Studies on powdery scab on potatoes in South Africa

Abstract

Spongospora subterranea (Wallroth) Lagerheim f. sp. subterranea Tomlinson (Sss) is the causal organism of potato powdery scab. This blemish disease is of economic concern to fresh and seed-tuber producers around the world, as it diminishes both tuber quality and marketability. In this study, techniques for detecting and quantifying Sss were evaluated. Comparison of enzyme linked immunosorbent assay (ELISA), conventional PCR and real-time PCR showed that conventional PCR is more sensitive than ELISA, as conventional PCR detected Sss inoculum from both tuber and soil samples whereas ELISA only detected Sss inoculum from tuber samples. Real-time PCR not only detected Sss DNA in a variety of sample types, but real-time PCR could also be used to quantify Sss DNA. The high sensitivity of real-time PCR gave consistent detection of standard DNA quantities ranging from 10 000 to as few as 1 sporeballs per ml. The benefit of real-time PCR is that it can be used for the study of the different life stages of Sss in a range of sample types. During 2008 to 2010 powdery scab samples were collected from Ceres, KwaZulu Natal, Mpumalanga and Sandveld and herbarium samples dated 1936 from the Sandveld were also obtained to investigate genetic variation in ITS1/2 sequences. Comparisons to known Sss Group Type I/II sequences were made to determine the specific Group Type/s found in South Africa. All these South African samples were identified as belonging to Sss Group Type II A bioassay was developed to investigate the dormancy period of Sss. The results showed that Sss zoospores are released four days after inoculation and infection of tomato roots occurs six days after inoculation. Plasmodia in tomato root hairs develop eight days after inoculation, followed by zoosporangia development by day 12. Several strategies are being used to minimize the risk of powdery scab as there is no single effective method for controlling the disease. One such strategy includes cultivar resistance. Cultivar susceptibility of six South African cultivars (Argos, BP1, Buffelspoort, Caren, Up-to-Date and Valor) was evaluated. Pot trials showed that all the evaluated cultivars are susceptible to infection by Sss and that there is a positive correlation between the severity of root galls and of tuber lesions. Various crops (cabbage, mustard, soybean and tomato) were assessed to determine the host range of Sss in rotation crops in South African potato growing regions and to identify possible trap crops for Sss. Of the crops evaluated, the pathogen was unable to complete its life cycle in wheat and cabbage, whereas soybean is a non-host crop. Both mustard and tomato are hosts of Sss. A severe outbreak of powdery scab occurred during 2006 in a potato mini-tuber production facility in Ceres, Western Cape, South Africa. A study was conducted in the production facility to detect Sss and to identify the source/s of contamination, so that corrective measures could be taken to eradicate the pathogen. Swab samples specified areas in the production facilities that were infested with Sss sporeballs. Following eradication efforts to improve facility hygiene, a second set of swab samples was taken in 2009 to determine the efficacy of the eradication methods. From 2009 onwards, disease-free mini-tubers have been harvested.</p. This comprehensive study will lead to a better understanding of Spongospora subterranea f.sp. subterranea and has emphasised the need for further research which will focus on strategies that will help to reduce the impact that powdery scab has on potato production, not only in South Africa, but globally too.