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First Report of Phytophthora × pelgrandis Causing Root Rot and Lower Stem Necrosis of Common Box, Lavender and Port-Orford-Cedar in Hungary

In 2008 and 2009, necrotic bark lesions at the root collar and lower stem associated with root rot, reduced growth, and wilting were observed on container-grown common box (Buxus sempervirens L.), lavender (Lavandula angustifolia Mill. ‘Hidcote’), and Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murray) Parl. ‘Columnaris’) in three ornamental nurseries in western Hungary. Number of affected plants ranged from approximately 100 (Port-Orford-cedar) to 250 (lavender). Isolations from necrotic root collars of each host plant species yielded four Phytophthora isolates developing uniform colonies on carrot agar with a maximum growth temperature of 35 to 36°C. The isolates were homothallic with smooth-walled oogonia (32.2 ± 2.3 to 35.9 ± 3.5 μm), aplerotic oospores (27.5 ± 1.8 to 32.1 ± 3.1 μm) and both amphigynous and paragynous antheridia, and produced chlamydospores (25.8 ± 3.9 to 29.1 ± 5.2 μm) and papillate sporangia (35.2 ± 2.5 to 43.5 ± 5.6 μm long and 27.6 ± 2.2 to 32.0 ± 3.8 μm wide), mostly obpyriform to nearly spherical or rarely distorted with two or three apices. In spring water, sporangia were both caducous with short pedicel and non-caducous. Multiplex ITS-PCR assay of DNA from all isolates, using primers specific for P. nicotianae (NICF1 and NICR2.1) and P. cactorum (CACTF1 and CACTR1) (1), amplified DNA fragments of the expected size for each Phytophthora species. In addition, isoenzyme analysis revealed a characteristic banding pattern of one heterodimer and two homodimer bands at both loci of the dimeric enzyme malate dehydrogenase. These bands comigrated with those of P. × pelgrandis (Gerlach et al.) (CBS 123385) and isolate PD 93/1339 (courtesy of W. A. Man in ‘t Veld), two natural hybrid strains of P. nicotianae and P. cactorum (2,3), proving that our four isolates can be referred to as this interspecific hybrid. Pathogenicity was tested on 1- or 3-year-old plants of the original host species and cultivars (for common box, cv. Faulkner was used). Cultures were grown for 4 to 6 weeks at 20°C on autoclaved millet grains moistened with V8 broth. Infested and uninfested grains were mixed with autoclaved soil in a ratio of 6% (w/v), and the mixes were used as potting media for transplanting five treated and five control plants per isolate, respectively. Plants were kept in a growth chamber (20°C, 70% RH, 12-h photoperiod). Pots were flooded for 24 h on the 1st and 21st day after transplanting. All plants in infested potting mix showed symptoms of wilt associated with basal stem and root necrosis, similar to those observed on the plants from the field, within 2 and 3 months on lavender and both common box or Port-Orford-cedar, respectively. Additionally, a reduction of root weight ranging from 35 to 68% compared to the control was recorded. Growth reduction was significant at P ≤ 0.019 according to Mann Whitney test. Control plants remained healthy. The same Phytophthora hybrid was reisolated solely from inoculated plants. In Europe, hybrid isolates of P. nicotianae × P. cactorum have been reported on several ornamental plants, including lavender, in the Netherlands and Germany (2,3). However, to our knowledge, this is the first report of this hybrid in Hungary and as a pathogen of common box and Port-Orford-cedar in the world.

References: (1) P. J. M. Bonants et al. Phytopathology 90:867, 2000. (2) W. A. Man in ‘t Veld et al. Phytopathology 88:922, 1998. (3) H. I. Nirenberg et al. Mycologia 101:220, 2009.