Rose Dieback Caused by Trichothecium roseum in Argentina.

Rose (Rosa sp.) is one of the most important ornamentals in Argentina. Since 2002, a severe disease has been observed on crops cultivated for cut flowers and garden plants in Escobar, San Pedro, Río Negro, and the surrounding area of Buenos Aires and La Plata. Symptoms consisted of a stem dieback progressing to plant death. In some cases, stem cankers were observed on the dieback limits. Mean incidence of stem dieback was 8% regardless of location. The disease was associated with pruning or harvest wounds. The objective of this study was to identify the causal agent of the described symptoms. Small pieces of diseased tissues from cvs. Rafaela, Merlise, Confeti, Mini rosal, Exótica, Macarena, and Peckowo were surfaced sterilized with a 2-min immersion in 0.2% NaOCl, washed with sterile distilled water, the tissue blotted dry, placed on 2% potato dextrose agar (PDA), and incubated at 22°C. Pure salmon-colored fungal colonies developed within 72 h. Hyaline, two-celled (the upper cell slightly larger), ovoid to ellipsoid conidia formed in chains at the apex of simple, long, slender, septate conidiophores. These characteristics are consistent with the description of Trichothecium roseum (Pers.) Link ex Gray (1) The pathogenicity tests were carried out on 10 plants of cv. Rafaela and 10 plants of cv. Mini Rosal using a conidial suspension (2.4 × 10⁵ spores/ml). All plants were pruned just before inoculation. Another 10 pruned plants (five from each cultivar) were sprayed with sterile water and served as controls. Inoculated and noninoculated plants were placed in a climatic chamber at 20°C and covered with polyethylene bags for 3 days to achieve a humid environment. Stem dieback was evident 7 days after inoculation on both cultivars and cankers appeared in 14 days. A dense, white mold that turned salmon-pink covered all the stems within 25 days. Inoculated plants died after 40 days. Symptoms did not develop on the control plants. The pathogen was recovered from inoculated stems, thus fulfilling Koch's postulates. To our knowledge, this is the first report of T. roseum causing a disease on rose in Argentina. Reference: (1) K. H. Domsch et al. Compendium of Soil Fungi. Academic Press. London, 1980.