FUNGAL DISEASES J Gen Plant Pathol (2005) 71:247–252 © The Phytopathological Society of Japan DOI 10.1007/s10327-005-0195-2 and Springer-Verlag Tokyo 2005 Harukuni Horita · Robert J. McGovern Tsutomu Komatsu · Shinji Yasuoka Effects of inoculum density, leaf age, moisture, temperature, and wetness duration on black streak of edible burdock Received: May 21, 2004 / Accepted: September 9, 2004 Introduction Edible burdock (Arctium lappa L.) has been cultivated in Japan for more than 1000 years and remains an economi- cally important vegetable. Hokkaido Island, the northern- most region of Japan, is one of the major edible burdock production areas, where about 12% of the commercial pro- duction in Japan occurs (Nishida 1998). Although edible burdock is susceptible to many patho- gens, including Xanthomonas campestris pv. nigromaculans, Rhizoctonia solani, Ascochyta phaseolorum, and Sphaer- otheca fusca (Narita et al. 1998), black streak on the leaf or petiole caused by Itersonilia perplexans Derx (Horita and Yasuoka 2002) is the most destructive disease on edible burdock in Hokkaido. During an extensive field survey in 1998 for edible burdock diseases in Hokkaido, the incidence of black streak disease was 100%. Initial symptoms of black streak on leaf vein and petiole are small (5 mm), pale brown, rectangular lesions that may elongate, fuse, and pro- duce characteristic black streaks. Diseased petioles easily snap off at the enlarged lesions. The fungus Itersonilia perplexans has been reported to be the causal agent of petal blight of chrysanthemum (McRitchie et al. 1973; Nishihara 1958), China aster (McGovern and Seijo 1999), and other Compositaceae plants (Cragg 1966; Sackston 1958; Seijo et al. 2000). The symptoms caused by I. perplexans in these hosts appear to be limited to floral parts. Parsnip (Pastinaca sativa L.) and dill (Anethum graveolens) also have been described as hosts of I. perplexans (Channon 1963; Koike and Tjosvoid 2001). The parsnip isolate of I. perplexans, formerly classified as I. pastinacae, produces characteristic symptoms on leaves, petioles, inflorescences, and roots (Channon 1969). Parsnip isolates differ from the chrysanthemum isolates of I. perplexans in that they are pathogenic to parsnip and not to chrysanthemum blooms. Although edible burdock isolates of Itersonilia infect chrysanthemum flowers and cause petal blight, they are unique strains of the fungus because of the characteristic symptoms they produce on the leaves and petioles of edible H. Horita (*) · T. Komatsu Hokkaido Ornamental Plants and Vegetables Research Center, Takikawa, Hokkaido 073-0026, Japan Tel. +81-125-28-2800; Fax +81-125-28-2299 e-mail: horitaha@agri.pref.hokkaido.jp R.J. McGovern Plant Pathology Department and Plant Medicine Program, University of Florida–IFAS, Gainesville, FL, USA S. Yasuoka Hokkaido Central Agricultural Experiment Station, Hokkaido, Japan Abstract Inoculum density, temperature, leaf age, and wet- ness duration were evaluated for their effects on the devel- opment of black streak (Itersonilia perplexans) on edible burdock (Arctium lappa L.) in a controlled environment. The effect of relative humidity (RH) on ballistospores pro- duction by I. perplexans was also evaluated. Symptoms of black streak on leaves increased in a linear fashion as the inoculum density of I. perplexans increased from 10 2 to 10 6 ballistospores/ml. Rugose symptoms on young leaves were observed at densities of ≥10 4 ballistospores/ml. Disease severity of I. perplexans in relation to leaf age followed a degradation curve when the leaves were inoculated with ballistospores. Disease severity was high in newly emerged leaves up to 5 days old, declined as leaf age increased to 29 days, and was zero when leaf age increased from 30 to 33 days. Disease development of edible burdock plants ex- posed to ballistospores of I. perplexans was evaluated at various combinations of temperature (10°, 15°, 20°, 25°C) and duration of leaf wetness (12, 24, 36, 48, and 72 h). Dis- ease was most severe when plants were in contact with the ballistospore sources at 15° or 20°C. The least amount of disease occurred at 25°C regardless of wetness duration. Ballistospores required 24–36 h of continuous leaf wetness to cause visible symptoms by infection on edible burdock. Ballistospores production in infected lesions required at least 95.5% RH. Key words Arctium lappa · Black streak · Environmental factors · Itersonilia perplexans