What was it all about?

From 2020 to 2022, this project investigated how soil pH and nutrition may play a part in reducing the impact of pink rot disease in potatoes. The research found that pink rot management is complex, and a clear connection between soil pH, nutrition and pathogen load/disease expression was not found. The project team recommend that future research should explore factors such as interactions with other soilborne pathogens, the role of key physical soil factors, strategic soil amendments and managing inoculum through a better understanding of the roles of potato volunteers and alternative hosts.

Pink rot caused by Phytophthora erythroseptica is a key soil-borne disease characterised by pink colouration of the cut tubers and a distinctly unpleasant odour. The rapid rotting of the tubers, whether in ground or in storage, results in significant yield losses and product downgrades or rejection, impacting the financial viability of growers in key potato production regions of Tasmania, and to a lesser extent, small regions on mainland Australia. 

With fungicides becoming less effective there was a need to assess other control options. A prior hydroponics study in the USA identified that calcium applications and/or the raising of soil pH to greater than 7.0 corresponded with reduced stolon and root infection by P. erythroseptica, although no work on pink rot tuber disease was undertaken. Combined with anecdotal evidence of the positive impacts of calcium application on soil and plant health in general, there was a motivation by industry to test whether calcium application and/or pH modification may provide a sustainable disease control option. Additionally, there was a need to test what other soil factors may contribute to increased likelihood of disease and catalogue through a review of the literature what is known of the disease and what knowledge and research gaps remain. Analysis of historical work on pink rot and industry perceptions were also utilized to confirm outcomes and identify future research priorities.

Field work from 19 different sites over the two growing seasons (2020/21 and 2021/22) provided ample opportunity to identify factors important in pink rot disease expression. While rainfall was a key driver, the intricacies of topography and importance of soil physical factors, including poor structural quality and reduced topsoil depth, were key determining factors of increased pink rot disease. Other physical factors, including row orientation, frequency of row intersections, and reduced mound height was also associated with a greater likelihood of pink rot. Physical crop damage, whether it be through tractor-traffic, irrigator runs, or stem wind damage also can predispose crops to a greater chance of pink rot as does the presence of other soilborne diseases including root galling and powdery scab, rhizoctonia canker, and Scerotinia, amongst others. All of the above factors are considered important avenues of future proposed work on pink rot.

Where specific chemical fertility treatments including calciprill, OzCal, nanocal, Platinum Plus, calcium nitrate, gypsum were tested at six different field sites and in two separate pot trials they provided no benefit in negating pink rot tuber symptoms and Phytophthora induced stem die-back. The trials also confirmed that non-practical large quantities of calcium product would be required in typical ferrosol potato growing soils to alter soil pH above 7.0.

Pink rot remains a complex disease with many factors contributing to potential epidemics. Future research should look at a number of these factors that will provide a better understanding of pathogen dynamics and disease outbreaks.