Occurrence of different species of fusarium from wheat in relation to disease levels predicted by a weather-based model in Argentina pampas region.

Fusarium head blight (FHB) is an important disease throughout many of the world wheat-growing areas that have humid to semi-humid climate. The infection happens mainly during the anthesis of the wheat, when there have been favorable conditions of moisture and temperature. The direct relation of the infection to environmental factors makes possible the formulation of mathematical models that predict the disease. The causal agent of the FHB of the spike of wheat is attributed principally to Fusarium graminearum. High economic losses due yield decrease have been recorded in Argentina. In the present work, 67 isolates of Fusarium spp. were obtained from samples of wheat grains from Pampas region from 15 locations distributed in Buenos Aires, Entre Ríos, Santa Fe and Córboba provinces during 2006 and 2007 wheat-growing seasons. The identification of species from monosporic isolates was carried out by morphological characterization and use of species-specific PCR-based assays. Both identification criteria were necessary and complementary for the species determination, since in some cases the molecular identification was not specific. Scanty presence of F. graminearum was observed in 2006 wheat-growing season coinciding with the lack of favorable meteorological conditions for producing FHB infection events. High presence of F. graminearum isolates was observed in 2007 wheat-growing season, in accordance with moderate incidence of the disease according to spatial distribution of FHB incidence values. The aim of this report was to identify the causal agent of the FHB disease by different taxonomic criteria and to relate its occurrence with disease incidence values predicted by a weather-based model in Argentina.

First Report of Rhizoctonia solani AG-4 on Epipremnum aureum in Buenos Aires, Argentina.

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.

First Report of Rhizoctonia solani AG-4-HG-II on Garden Pink in Buenos Aires, Argentina.

During 2001, basal stem rot, wilt, and plant death were observed on 30% of the plants in a crop of Dianthus plumarius L. 'Telstar' in Buenos Aires. Pieces of diseased stems ≈1 cm long were surface-disinfested in 2% NaOCl for 1 min and cultured on 2% potato dextrose agar (PDA), pH 7, at 22 to 24°C. After 7 days, an identical fungus was consistently isolated from pieces of infected tissue. Colonies initially were white, turned brown after 2 to 3 days, and eventually formed irregularly shaped sclerotia. Cultures exhibited morphological characteristics typical of Rhizoctonia solani Kühn (2) and were identified with known anastomosis group tester isolates (1). Positive anastomosis was observed with tester strains of R. solani AG-4-HG-II. One isolate was tested for pathogenicity by placing two pieces of PDA (1 cm2) containing 7-day-old mycelial growth ≈0.5 cm from the base of healthy 2-month-old plants. Control plants were treated with sterile pieces of PDA using the same procedures. Ten replicate plants were used for each treatment. Plants were maintained at 22 to 24°C under 95 to 100% relative humidity and a 12-h light/dark photoperiod. After 7 days, symptoms developed that were similar to those originally observed, and Koch's postulates were satisfied by reisolating the fungus. To our knowledge, this is the first report of R. solani AG4-HG-II causing disease on D. plumarius in Argentina. References: (1) B. Sneh et al. Identification of Rhizoctonia Species. The American Phytopathological Society, St. Paul, MN, 1991. (2) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.

First Report of Petunia Root Rot Caused by Rhizoctonia solani in Argentina.

Common garden petunias (Petunia × hybrida Hort. Vilm.-Andr.) are herbaceous annual plants with brightly colored flowers up to 10 cm in diameter. During the winter of 2002, crown and root rot were observed on plants (cv. Ultra) growing in five greenhouses in Buenos Aires. Affected plants were randomly distributed in the greenhouses, and mean disease incidence in all the greenhouses was 26%. Basal leaves turned yellow and gradually became necrotic, and infected plants were often killed. Small pieces of affected tissues were disinfested in 2% sodium hypochlorite for 1 min and plated on 2% potato dextrose agar (PDA). Fifteen isolates identified to the genus Rhizoctonia were obtained. Fungal colonies were initially white, turned brown with age, and produced irregularly shaped, brown sclerotia. Hyphal branched at right angles, were constricted at the base of the branch near the union with main hyphae, and septate near the constriction. Basidia were not observed in the greenhouses or on the plates. Isolates were cultivated on water agar and incubated at 25°C for 3 days. Hyphal cells were determined to be multinucleate when stained with 1% aniline blue solution (2) and examined at ×400. Anastomosis group of one isolate was determined by using AG-4 HG II, AG-1 IA, AG-1 IB, AG-1 IC, AG-2 2-1, and AG-2 2IIIB tester strains of Rhizoctonia solani that includes isolates reported to be pathogenic on ornamentals (1). Anastomosis was observed only with strains of AG-4 HG II. Pathogenicity on this isolate was conducted on potted, healthy, adult plants that were 10 to 22 cm high and flowering. Thirty-five plants were inoculated by placing 1 cm2 pieces of PDA from 7-day-old mycelial cultures near the base of the stem. Twelve control plants were treated with 1 cm2 PDA plugs. Plants were kept at 22 to 24°C, >95% relative humidity, and 12 h of fluorescent light. Wilt symptoms due to basal stem rot appeared 7 days after inoculation, and all the inoculated plants died within 27 days. Control plants remained disease free. The pathogen was reisolated from symptomatic tissues, completing Koch's postulates. To our knowledge, this is the first report of R. solani causing disease on petunia in Argentina. References: (1) D. M. Benson and D. K. Cartwright. Ornamental diseases incited by Rhizoctonia spp. Pages 303-314 in: Rhizoctonia species: Taxonomy, Molecular Biology, Ecology, Pathology and Disease Control. B. Sneh et al., eds. Kluwer Academic Publishers, London, England, 1996. (2) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.