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First Report of Rust Caused by Puccinia carduorum on Italian thistle in Tunisia

Italian thistle (Carduus pycnocephalus L.) is a common and increasingly important weed in Tunisia. It is also problematic in the western United States and a target of biological control. In surveys conducted in northern Tunisia from 2003 to 2005, Italian thistle plants in many locations were found diseased by rust. Eighty-five isolates of rust were collected from Italian thistle during these surveys. Each isolate was collected from a single plant and stored individually as mixtures of urediniospores and teliospores at 4°C or in liquid nitrogen. Urediniospores and teliospores of all isolates were similar in morphology and matched the description of Puccinia carduorum Jacky (3). Isolate B1003 (BPI No. 878207), collected from Béja, Tunisia, was arbitrarily selected for further study. Comparison of internal transcribed spacer (ITS) regions of B1003 (GenBank Accession No. EF050059) with other ITS sequences indicated a 97% similarity to P. carduorum (GenBank Accession No. PCU57351) from Carduus nutans subsp. leiophyllus. Eight Italian thistle plants, grown from seeds collected in Béja, were inoculated in the 3- to 5-leaf stage with urediniospores of B1003 by spraying plants with an aqueous suspension of urediniospores at 10⁶ spores per ml with approximately 0.03 ml of surfactant until they were thoroughly wet. Four plants were sprayed with water plus surfactant only. All plants were covered with plastic bags and placed in a growth chamber at 18/20°C night/day temperatures. Bags were removed 24 h after inoculation and plants were monitored daily for symptoms. Plants sprayed with water plus surfactant only did not develop symptoms. Six inoculated plants developed disease symptoms similar to those observed on samples collected during the surveys. White flecks appeared within 7 days of inoculation and developed into brown pustules 10 days after inoculation. Pustules enlarged and produced urediniospores until they covered both sides of diseased leaves. One month after appearance of symptoms, diseased leaves turned yellow and died. Urediniospores from these plants were used to inoculate six plants each at the 2- to 5-, 6- to 8-, and >8-leaf stages. All plants became diseased and produced uredinia. Plants in the 2- to 5-leaf stage were more severely diseased than other plants. P. carduorum was introduced to the United States in a field test for control of C. nutans subsp. leiophyllus (musk thistle) and has become established in a number of states (1). An isolate of this fungus has also been found from C. tenuiflorus (slender-flower thistle) in California (4). However, neither isolate causes substantial disease on Italian thistle from California (2,4). Isolates of P. carduorum from C. pycnocephalus in Greece, Italy, and Turkey caused little disease on most Italian thistle collections from California (2), indicating variability in susceptibility among plants as well as in virulence among rust isolates from different geographical locations. To our knowledge, this is the first report of P. carduorum parasitizing Italian thistle in Tunisia. Tests will be conducted to determine the host range of this isolate among C. pycnocephalus collections and other species.

References: (1) A. B. A. M. Baudoin and W. L. Bruckart. Plant Dis. 80:1193, 1996. (2) W. L. Bruckart and G. L. Peterson. Phytopathology 81:192, 1991. (3) D. B. O. Savile. Can. J. Bot. 48:1553, 1970. (4) A. K. Watson and K. Brunetti. Plant Dis. 68:1003, 1984.