RESUMO
Poinsettia (Euphorbia pulcherrima Will. ex. Klotzsch) is a worldwide potted or landscape ornamental plant that belongs to the Euphorbiaceae family. During 2003 and 2004, several symptoms were observed on poinsettia potted plants in nurseries and crops near Buenos Aires. Symptoms included irregular, brown, water-soaked spots on adult plants and leaf spots that extended causing stem blight in seedlings. Small pieces of diseased tissues were surface disinfected for 2 min in 2% sodium hypochlorite, plated on 2% potato dextrose agar (PDA), and incubated at 22°C for 48 h. Dense, whitish mycelia developed on PDA and then turned gray when asexual structures were formed. The fungus conidia were ellipsoid, hyaline, nonseptate, and were formed on botryose heads. The pathogenicity test was carried out on 10 plants using a conidial suspension (2 × 106 spores per ml) that was sprayed on leaves with and without injuries. All plants were incubated in a moist chamber at 22 ± 2°C for 48 h and then maintained in a greenhouse. After 3 days, symptoms similar to the original were observed on the inoculated plants. Control plants sprayed with distilled water remained symptomless. Koch's postulates were confirmed by reisolating the same fungus from diseased plants. In accordance with conidial and cultural characteristics, the pathogen was identified as Botrytis cinerea Pers: Fr. (1). To our knowledge, this is the first report of B. cinerea causing leaf spot and stem rot on poinsettia in Buenos Aires, Argentina. Reference: (1) M. V. Ellis and J. M. Waller. No. 431 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1974.
RESUMO
Root and basal stem rot, blighting, and wilting have been observed on Epipremnum aureum (Linden ex André) plants in many nurseries in and near Buenos Aires since 1997. Infected stem tissues show an intense dark brown discoloration and water soaking near the stem base that eventually leads to plant death. To determine the causal agent of the disease, small pieces of diseased tissue were surface-sterilized for 2 min in 2% sodium hypochlorite and plated on potato-dextrose agar (PDA). Whitish colonies that eventually turned brown developed in 2 to 3 days at 22 to 24°C. Irregularly shaped sclerotia were observed. Isolates typical of Rhizoctonia solani Kuhn exhibited mycelia with branches inclined in the direction of growth, constricted at the point of union with the main hyphae, with a septum in the branch near the constriction. No telemorph was observed. Nuclei in living hyphal mats were stained directly on a microscope slide coated with water agar according to the method of Tu and Kimbrough (4) and were examined at 400× magnification. The cells were multinucleate. Anastomosis group was determined by using known tester isolates of Rhizoctonia spp. (3). Positive anastomosis was observed with tester strains of AG-4 HG-II. The polymerase chain reaction was performed according to the protocol of Boysen et al (1) in order to confirm the anastomosis group. Primers used for the amplification of the ITS region were ITSI and LROR. Amplification of the ITS region indicated lack of variation with AG-4 tester strain. The pathogenicity of the isolate was determined with the inoculum-layer technique (2), consisting of a 7-day-old petri plate culture of the pathogen in PDA that is removed from the dish and placed intact on the soil, 2 to 4 cm under the roots of 10 healthy plants. Some leaves of the plants were placed in contact with the inoculated substratum. For a control, PDA was placed under the roots of other plants. Plants were maintained at 22 to 24°C, with close-to-saturation humidity. After 6 to 10 days, symptoms were similar to those previously observed. Initially leaves that had been placed in contact with the substratum showed dark areas with a watersoaked area 2 to 3 cm in diameter. These lesions expanded over the entire leaf blade moving into the petioles and stems killing the plant. One hundred percent of inoculated plants were infected. Koch's postulates were satisfied after reisolating the fungus. The characteristics of the causal agent are those of multinucleate isolates of R. solani belonging to the anastomosis group AG-4 HG-II (3). This is the first report of R. solani causing disease on E. aureum in Argentina. References: (1) M. Boysen, M. Borja, C. Del Corral, O. Salazar, and V. Rubio. Curr. Genet. 29:174-181, 1996. (2) A. F. Schmitthenner and J. W. Hilty. Phytopathology 52:177-178, 1962. (3) B. Sneh, L. Burpee, and A. Ogoshi. 1991. Identification of Rhizoctonia Species. The American Phytopathological Society, St. Paul, MN. (4) C. C. Tu and J. W. Kimbrough. Mycologia 65:941-944, 1973.