Powdery scab caused by Spongospora subterranea f. sp. subterranea (Sss) has recently become one of the most devastating potato diseases of economic importance in South Africa. The use of resistant cultivars has long been considered the most effective and sustainable strategy to manage the pathogen. However, little is known about the molecular mechanisms underlying resistance of potato tubers to Sss. Using RNA-sequencing (RNA-seq), 2058 differentially expressed genes (DEGs) were identified from two potato cultivars (tolerant and susceptible) in response to Sss infection. Analysis of the expression patterns of 10 selected defence-response genes was carried out at two different stages of tuber growth using RT-qPCR to validate the RNA-seq data. Several defence-related genes showed contrasting expression patterns between the tolerant and susceptible cultivars, including marker genes involved in the salicylic acid hormonal response pathway (StMRNA, StUDP and StWRKY6). Induction of six defence-related genes (StWRKY6, StTOSB, StSN2, StLOX, StUDP and StSN1) persisted until harvest of the tubers, while three other genes (StNBS, StMRNA and StPRF) were highly up-regulated during the initial stages of disease development. The results of this preliminary study suggest that the tolerant potato cultivar employs quantitative resistance and salicylic acid pathway hormonal responses against tuber infection by Sss. The identified genes have the potential to be used in the development of molecular markers for selection of powdery scab resistant potato lines in marker-assisted breeding programmes.

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Plants defend themselves from pathogens by producing bioactive defense chemicals. The biochemical mechanisms relating to quantitative resistance of potato to root infection by Spongospora subterranea f. sp. subterranea (Sss) are, however, not understood, and are not efficiently utilized in potato breeding programs. Untargeted metabolomics using ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS) was used to elucidate the biochemical mechanisms of susceptibility to Sss root infection. Potato roots and root exudate metabolic profiles of five tolerant cultivars were compared with those of five susceptible cultivars, following Sss inoculation, to identify tolerance-related metabolites. Comparison of the relative metabolite abundance of tolerant versus susceptible cultivars revealed contrasting responses to Sss infection. Metabolites belonging to amino acids, organic acids, fatty acids, phenolics, and sugars, as well as well-known cell wall thickening compounds were putatively identified and were especially abundant in the tolerant cultivars relative to the susceptible cultivars. Metabolites known to activate plant secondary defense metabolism were significantly increased in the tolerant cultivars compared to susceptible cultivars following Sss inoculation. Root-exuded compounds belonging to the chemical class of phenolics were also found in abundance in the tolerant cultivars compared to susceptible cultivars. This study illustrated that Sss infection of potato roots leads to differential expression of metabolites in tolerant and susceptible potato cultivars.

Rhizoctonia solani is considered a destructive and widely distributed soil borne pathogenic fungus causing black scurf on potato tubers globally.  Morphological, microscopic characteristics and virulence studies were done on ten R. solani isolates isolated from four districts of Maseru, Thaba-tseka (Mantsónyane), Quthing and Berea representing three agro-ecological zones; the mountains, lowlands and Senqu river valley.  The characteristics include colony colour, hyphal orientation, number of nuclei, and presence of constrictions were studied after five days while colony growth was observed over 24 hour intervals. Variations were observed on all cultural and morphological characters studied in different geographical areas. Among the thirty isolates studied, thirteen of them had pale brown colonies in colour, while eleven had colonies with cream colour, and only 6 isolates had pale to brown coloured colonies. Isolate RB2A  had a number of differentiating features such as faster mycelium growth rate of 4 cm at 24 hours, compared to other isolate with 2 cm between 24 hours and 72 hours which progressed to medium (5cm) after 72 hours and then faster after 96 hours. Mycelium growth rate was observed to be independent of the sample collection site (Figure 4; Table 2). The highest nuclei number of 12 was observed with RM3B isolate, while the lowest number of four was observed with RM1A isolate. A pot experiment to evaluate the virulance strenghth of collected isolates was also conducted under greenhouse conditions, with three cultivars commonly grown in Lesotho, inoculated with the isolates collected from different geographical areas were tested. Isolates showed differences in aggressiveness among and within different potato cultivars grown. All the isolates collected were aggressive in causing black scurf of potatoes with percentage disease incidence ranging from 22 to 51 in Fandango, 70 to 92 in Panamera and 4 to 51 in Savannah. However, different cultivars responded diferently to infection by the isolates. Panamera was the most susceptible cultivar with the highest disease incidence of 83% and disease severity index of 85%. This is the first report of R. solani causing black scurf on potatoes  in Lesotho. Information of Rhizoctonia solani’s prevelence is crucial in the development of effective and timely potato diseases control strategies and growers can make informed cultivar choices  for management of Rhizoctonia solani