The winter strawberry (Fragaria ananassa Duch.) industry in subtropical regions is characterized by early-season fruit production, a time when crop value is the highest. This is the case of the northwestern province of Tucuman (27°S), Argentina's largest winter strawberry growing area (2). Collapsed and dying strawberry plants of cv. Camarosa were observed in commercial farms in Tucumán in September 2007 with an incidence of 16%. Crowns and roots of affected plants revealed dark, external and internal browning tissue with several dark, oblong, subepidermal sclerotia. Pieces of necrotic crowns and roots disinfested with 0.6% chloride for 30 s were placed on potato dextrose agar (PDA). After 4 days of incubation at 25°C with constant fluorescent lighting, dark cultures with numerous, dark, oblong sclerotia immersed in the isolation medium were observed. Sclerotia size ranged from 70 to 120 μm in 7-day-old cultures. The isolated fungus was identified as Macrophomina phaseolina (3). Ostiolate pycnidia bearing ellipsoidal, hyaline conidia were rarely observed on the affected host tissue (1). Pathogenicity tests were performed on micropropagated nursery plants grown in sterile potting soil under greenhouse conditions for 3 months. Single-sclerotia culture was grown in PDA at 25°C for 4 days. Then, sterile wooden toothpicks were placed on the medium for seven additional days to allow fungal colonization. Prior to use, toothpicks were autoclaved twice in deionized water and once in V8 juice (4). Four plants of Camarosa, Camino Real, Pajaro, and Chandler cultivars were inoculated by inserting a colonized toothpick into the crown. Sterile toothpicks were inserted into crowns of control plants. All plants were incubated under greenhouse conditions. The experimental setting was a randomized complete design with two replicates per cultivar. Inoculated plants developed wilting in one or more leaves 6 days after inoculation. Most of the inoculated plants (90%) collapsed or died within 2 weeks, while control plants remained healthy during the observation period. The reisolated fungus from affected plants was morphologically and culturally identical to the inoculated isolates. M. phaseolina has been reported in the province of Santa Fe (~32°S) where winters are humid and cold (1), contrasting with Tucumán winters, which are dry and cold, both highly conducive conditions for the development of the disease. To our knowledge, this is the first report of M. phaseolina, an emerging pathogen, considered as a potential threat for northwestern Argentina's strawberry industry, which is recognized as a major source of employment and income in this region.
References: (1) C. J. N. Carrera. Ficha Fitopatológica 000867 INTA IMIZA Castelar Bs.As., 1972. (2) D. S. Kirschbaum and J. F. Hancock. HortScience 35:807, 2000. (3) J. Maas. Page 26 in: Compendium of Strawberry Diseases. 2nd ed. J. L. Maas, ed. The American Phytopathological Society, St. Paul, MN, 1998. (4) J. Mertely et al. Plant Dis. 89:434, 2005.
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