Elucidating the impact of environmental factors on the growth of Colletotrichum coccodes strains isolated from potato tubers in Great Britain

Black dot and silver scurf caused by Colletotrichum coccodes and Helminthosporium solani, respectively, are tuber blemish diseases affecting quality in the fresh and pre-pack potato industry. In the last 20 years, the importance of high-quality tuber appearance has increased considerably due to the growing demand for washed and prepacked potatoes in the UK. Changing climate characterised by rising temperatures and wetter summers is a threat as this will favour the development of pathogens such as C. coccodes in the soil increasing the risk of food spoilage. Moreover, both diseases can develop not only in the field but also after harvest, with postharvest storage temperatures being a crucial factor in controlling fungal growth. Furthermore, anecdotal evidence showed differences on the aggressiveness of black dot depending on its origin (i.e. England and Scotland) on potato tubers. Silver scurf and black dot are difficult to differentiate as they present similar phenotypes characterised by silvery lesions making it challenging for managers to take the necessary corrective action during storage. Hence, the aim of this study was to give a general insight into the ecological conditions affecting the establishment of the causal agent of potato black dot in the field, and black dot and silver scurf during the supply chain. Therefore, in vitro experiments were designed to study the growth rate and lag times simulating both scenarios respectively: on soil extract agar (SEA) media at different temperatures (4, 11, 15 degrees C) and matric potentials (control [unmodified] and - 1.4 MPa [modified]); and on natural potato dextrose agar (NPDA) for different temperatures (4, 11, 15 and 20 degrees C) at 99 % relative humidity (RH) for 25 days. When simulating the field environment, drier conditions (matric potential = -1.4 MPa) reduced fungal growth for both isolates by 0.1 cm day- 1 at the temperature of 15 degrees C, suggesting temperature as the main limiting factor for the growth of C. coccodes in the soil. The causal agent of black dot exhibited a faster growth rate under retailer-like conditions (i.e., 15 degrees C) compared to H. solani. Understanding the environmental influence on both the pathogen and the crop is vital for proper disease management to help reduce food loss and waste.