P. boehmeriae. Pathogenicity tests were done by inoculating five 1-year-old wattle plants with 7 mm mycelial discs of 5-day old cultures of the three isolates used in this study. The mycelial discs were placed in 7 mm diameter holes made in the bark with a cork borer, at 5 cm above the soil. Plants were maintained at approximately 25°C and were assessed 45 days after inoculation. All three isolates were pathogenic to black wattle and were re-isolated from the lesions. Phytophthora boehmeriae was reported as one of the causal agents of the gummosis complex on black wattle in South Africa (TPCP, 2004) and is of quarantine importance for the citrus industry in Brazil. This is the first report of the involvement of P. boehmeriae in the aetiology of the gummosis complex of black wattle in Brazil.