CEREAL disease authorities are calling for a whole-of-industry approach to manage and reduce the impact of Septoria tritici blotch (STB).
The stubble-borne wheat disease is of major concern to growers in the high rainfall zones of South-East South Australia, southern Victoria and Tasmania.
Dr Andrew Milgate, a research scientist with the NSW Department of Primary Industries (NSW DPI), says STB is a triple threat in that it is able to rapidly overcome host resistance, it has evolving fungicide resistance and its spores can travel long distances.
“These features make it a social disease which requires a whole-of-industry response through integrated disease management,” Dr Milgate said.
His research into cereal diseases is funded in partnership by NSW DPI and the Grains Research and Development Corporation (GRDC).
Speaking at recent GRDC grains research Updates and Septoria technical workshops in the southern cropping region, Dr Milgate said the presence of mutations in the CYP51 gene – known to cause changes to sensitivity to triazole fungicides – had been confirmed in Victoria, South Australia and Tasmania.
He said several factors had most likely brought about the selection of fungicide resistance in the Australian STB population.
“Firstly is the expansion of intensive wheat production in the high rainfall zones as these environments provide ideal cool and wet conditions for STB to survive and thrive.
“The increasing adoption of stubble retention in farming systems has also provided the pathogen with the ability to survive from one season to the next in large population sizes.
“The widespread adoption of cheap fungicides to counter wheat stripe rust epidemics in the early 2000s is another factor.
“The collision of these factors all occurring in the same decade created an environment which has led to the evolution of fungicide resistance with the Australian STB population.”
Not all triazoles are affected by resistance in STB in the same way, and strobilurin resistance has not yet been detected.
STB belongs to a special class of pathogens known as hemibiotrophs, and spends between 14 and 21 days of its life living inside wheat leaves without causing visible symptoms.
The pathogen then transitions to causing visible disease symptoms which are necrotic lesions in which the characteristic black fruiting structures are observed.
Dr Milgate said integrated disease management involving cultural practices, varieties and fungicides was needed to combat STB.
A number of cultural practices can be used to decrease the amount of inoculum and as a result reduce disease pressure within a crop: Infected stubble is the major source of disease within a crop so not sowing wheat on wheat removes the immediate sources of infection. If sowing wheat on wheat cannot be avoided, removal of wheat stubble before sowing of the next crop by grazing, baling, incorporating and/or burning should be considered. Delayed sowing has been shown to reduce the rate of infection early in the season due to avoiding spore showers released from the stubble, however, this needs to be balanced with yield penalties associated with later sowing times.
The choice of variety can have a major impact on disease development within a crop:
The more resistant a variety is the better the expected disease control, and the need for fungicides is lessened There are STB disease ratings available for NSW, Victoria and South Australia. Because of regional differences in the performance of some varieties, caution must be used when choosing a variety for Tasmania using ratings from other regions.
Current advice about fungicide selection and spray timings is based on the most economical options to reduce the yield impact of the disease, however, these strategies have not been developed in the context of how best to reduce the risk of fungicide resistance, according to Dr Milgate.
He said early disease control was important in lowering the population size and this could be achieved through a fertiliser or seed-applied fungicide, in addition to foliar fungicides. “This early control can also reduce the need for follow-up foliar fungicide applications.”
Dr Milgate said that developing a better understanding of mutations in the CYP51 gene which lead to fungicide resistance would assist in formulation of strategies to hopefully delay further erosion of fungicide efficacies.
“Our goal now is to develop a strategy that will enable us to sustain the effectiveness of triazole fungicides in combination with different modes of action such as strobilurins while at the same time achieve good disease management.”
The current advice is not to use an active ingredient from the same fungicide resistance group more than once in a growing season on the same crop.
Dr Milgate said growers and advisers must follow label directions for the application of all products and the relevant state regulations pertaining to the usage of fungicides.