Progressing Stubble-Borne Disease Management of Winter Cereals in the Northern Grain Region of Australia

Abstract

<p>Stubble-borne diseases are a major constraint to wheat and barley production in the northern grain region (NGR) of Australia, a problem that is becoming worse with the increased adoption of conservation agricultural practices. Current management strategies rely largely on controlling disease in the pathogenic phase using resistant or tolerant cultivars and crop rotation with non-hosts to reduce inoculum levels in the field over multiple seasons.</p> <p>However, if weather conditions permit, the fungal pathogens responsible can continue to colonise retained cereal stubble (saprotrophic colonisation), which may further increase inoculum levels after harvest. Improving our understanding of what drives saprotrophic colonisation of the pathogens which cause these diseases, such as Fusarium crown rot (<i>Fusarium pseudograminearum</i>), common root rot (<i>Bipolaris sorokiniana</i>) and yellow leaf spot (<i>Pyrenophora tritici-repentis</i>), could lead to development of new field management strategies for reducing inoculum carry-over into subsequent seasons. This concept was explored in detail using a controlled relative humidity (RH) laboratory experiment, a glasshouse experiment, two three-year field experiments, and two laboratory-based microwave radiation experiments. In the RH experiment, Fusarium pseudograminearum demonstrated increased relative saprotrophic fitness, colonising 20 to 42% more stubble length over a 7-day period at 100% RH compared with B. sorokiniana and P. tritici-repentis, respectively. Saprotrophic colonisation by all three fungal cereal pathogens in sterilised cereal stubble was also significantly faster under higher moisture conditions of 97.5 % RH and above and was largely unaffected by cereal crop type or level of genetic resistance expressed during the pathogenic phase. Further glasshouse experimentation with <i>F. pseudograminearum</i> confirmed that genetic resistance traits (e.g., growing oat instead of wheat or barley) can reduce the height of pathogen colonisation by up to 80.3% in the main tiller prior to senescence. However, genetic resistance did not limit the extent of saprotrophic colonisation of cereal stubble after harvest. After plant senescence, <i>F. pseudograminearum</i> colonised as much as 62.6% higher within the main tiller of highly resistant wheat germplasm LRC2012-122 compared with the most susceptible wheat cv. Kittyhawk. These findings confirmed that crop selection remains a useful tool to limit Fusarium crown rot symptoms but cannot be used to manage saprotrophic colonisation of cereal stubble after harvest. Two new management strategies were therefore investigated to control or reduce inoculum in retained cereal stubble. Lowering the cereal harvest height of a durum wheat crop infected with <i>F. pseudograminearum</i> effectively restricted post-harvest saprotrophic colonisation of stubble by 61 to 70% at two field sites in the NGR. In the shorter-term, harvest height modification could therefore be developed as a useful integrated disease management strategy to improve stubble-borne disease management in this region. In the longer-term, using microwave radiation to kill stubble-borne pathogens appears promising to remove pathogens from cereal stubble <i>in situ</i> whilst retaining the associated system benefits of stubble retention. For instance, reduction of pathogens <i>B. sorokiniana</i>, <i>F. pseudograminearum</i> and <i>F. cerealis</i> required 150 to 300 J per gram of spore solution to eliminate 99% of pathogen populations using microwave radiation. Heating of wheat and barley stubble using microwave radiation also appears feasible, particularly when applied to stubble with a higher moisture content (30-100% moisture by weight). These findings have improved the fundamental understanding of the saprotrophic phase of key stubble-borne cereal disease cycles whilst also offering new and transformational future directions for stubble-borne disease management. Ultimately, future management strategies need to consider the role of inoculum production in both the pathogenic and saprotrophic phases to better control stubble-borne cereal diseases in the NGR.</p>