Synergy Between Puccinia xanthii and Colletotrichum orbiculare on Xanthium occidentale

Successful infections of Noogoora burr (Xanthium occidentale) by Colletotrichum orbiculare and subsequent development of necrotic lesions always occurred on or in proximity of leaf or stem lesions of Puccinia xanthii. Microscopic observations indicated that C. orbiculare behaved as a hemibiotrophic parasite on rust-infected tissue of Noogoora burr. Following infection, C. orbiculare spread necrotrophically in the rust lesions, thus destroying the living plant cells which are essential for the growth of rust, and spread subsequently beyond the rust lesions. These findings suggest that P. xanthii alters the physiology of the infected and neighboring host cells and renders them susceptible to other nonpathogens. A minimum of 14 h dew was required for C. orbiculare to infect leaves of Noogoora burr via rust lesions. The isolates of C. orbiculare obtained from Xanthium italicum or X. occidentale were slightly more aggressive in infecting Noogoora burr via rust leaf lesions than the isolate of the fungus being developed as a potential bioherbicide for Bathurst burr (Xanthium spinosum). Colletotrichum orbiculare was most effective in infecting leaves of Noogoora burr when it was inoculated on plants more than 3 days after inoculation with P. xanthii. Infection of the stem of Noogoora burr by C. orbiculare, via rust lesions, resulted in severe disease symptoms and, in some cases, death of the plant. The occurrence and severity of lesions caused by C. orbiculare on rust-infected stems of plants increased as the dew period or the number of days between inoculation with P. xanthii and C. orbiculare was increased but were not affected by increased inoculum densities of C. orbiculare. The use of C. orbiculare on rust-infected populations of Noogoora burr may have potential to control this weed in the field. Alternatively, investigations of the synergy between these fungi at the cellular and biochemical levels may lead to the development of a bioherbicide effective on Noogoora burr plants that are not necessarily infected by rust.