Phenotypic variation of cabbage white mold pathogen, Sclerotinia sclerotiorum in the upcountry commercial cabbage fields in Sri Lanka

Sclerotinia sclerotiorum is a cosmopolitan, necrotrophic and soil borne plant pathogen, capable of infecting many economically important crops. Although it is a wellstudied pathogen elsewhere, there is no research conducted in Sri Lanka. The objectives of the current research were to study the phenotypic variations among isolates within the pathogen population in the upcountry commercial cabbage fields in Sri Lanka and to determine their sensitivity to a commonly applied fungicide, Mancozeb. Forty-six isolates obtained from infected cabbage heads were identified as S. sclerotiorum based on its unique morphological characteristics. Colony diameter was measured on potato dextrose agar (PDA) plates at 23 ºC and mycelial colour was observed after two-week incubation. All the isolates were grown in pH indicator media to determine their acid producing abilities. Sensitivity of the isolates to Mancozeb was assayed in vitro. There was a significant difference in colony growth among the isolates and sensitivity to Mancozeb. Two isolates showed abnormal mycelial growth characteristics; a slower growth rate on PDA and reduced pathogenicity on cabbage leaves. Two levels of mycelial pigmentations were observed on PDA. All the isolates were acid producers, inferring that all of them were pathogenic. In summary, the upcountry S. sclerotiorum pathogen population was found to be diverse for selected phenotypic traits, especially for mycelial growth, and fungicide sensitivities inferring that the pathogen population is capable of adapting to changing environmental conditions. Therefore, disease management will be challenging if an epidemic occurs, and it is recommended that a proper management system should be identified before further expansion of upcountry cabbage cultivation.