The potential of pathogens as biological control of parthenium weed (Parthenium hysterophorus L.) in Ethiopia.
P. hysterophorus is an exotic invasive annual weed causing severe infestation in Ethiopia. Studies on diagnosis, incidence and distribution of pathogens associated with the parthenium weed in Ethiopia were carried out from 1998-2002 in Wollo, Hararghe and Wollega. Several fungal isolates were obtained from seeds and other parts of parthenium plants. Among them were putative pathogenic fungal species of the genus Helminthosporium, Phoma, Curvularia, Chaetomium, Alternaria and Fusarium. However, pathogenicity test of the isolates obtained showed no or nonspecific symptoms. It was concluded that these pathogens could be opportunistic with insignificant potential for biological control of Parthenium hysterophorus. The 2 most important diseases associated with Parthenium were a rust disease, caused by Puccinia abrupta var. partheniicola, and a phyllody disease, caused by a phytoplasma of fababean phyllody (PBP) phytoplasma group. The rust was commonly found in cool mid-altitude (1500-2500 m) areas while phyllody was observed in low to mid-altitude regions (900-2500 m) of Ethiopia, with a disease incidence up to 100 and 75%, respectively, in some locations. Study of the individual effects of the rust and phyllody diseases under field conditions showed a reduction on weed morphological parameters (plant height, leaf area, and dry matter yield). Parthenium hysterophorus seed production was reduced by 42 and 85% due to rust and phyllody, respectively. Phyllody and rust diseases of parthenium showed significant potential for classical biological control of Parthenium hysterophorus after further confirmation of insect vectors that transmit phyllody and host range of phyllody disease to the related economic plants in Ethiopia.