Fungal pathogens of parthenium (Parthenium hysterophorus L.), an invasive weed in India, have been under intensive investigation at our Directorate since 1996 for development of mycoherbicides. During the southwest monsoon month of August 1997, a severe foliar disease of the weed was observed near Hunsur (12° 16′N; 76° 18′E; 779.37 masl) in Mysore district of Karnataka State. The primary symptoms consisted of circular to irregular, light brown leaf spots with a diameter up to 8 mm and surrounded by a diffuse chlorotic margin. Isolations on potato dextrose agar (PDA) yielded a relatively fast-growing (approximately 11.4 mm/day) white fungus. The same fungus was isolated consistently from diseased leaves but not from symptomless leaves collected from the vicinity. The mycelium of the fungus on PDA was immersed, branched, septate, and hyaline to pale brown. Conidia (approximately 10.50 × 2.75 µm) were hyaline, thin-walled, smooth, straight, and ellipsoid with obtuse apex and base abruptly tapered to a distinct truncate scar. Pathogenicity was assessed by inoculating healthy, surface-sterilized, mature detached leaves and whole plants (40-day-old) with spore (1 × 106 conidia/ml) or mycelial (0.2 g/ml) suspensions. For both kinds of inoculation, 10 replicates each for detached leaves and whole plants were maintained with equal number of uninoculated controls. Inoculated leaves and plants were incubated at ambient temperature (28 ± 2°C) and 48 h of continuous leaf wetness. Lesions resembling symptoms that occurred in the field were observed on all detached leaves and whole plants within 3 weeks of inoculation. The same fungus was recovered from the diseased tissues excised from all the inoculated material, satisfying Koch's postulates. All the controls remained symptomless and were negative for the fungus. A virulent isolate of this fungus was referred to CABI Bioscience, UK Centre (Egham) and was identified as Cryptosporiopsis sp. (IMI 378270). Although there are at least 12 described species of Cryptosporiopsis Bub. & Kabát (1), no species has been described on parthenium. This pathogen is being studied for its usefulness as a mycoherbicide for parthenium.
Reference: (1) B. C. Sutton. 1980. The Coelomycetes. Commonwealth Mycological Institute, Kew, Surrey, England.
