Parthenium (Parthenium hysterophorus L.) has been a target of weed researchers in India for more than four decades. The weed is attacked by many pathogens, some of which have been tried as mycoherbicides with mixed results (1). In an effort to identify more pathogens of parthenium, surveys were conducted in different regions. A typical foliar disease was frequently observed in several districts of Karnataka State, especially during the winter months of 1996 to 1998. The primary symptoms were circular to irregular, light brown to gray leaf spots, ranging from 2 to 8 mm in diameter. Careful observation of diseased plants revealed that the development of several lesions in close proximity resulted in a rapid necrosis and withering of the entire leaf. The suspected causal organism, which was a dark, chocolate-brown fungus, grew well on potato-dextrose agar at 25°C and a 12-h photoperiod. It produced slowly maturing conidia (20 to 27 × 12 to 14 μm), which were dark brown, ellipsoid, thick-walled, and single-septate with longitudinal striations. A dense conidial suspension (108 conidia per ml) of the fungus was evenly applied (250 μl per leaf) on detached, surface-sterilized, healthy parthenium leaves (N = 10), and infection was ascertained after 4 days of incubation at 26 ± 2°C in petri dishes lined with moist cotton. When 45-day-old pot-grown parthenium plants (N = 50) were similarly inoculated and maintained above 95% RH for at least 48 h, typical symptoms resembling those of naturally infected plants were visible on 88% of the plants within 3 weeks of incubation at 28 ± 2°C. Whereas the fungus was consistently isolated from diseased leaves of inoculated plants, with 80% frequency of reisolation, uninoculated controls neither displayed symptoms nor yielded the pathogen. The experiment was performed three times with similar results, satisfying Koch's postulates. The identity of one of the virulent isolates, WF(Ph)8 (ex Siddeswaranadurga, Chitradurga district, December 1997), was confirmed as Lasiodiplodia theobromae (Pat.) Griffon. & Maubl. (=Botryodiplodia theobromae Pat.) (IMI 378919a) at CABI Bioscience, UK Centre (Egham). Although it is a ubiquitous tropical and subtropical plant pathogen that occurs between 40°N and 40°S (2), it was not previously recorded as a cause of leaf disease on parthenium. Investigations on the possible use of the pathogen as a mycoherbicide have given encouraging results.
References: (1) P. Sreerama Kumar. 1998. Pages 192-210 in: Biological Suppression of Plant Diseases, Phytoparasitic Nematodes and Weeds. S. P. Singh and S. S. Hussaini, eds. Project Directorate of Biological Control, Bangalore, India. (2) B. C. Sutton. 1980. The Coelomycetes. Commonwealth Mycological Institute, Kew, England.
