Characterization of a new cultural type (LP) of Rhizoctonia solani AG2-2 isolated from warm-season turfgrasses, and its genetic differentiation from other cultural types

Abstract

Isolates of Rhizoctonia solani AG2-2 obtained from turf with symptoms of large-patch disease of warm-season turfgrasses were compared with known AG2-2 isolates belonging to cultural types IIIB and IV. Some isolates that were previously identified as type IV have been separated here and named LP isolates. Comparisons among isolates were based on cultural morphology, hyphal growth rate, pathogenicity and restriction fragment length polymorphism (RFLP) analysis in the nuclear encoded ribosomal DNA (rDNA) genes. The cultural characteristics of LP isolates varied from those of types IIIB and IV. LP isolates did not show distinct sclerotial formation and zonation, and the colour of their mycelia and pigment deposition was dark brown. LP isolates had slower hyphal growth rates than types IIIB and IV, with an optimum temperature of 25°C compared with 28°C for types IIIB and IV. LP isolates were less virulent on radish but highly virulent on zoysia grass when compared with isolates of types IIIB and IV. Genomic DNA was digested separately with Eco RI, Ban III, Xba I and Sal I, and probed with cloned rDNA from Alternaria alternata in Southern hybridizations. LP isolates had one RFLP pattern, while both IIIB and IV possessed four different patterns each. Cluster analysis of RFLPs showed that R. solani AG2-2 is divided into three genetic subgroups, consisting of the IIIB, IV and LP isolates, respectively. The polymerase chain reaction (PCR) amplified rDNA internally transcribed spacer (ITS) regions of the IIIB, IV and LP isolates had the same length but produced different restriction patterns when digested with Msp I and Taq I. These results indicate that there are three cultural types in R. solani AG2-2, namely IIIB, IV and LP.