Elucidating the role of microbial factors and water stress in causing corky crack blemishes on potato tubers in South Africa

Abstract

Potato (Solanum tuberosum L.) is the most consumed and economically important vegetable crop in the world and is a staple crop in many communities. However, the potato crop is susceptible to devastation by various diseases that reduce the yield and quality of fresh produce. Blemishes, rots and deformation are some of the tuber symptoms caused by fungi, bacteria, viruses and nematodes. Abiotic factors such as water stress (dry conditions and thereafter high moisture conditions), has also been associated as causal agents of tuber blemishes. Potato tuber blemishes constitute a persistent quality problem in the production of potato throughout the world and can be a result of known causes (typical blemishes), which are manageable; or unknown causes (atypical blemishes), which are difficult to manage. Atypical corky crack blemishes are an emerging problem of potatoes in South Africa. There has been a large debate in the South African potato industry regarding the causal agents of corky cracks. This study investigated the role of microbial factors and water stress in causing corky crack blemishes in South Africa. In a pilot trial, R. solani AG2-2IIIB and various Streptomyces species were isolated from corky crack blemishes on potatoes in South Africa. However, none of these microorganisms, alone or in combination resulted in the development of corky cracks when tested in pot trials. Therefore, in an effort to confirm the causal agent(s) of corky crack blemishes, additional tubers with symptoms of either growth cracks or corky cracks were sampled from different potato growing regions of South Africa and further isolations of bacteria and fungi were made from the different symptoms. Water stress and the detection of Potato Virus Y (PVY) was also studied to describe the association with corky crack blemishes in South Africa. Binucleate Rhizoctonia (BNR) AG-A and BNR AG-R as well as Fusarium oxysporum were the most predominant fungal species isolated from the corky crack symptoms, while no fungal species were isolated from growth cracks. Streptomyces collinus, S. yaanensis, S. corchorussi, S. viridochromogenes and S. griseorubens were the most predominant bacterial species isolated from growth cracks and no Streptomyces species were isolated from corky cracks. The Streptomyces species identified are not known to be pathogenic to potatoes and are, therefore, not likely causal agents of the corky cracks. PVY was not amplified from corky crack tissue using molecular techniques and it was, therefore, assumed that PVY is not associated with corky cracks on the tested samples of potato tubers in South Africa. Greenhouse pathogenicity trials using the isolated microorganisms alone and in combination were done to confirm Koch’s postulates. Corky crack blemishes were not observed on progeny tubers in single inoculations for fungal or bacterial isolates; however, corky crack blemishes were observed on the progeny tubers inoculated with a combination of Rhizoctonia species. This suggests a synergistic interaction between the three Rhizoctonia species tested in this study. The findings of this study are crucial in the development of effective integrated strategies for the management of corky crack disease and to improve the tuber quality in the South African market and globally.