From 2007 to 2009, necrotic bark lesions at the root collar and lower stem associated with root rot were observed on container-grown cork-bark fir (Abies lasiocarpa var. arizonica, Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murray) Parl. ‘Barabits Gold’), lavender (Lavandula angustifolia Mill. ‘Hidcote’), flowering currant (Ribes sanguineum Pursh ‘King Edward VII’), and lilac (Syringa vulgaris L. ‘Belle de Nancy’) in six ornamental nurseries in western Hungary. Symptoms also included reduced growth, wilting, and desiccation of branches. Mortality of affected plants ranged from a low level in flowering currant to a high frequency in cork-bark fir (1,000 plants; 50%) and lavender (2,500 plants; 50%). Isolations from necrotic tissues onto PARPB medium (1) yielded 13 isolates of a Phytophthora sp. None of the isolates produced sexual structures, sporangia, or chlamydospores but developed slightly stellate or patternless colonies with loose, aerial mycelium on nonselective carrot agar (CA) at 25°C. One isolate from each host species was further characterized and tested for pathogenicity. Growth on CA was fastest at 25°C (7.9 to 8.6 mm per day) and no growth occurred below 5°C or above 34°C. In nonsterile stream water, persistent, mono- and bipapillate, mostly ovoid, rarely distorted sporangia, measuring 40.5 to 49.4 × 29.8 to 37.3 μm were produced. In pairings on CA with A1 and A2 strains of P. cambivora and P. nicotianae, used as testers, none of our isolates produced gametangia. On the basis of these characteristics, the pathogen from ornamentals appeared to be P. citrophthora (Smith & Smith) Leon. (1). Species identity of all 13 isolates was determined in single-round PCR assays with the P. citrophthora-specific primer-pair Pc2B/Pc7 (2) and/or by sequencing the rDNA internal transcribed spacer (ITS) regions amplified with the universal ITS1/ITS4 primers. The primers Pc2B and Pc7 generated a single DNA fragment of the expected size (approximately 210 bp), and the rDNA ITS sequences (NCBI Accession Nos. GU723282, GU723284, GU723285, and GU723287) showed 99 to 100% homology with many GenBank sequences (e.g., HQ697232) of P. citrophthora as the closest match. Pathogenicity to the original host plant cultivars was tested on 2- or 3-year-old healthy plants potted in sterile soil and inoculated at the root collar (2 replicates per isolate). A 4-mm-diameter bark plug was removed and a mycelial disc of the same size from an actively growing CA culture was placed into the hole. Control plants received sterile CA plugs. Inoculation points were sealed with sterile moist cotton and Parafilm, covered with sterile soil, and then the plants were kept in a greenhouse at 24 ± 4°C. Symptoms identical to those observed on the naturally diseased hosts developed on inoculated plants within 3 months. Control plants remained healthy. P. citrophthora could be reisolated only from the infected plants. The pathogen is polyphagous, widely distributed (1), and has been associated with woody ornamentals in nurseries (3). However, to our knowledge, this is the first record of P. citrophthora in Hungary. The pathogen has to be considered as a threat to ornamental production within Hungary.
References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (2) A. Ippolito et al. Eur. J. Plant Pathol. 108:855, 2002. (3) B. W. Schwingle et al. Plant Dis. 91:97, 2007.
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