Morphological and Pathological Variability of Intra-field Rhizoctonia solani Isolates in a Rice-Potato Rotation and their Sensitivity to Fungicides

Rhizoctonia solani is a destructive soil-borne plant pathogen having a vast host range that infects various agricultural and horticultural crops. A total of 25 R. solani isolates were collected from a field of rice-potato rotation to determine their morphological variability, mycelial compatibility, cross-infectivity and fungicide sensitivity. Considerable variability in cultural and morphological features was observed among R. solani isolates collected from rice and potato crops. Rice isolates grew faster than potato isolates and hyphal width of both the host isolates varied from 6.67 to 9.37 mu m. Significant variability was observed in sclerotial characteristics such as sclerotia colour, size, intensity and sclerotial distribution pattern on the Petri plates. Most of the isolates from both hosts produced micro (<= 1.25 mm) size sclerotia and only few isolates produced macro (> 1.25 mm) size sclerotia. The sclerotial colour of most of the rice isolates was brown and some isolates exhibited black sclerotial colour. All the potato isolates produced brown-coloured sclerotia. Hyphal interaction studies revealed that potato isolates belonged to AG-3 and rice isolates belonged to AG1-IA group. Mycelial compatibility studies showed that 80.7% of the pairings were non-compatible, while 19.3% of the pairings exhibited a compatible interaction. Cross-infectivity revealed that R. solani potato isolates did not infect rice plants whereas rice isolates showed a varied degree of pathogenicity on potatoes and Rhizoctonia-like atypical symptoms were observed on the tuber surface. The EC50 and EC90 values of representative isolates from both hosts exhibited considerable variation with regard to sensitivity level against fungicides thifluzamide 24% SC and mancozeb 50% + carbendazim 25% WS. The outcomes of this study will help to develop management strategies and breeding programmes on varietal evaluation for effective management of Rhizoctonia diseases of potatoes and rice.