Impact of increasing temperature on the fitness of Fusarium pseudograminearum causing crown rot in wheat

Abstract

Warmer temperatures associated with climate change are expected to have a direct impact on plant pathogens, challenging crops and altering plant disease profiles in the future. In Australia, the fungal pathogen Fusarium pseudograminearum associated with causing crown rot, a major disease in wheat, has been estimated to cost an average AUD$79 million in losses each year. In addition, Fusarium produces a variety of mycotoxins, such as the trichothecence deoxynivalenol (DON) in infected tissue. When produced in high concentration and consumed by animals and/or humans, DON can cause serious health effects. In Australia, a lack of knowledge regarding the effects of individual weather variables, such as temperature, on Fusarium crown rot has impeded the development of management plans to safeguard the wheat industry from disease under future climates. The experiments reported in this thesis were undertaken to: determine the effect of increasing temperature on the fitness, of F. pseudograminearum during the saprophytic and pathogenic stages of its lifecycle; explore the relationship between temperature and wheat line to determine if host resistance plays a part in influencing the overall pathogenic fitness of F. pseudograminearum; assess the impact of increasing temperature on crop growth and productivity of crown rot infected wheat lines; and to explore the relationship between different measures of pathogen fitness, and crop productivity and growth. F. pseudograminearum has a complex life cycle and focusing on both the saprophytic and pathogenic stage offers a more realistic assessment and biologically meaningful understanding to overall pathogen fitness. Using temperature controlled incubators maintained at four diurnal temperatures 15/15oC, 20/15oC, 25/15oC and 28/15oC, saprophytic measures of fitness including mycelial growth and fecundity, in vitro, of F. pseudograminearum were determined. Overall, both measures were significantly influenced by temperature. Growth was favoured under 25/15oC, whilst fecundity was optimal at 20/15oC on full strength potato dextrose agar (PDA). As 15/15oC closely resembles current wheat farming conditions in the southern wheat belt of Australia these results suggest warming could lead to increased saprophytic fitness of F. pseudograminearum. A temperature controlled glasshouse experiment using the same four diurnal temperatures as above, was designed to study the pathogenic fitness of F. pseudograminearum. Measures of disease severity, pathogen biomass and DON of mature plants showed the pathogen had superior overall fitness at 15/15oC, and this was reduced with increasing temperature. Pathogen fitness was significantly influenced by the level of crown rot resistance of wheat lines, but the influence declined with increasing temperature. In addition, this experiment found a strong correlation between DON in stem base tissue and disease severity, but length of browning was not a good predictor of Fusarium biomass in the stem base. Based on the findings from this experiment, warmer temperatures associated with climate change may reduce overall pathogenic fitness of F. pseudograminearum. The temperature controlled glasshouse experiment mentioned above, was also used to determine whether increasing temperatures will affect crop productivity (grain weight) and growth (tiller length and tiller number) of crown rot infected wheat under future climates. For crown rot infected plants, grain weight and tiller length, were highest at the coolest temperature treatment of 15/15oC and were reduced with warmer temperatures (20/15oC, 25/15oC and 28/15oC), whilst tiller number showed the opposite trend. By examining the percentage change of the mean grain weight per plant from control versus inoculated plants it was found that increasing temperatures may reduce the impact of crown rot yield loss. Overall, there were significant correlations between most measures of crop growth and productivity of crown rot infected wheat. Furthermore, increasing disease severity was associated with a reduction in grain weight in this study. The results from these experiments have provided significant insight into the effects of increasing temperature on an important host-pathogen system in Australia. As the global mean surface temperature continues to rise over the 21st century, this study has shown Fusarium crown rot may be reduced with careful selection of host cultivars, and improved agronomic practices.