First Report of Phytophthora citricola on False Cypress in Hungary.

Abstract

In May 2005, an estimated 10 to 15% mortality of various cultivars of false cypress (also named Lawson cypress or Port-Orford-cedar [Chamaecyparis lawsoniana]) with severe wilt was observed in field stands of an ornamental nursery in western Hungary. Wilted plants had rot-associated reduction of their root system. Root discoloration and occasional chlorosis of lower leaves commenced on potted 3-year-old plants that were held in the open air for 10 to 12 months before planting. Four species of Phytophthora (P. lateralis, P. eriugena, P. hibernalis, and P. cinnamomi) have been reported on this host (2). Direct plating of discolored roots from the most susceptible cultivar (Silver Globus) onto a selective potato dextrose agar or carrot agar medium yielded pure cultures that developed white, stellate colonies with sparse aerial mycelia. The hyphal growth was optimal at 25°C, but the growth above 32°C and below 4°C was completely inhibited. Single, terminal sporangia on simple (occasionally sympodial) sporangiophores formed abundantly in nonsterile soil filtrate but not in agar. Sporangia, 31 to 67 μm (59.1 ± 9.3 μm) long and 25 to 39 μm (31.5 ± 4.0 μm) wide, were noncaducous and semipapillate, variable in shape, mostly obpyriform, rarely obovoid, ovoid-ellipsoid and spherical or bifurcated and distorted, and the exit pore was narrow (7.2 ± 0.8 μm). No external or internal proliferation and no hyphal swellings or chlamydospores were observed. The isolates were homothallic with smooth-walled oogonia (27.3 ± 3.4 μm in diameter) and paragynous antheridia. The oospores (24.7 ± 2.1 μm in diameter) were plerotic. The morpho-physiological features suggested that our isolates belonged to Waterhouse's Group III, and in particular, represented P. citricola. This was confirmed by cellulose acetate electrophoresis of malate dehydrogenase; the isozyme pattern of false cypress isolate was identical to that of the ITS-sequenced (NCBI Accession No. AY366193) P. citricola isolate from a Hungarian alder forest (1). Pathogenicity tests on four 3-year-old potted false cypress (cv. Silver Globus) plants in the greenhouse resulted in rapidly developing (within 2 weeks) sunken, necrotic lesions at the stem base around the site of wound inoculation with a 5-mm-diameter mycelial agar plug. After 12 weeks, each inoculated plant wilted and died. The causal agent was consistently reisolated from necrotic tissues. In Hungary, P. citricola was first isolated and identified from alder forest soil (1). Nonetheless that false cypress has been listed as the host of P. citricola in Norway and Poland (3,4), to our knowledge, this report is the first definitive description of this Phytophthora sp. on this host.