Lavender Cotton Root Rot: A New Host of Phytophthora tentaculata Found in Spain

Abstract

Lavender cotton, Santolina chamaecyparissus, is an evergreen shrub growing primarily in dry, calcareous habitats and is grown in rock gardens and mixed borders mainly for its ornamental and aromatic foliage. During 2004, several commercial nurseries in Valencia Province (eastern Spain) reported high mortality of lavender cotton. The foliage of the diseased plants turned brown, wilted, and died. A Phytophthora sp. was isolated consistently from the soil and roots of infected plants using apple baits and the selective medium PARBH (1), respectively. Four pure cultures (PS-31, PS-32, PS-33, and PS-34) were established from hyphal tips and characterized. Colony morphology on potato dextrose agar (PDA) at 24°C was stoloniferous (short stubby branches) with a growth rate of 2.2 mm per day. Sporangia, chlamydospores, and oospores were produced on V8 agar. The sporangia were ovoid to obpyriform, 27.5 to 64.8 (48.3) × 25 to 52.5 (37.5) μm, length/breadth ratio of 1.3:1, and papillate, from which 20% were caducous with a short pedicel (<5 μm). Hyphal swellings and chlamydospores (22 to 38 μm in diameter) were present. Isolates were homothallic, oogonia were globose, mostly terminal 27.5 to 40 (36.2) μm in diameter, 88% of the antheridia were paragynous, monoclinous, or diclinous, and occasionally with two paragynous antheridia per oogonium. Amphigynous antheridia (12%) were also observed. Oospores were aplerotic, 25 to 35 (32.3) μm in diameter, and thin walled. These characteristics and measurements conformed to the description of P. tentaculata described by Kröber and Marwitz (2). Sequencing the internal transcribed spacer region of Santolina isolates PS-32 and PS-34 and comparison of these sequences with other sequences available in GenBank revealed that they were identical to P. tentaculata (AF266775). Pathogenicity tests used 10 4-to-5-month-old potted lavender cotton and two methods. In the first method, inoculum was prepared on a media of 200 g of oats and 120 ml of V8 juice to 1 liter of distilled water. The medium was inoculated with P. tentaculata grown on PDA and incubated in the dark at 20°C for 4 weeks. Inoculum was buried into the compost mixture around the roots at a rate of 3% (w/v). The second method applied a zoospore drench of 50 ml per plant (1 × 104 zoospores per ml) obtained by inducing zoospores in sterile soil extract from cultures of V8 juice agar. The control plants were inoculated with sterile media and sterile distilled water. The following day, the pots were flooded for 2 days, plants were maintained in a glasshouse at 24 ± 5°C, and watered twice a week. All plants inoculated with the first method had wilted foliage and died within 2 months after inoculation, while plants inoculated with zoospores died after 3 months. P. tentaculata was reisolated and the test was repeated twice. The control plants did not show any symptoms of the disease. P. tentaculata was first reported causing root and stalk rot on Chrysanthemum frutescens hybrids, C. leucanthemum, Delphinium ajacis, and Verbena hybrids in Germany (2). It has also been reported on Verbena hybrids in Spain (3). To our knowledge, this is the first report of P. tentaculata causing root rot on lavender cotton. References: (1) S. N. Jeffers and S. B. Martin. Plant Dis. 70:1038, 1986. (2) H. Kröber and R. Marwitz. Z. Pflanzenkr. Pflanzenschutz 100:250, 1993. (3) E. Moralejo et al. Plant Pathol. 53:806, 2004.