Genetic variability and pathogenicity of R hizoctonia solani associated with black scurf of potato in N ew Z ealand

Isolates (a total of 129) of R hizoctonia solani were collected from black scurf on potato tubers from different potato‐growing regions in N ew Z ealand. Sequence analysis of the nuclear ribosomal DNA internal transcribed spacer (r DNA – ITS ) regions from these isolates identified three anastomosis groups ( AG s), AG ‐3 PT , AG ‐2‐1 and AG ‐5. Isolates classified as AG ‐3 PT were widely distributed, whereas AG ‐2‐1 and AG ‐5 were confined to distinct locations. Sequence heterogeneity was identified in the ITS regions of 100 AG ‐3 PT and AG ‐2‐1 isolates. Variation in the sequence and length of the r DNA – IGS 1 region was also observed for selected isolates of AG ‐3 PT and AG ‐2‐1. Phylogenetic studies found all AG ‐2‐1 isolates belong to AG ‐2Nt, a subset of AG ‐2‐1 previously associated with solanaceous crops in other countries. AG ‐2‐1 isolates were consistently more aggressive than those of AG ‐3 PT . Delayed emergence, severe infection on stolons, formation of aerial tubers and considerable yield losses were associated with AG ‐2‐1, but they caused negligible black scurf. In contrast, AG ‐3 PT caused black scurf on progeny tubers but variable effects on stem emergence and stolons. Furthermore, AG ‐2‐1 isolates caused severe tuber malformation, but isolates of other AG s did not. This is the first report on the AG composition, genetic variability and pathogenicity of R . solani isolates associated with black scurf of N ew Z ealand potatoes.