Genetic variation and pathogenicity of anastomosis group 2 isolates of Rhizoctonia solani in Australia

A collection of isolates of Rhizoctonia solani anastomosis group (AG) 2 was examined for genetic diversity and pathogenicity. Anastomosis reactions classified the majority of isolates into the known subgroups of AG 2-1 and AG 2-2 but the classification of several isolates was ambiguous. Morphological characters were consistent with the species, with no discriminating characters existing between subgroups. Vertical PAGE of pectic enzymes enabled the separation of zymogram group (ZG) 5 and 6 within AG 2-1, but not the separation of ZG 4 and 10 within AG 2-2. PCR analysis using inter-simple sequence repeats (ISSR) and the intron-splice junction (ISJ) region supported the separation of ZG 5 and 6, while the AG 2-2 isolates were separated by geographic region. A comparison of distance matrices produced by the zymogram analysis and PCR indicated a strong correlation between the marker types. Pathogenicity studies suggested canola (Brassica napus) cultivars were most severely affected by AG 2-1, while cultivars of two species of medic (Medicago truncatula cv. Caliph and M. littoralis cv. Herald) were susceptible to both AG 2-1 and 2-2. The results indicate that AG 2 is a polyphyletic group in which the classification of subtypes is sometimes difficult. Further investigation of the population structure within Australia is required to determine the extent and origin of the observed diversity.