Cornelian cherry dogwood (Cornus mas) is a widespread species in Bulgaria and some cultivars with large fruits are the subject of propagation. In the springs of 2007 and 2008, severe, unusual damages were observed on sporadically scattered plantlets of ‘Kazanlashki’ (known also as ‘Kazanlaker’) in a nursery located near Vratza in northwestern Bulgaria. Symptoms were identical in both years and expressed on the leaves, young shoots, and adjacent rootstock wood. Dark brown, necrotic leaf spots initiated most often from the leaf periphery and quickly covered more than half of the leaf area. Necrosis of the leaves and shoots spread toward the older woody tissues and the plantlets died within a couple of weeks. Isolations from symptomatic leaves, shoots, and rootstocks (three to five samples per plant organ) on potato dextrose agar always revealed a fungus-like organism that formed relatively fast-growing white, radial, petaloid colonies. Numerous, ovoid to obpyriform, noncaducous, semipapillate sporangia occasionally with two papilla were observed after 1 or 2 days of incubation at 20°C in nonsterile soil extract (1). Average sporangium size was 39 (35 to 45) × 31 (20 to 35) μm with a ratio between both parameters of approximately 1.26. The pathogen's paragynous antheridia and smooth-walled spherical oogonia (20 to 32 μm in diameter) yielded spherical aplerotic to almost plerotic oospores on V8 medium with an average size of 25 μm. The morphological data identified the organism as Phytophthora citricola (1). Isolates had identical cultural and morphological characteristics, and pathogenicity was tested by laboratory inoculations carried out in 2007 (two isolates) and twice in 2008 (three isolates). Separately, detached leaves of C. mas seedlings and ‘Kazanlashki’ were wiped with 70% ethanol, punctured with a needle, and the wounds inoculated with 5-mm mycelial plugs from a 7-day-old V8 growth plate. Sterile V8 plugs were placed onto similar wounds of control leaves. Leaf samples were incubated at 20°C in a humidified chamber. Necrosis similar to that observed in the field became visible around the mycelia plugs 4 days after inoculation. The necrotic lesions enlarged to 20 to 25 mm in diameter within the next 2 days, whereas the control leaves did not show any symptoms. Subsequently, the pathogen was reisolated solely from all the mycelium-inoculated samples. By means of the same inoculation method, pathogenicity was also demonstrated on shoots and mature fruits of C. mas. DNA was isolated from mycelium of an isolate and the internal transcribed spacer (ITS) region was amplified using ITS6 and ITS4 primers. The PCR product was sequenced (GenBank Accession No. FJ269034) and the BLAST search showed 100% homology with P. citricola, type II (2). To our knowledge, this is the first report of P. citricola on C. mas in Bulgaria, thus confirming its ability to attack Cornus spp. (3). Taking the lethal results of the disease and the polyphagous nature of the causal agent into consideration, this report is a serious warning for nurserymen and consumers.
References: (1) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996. (2) M. E. Gallegly and C. X. Hong. Phytophthora: Identifying Species by Morphology and DNA Fingerprints. The American Phytopathological Society, St. Paul, MN, 2008. (3) F. N. Martin and P. W. Tooley. Mycologia 95:269, 2003.
