Losses from ratoon stunting disease (RSD) are heavily underestimated in Australia and likely exceed $200 million in annual direct losses

Recent debate regarding the management of ratoon stunting disease (RSD) has moved from underestimates of disease incidence to underestimates of its associated yield losses. Using a self-developed yield-loss equation, Young and Knight (2020) estimated $126 million annual direct losses based on 30% incidence, losses of 15% at plant and 30% at ratoon, and a CCS of 13.7 applied at the then-current sugar price of $AU409/tonne. However, with the same yield penalties (15%P, 30%R), the following year Magarey et al. (2021) estimated a much lower figure of $25 million. A forensic review of the data presented revealed that the yield-loss equation used by Magarey et al. (2021) was simply a rearrangement of the one presented by Young and Knight (2020). Therefore, the disparity of yield losses must have been subject to the input parameters. Through calculation it was found that Magarey et al. (2021) had used a CCS of 9.0, which was 38% lower than Australia’s average CCS in 2020. The CCS figure was based on local cane prices and does not account for the 4 CCS units that go to the sugar mill. Furthermore, they applied their yield loss to 13% incidence, an unrealistic figure skewed towards plant-source inspection incidences that is not even supported by their own data. A new yield-loss formula is developed to more accurately calculate direct losses, including to the milling sector. These amounted to an average direct annual loss of $172 million at 30% incidence in Australia in 2020. Using the 2022 sugar price and industry figures, this rises to $228 million annual direct losses. These losses should alert policy makers, research bodies and stakeholders to take initiatives to reduce RSD impacts in sugarcane production in Australia.