Kordyana brasiliensis on the invasive weed Tradescantia fluminensis: ability of conidia to infect and the effect of temperature and wetness period on basidiospore germination

Kordyana brasiliensis is a pathogenic biotrophic fungus native to Brazil. It was deliberately released in New Zealand and Australia for the biological control of the invasive environmental weed Tradescantia fluminensis. The fungus produces two different spore types, basidiospores and asexual conidia, but the infection ability of conidia and the microclimatic variables that influence basidiospore germination processes are unknown. Results of controlled environment trials showed that conidia produced on either solid or liquid media were able to cause leaf lesions on T. fluminensis plants, but not when applied at the lowest suspension density tested (1 × 10⁴ conidia/ml) or when conidia were harvested from young (<7 days) cultures. The successful infection of T. fluminensis plants inoculated with conidia (derived from a single basidiospore culture) and subsequent development of sexual structures in lesions indicate that K. brasiliensis is homothallic. The mechanism of infection, however, remains unclear because penetration of the host by thin hyphae produced by conidia on leaf surfaces was not observed in the samples examined by light microscopy. The influence of temperature and wetness period on basidiospore germination processes in vitro was investigated in a second set of experiments. Basidiospore germination and germ-tube elongation were significantly influenced by the interaction of both temperature and wetness period duration (optimal germination at 15-25 °C and germ-tube elongation at 20-22.5 °C). Extended wetness periods buffered the effects of suboptimal temperatures on basidiospore germination until the maximum threshold temperature of 30 °C was reached. Conidia formed on sterigmata-like outgrowths on basidiospore germ-tubes after exposure to extended wetness periods (>18 h) and a relatively broad temperature range (15-25 °C). The potential role of conidia in the life cycle of the fungus is discussed. This is the first study of a Kordyana species to demonstrate the ability of conidia to cause infection on its host and investigate the influence of microclimate variables on basidiospore germination processes. Results of this study can be used as a basis for future research to examine the life cycle, mating system genetic basis and epidemiology of K. brasiliensis.