First Report of Rhizoctonia solani Associated with Black Scurf of Potato Tubers in Lesotho

Abstract

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