First Report of Phyllachora ambrosiae in Europe Causing Epidemics on Common Ragweed.

Abstract

Common ragweed (Ambrosia artemisiifolia var. elatior (L.) Descourt.) was introduced to Europe from North America during the nineteenth century. Since the early 1990s, it has become the most widespread and most important allergenic weed in Hungary. In July 1999, during the annual survey of fungal diseases of ragweed in Hungary, plants exhibiting irregular brown spots surrounded by yellowish halos with small black spherical bodies on the upper surface of leaves, especially along the vessels, were collected from three roadside sites. Light microscopy revealed intracellular hyphae in the cells of leaf tissues, including the vessels, and perithecia (186 to 262 μm in diameter) containing paraphyses and asci (93 × 15 μm) with eight hyaline, unicellular ascospores (14 × 7.5 μm). Hyphae also were observed in asymptomatic leaf tissues and the petioles of infected plants. Mature perithecia commonly found in the necrotic spots also were present in asymptomatic, green leaf tissues. Based on the literature (1,2) and on the morphological examination of two herbarium specimens, BPI 636220 and BPI 636225, borrowed from the U.S. National Fungus Collection, the fungus was identified as Phyllachora ambrosiae (Berk. & M.A. Curtis) Sacc., a holobiotrophic pathogen of ragweed in North and South America. This is the first report of P. ambrosiae on ragweed in Europe. To confirm pathogenicity, leaves of five potted ragweed plants, grown from seed in pots, were inoculated with 0.1 ml of an aqueous suspension containing 5 × 105 ascospores per ml. Inoculated plants were placed in a moist chamber for 48 h and kept in the greenhouse at 20 to 25°C. Noninoculated plants served as controls. Two weeks after inoculation, yellowish spots, which later became brown and necrotic, and perithecia appeared on each of the inoculated leaves. Infected leaves died 3 to 4 weeks after inoculation. Symptoms were similar to those seen in the field. The fungus in tissues of inoculated plants was morphologically identical to the original fungus on plants with spontaneous infections. Control plants did not develop symptoms. To determine the distribution of P. ambrosiae in Hungary, a total of 500 ragweed plants were collected at random from 21 locations in all regions of the country between August and October 1999. Symptoms characteristic of P. ambrosiae infections and perithecia of the fungus were found in 92% of all 500 collected plants. From mid-September, all ragweed plants examined had dead leaves and inflorescences. Perithecia of P. ambrosiae were found in leaves, stems, and flowers.