A total of 119 isolates of Rhizoctonia were collected from stem canker lesions, stolon and root lesions, hymenia on stems, or from black scurf on tubers of potato plants ( Solanum tuberosum ) in Finland (latitudes 60 -67 ° N). All isolates except three belonged to anastomosis group 3 (AG-3) of R. solani , as determined by phylogenetic analysis of the internal transcribed spacer sequences (ITS1 and ITS2) of ribosomal RNA (rRNA) genes. Sensitivity of the 119 isolates to the fungicide flutolanil was tested in vitro (EC 50 values 0·14-0·75 µ g active ingredient mL − 1 ). The isolates also varied considerably in growth rate (5·1-14·8 mm day). The severity of disease caused by 99 isolates was determined based on the proportion of potato sprouts affected by lesions, discoloration or death, which was c . 1 -60%. Only two isolates that were able to cause severe symptoms showed particularly low sensitivity to the fungicide and rapid growth rate. One isolate each of anastomosis groups AG-2-1 and AG-5 and an unknown, binucleate Rhizoctonia sp. were detected. The AG-5 isolate and the binucleate isolate caused mild symptoms on potato sprouts, whereas the AG-2-1 isolate was not pathogenic. Taken together, AG-3 of R. solani was the predominant causal agent of the stem canker and black scurf diseases of potato in Finland and showed considerable variability in disease severity, fungicide sensitivity and growth rate in vitro .

Stem canker and black scurf are diseases of potato caused by the fungus Rhizoctonia solani . Spatiotemporal experimentation and empirical modelling were applied for the first time to investigate the effect of antagonistic Trichoderma harzianum on the dynamics of soilborne R. solani on individual potato plants. Trichoderma harzianum reduced the severity of symptoms, expressed as 'rhizoctonia stem lesion index' (RSI), during the first 7 days post-inoculation when the inoculum of R. solani was placed at certain distances (30 -60 mm) from the host. For example, with inoculum at 40 mm from the host, RSI was 6 and 40 with and without T. harzianum , respectively. At later observation times, the antagonistic effect was overcome. Trichoderma harzianum reduced the severity of black scurf on progeny tubers. Furthermore, the mean number of progeny tubers per potato plant was reduced by the biocontrol treatment (means of 6·5 ± 1·1 and 9·9 ± 2·7 tubers per plant with and without T. harzianum , respectively), as was the proportion of small (0·1-20·0 g) tubers (48% and 66% with and without T. harzianum , respectively). Additionally, there were fewer malformed and green-coloured tubers in pots treated with T. harzianum than in those without T. harzianum .

The rRNA cistron (18S-ITS1-5.8S-ITS2-28S) is used widely for phylogenetic analyses. Recent studies show that compensatory base changes (CBC) in the secondary structure of ITS2 correlate with genetic incompatibility between organisms. Rhizoctonia solani consists of genetically incompatible strain groups (anastomosis groups, AG) distinguished by lack of anastomosis between hyphae of strains. Phylogenetic analysis of internal transcribed spacer (ITS) sequences shows a strong correlation with AG determination. In this study, ITS sequences were reannotated according to the flanking 5.8S and 28S regions which interact during ribogenesis. One or two CBCs were detected between the ITS2 secondary structure of AG-3 potato strains as compared to AG-3 tobacco strains, and between these two strains and all other AGs. When a binucleate Rhizoctonia species related to Ceratobasidiaceae was compared to the AGs of R. solani, which were multinucleate (3-21 nuclei per cell), 1-3 CBCs were detected. The CBCs in potato strains of AG-3 distinguish them from AG-3 tobacco strains and other AGs yielding further evidence that the potato strains of AG-3 originally described as R. solani are a species distinct from other AGs. The ITS1-5.8S-ITS2 sequences were analyzed by direct sequencing of PCR products from 497 strains of AG-3 isolated from potato. The same 10 and 4 positions in ITS1 and ITS2, respectively, contained variability in 425 strains (86%). Nine different unambiguous ITS sequences (haplotypes) could be detected in a single strain by sequencing cloned PCR products indicating that concerted evolution had not homogenized the rRNA cistrons in many AG-3 strains. Importantly, the sequence variability did not affect the secondary structure of ITS2 and CBCs in AG-3.

Rhizoctonia solani causes stem canker and black scurf diseases on potato and negatively affects the yield in all potato-growing areas. While seed-borne infection can be efficiently controlled by dressing with fungicides, few means of effective control are available against soil-borne infection. In this study, commercially available antagonistic fungi and bacteria, and the combination of antagonistic Trichoderma harzianum and seed dressing with flutolanil, were tested for their efficacy in the control of soil-borne infection of R. solani in the field. Combined use of flutolanil and T. harzianum was found feasible because even the highest tested concentration of flutolanil [13.0 lg active ingredient (a.i.) mL 21 ] had little effect on the growth of T. harzianum in vitro, whereas over 100-fold lower concentrations (0.1 lg a.i. mL 21 ) were sufficient to strongly inhibit the growth of R. solani (EC 50 0.045 AE 0.0068 lg a.i. mL 21 ). The variables under focus in plants inoculated with R. solani were the relative stem lesion index; sprout/stem number; stolon number, weight and incidence of symptoms on stolons; total yield and the yield of marketable sized tubers, and incidence of black scurf on the marketable-sized tubers. Flutolanil and its combined application with T. harzianum reduced the damage to sprouts and severity of stem canker at the early stages of growth (up to 30 days postplanting). Towards the end of the growing season, T. harzianum was required to reduce disease severity. When applied in-furrow alone or in combination with flutolanil-dressed seed potatoes, T. harzianum increased the proportion of marketable-sized tubers in yield from 35% to 60% and decreased the incidence of black scurf on progeny tubers from 31% to 11%, which was not achieved using flutolanil alone. The number and weight of stolons and the yield of plants remained lower in the inoculated plants than un-inoculated control plants regardless of the method of control used. The other two antagonists tested, Streptomyces griseoviridis and Gliocladium catenulatum, showed no consistent control of R. solani. Taken together, the results suggest that combining the application of the antagonist T. harzianum with seed dressing with flutolanil may provide the best protection of the potato crop against damage caused by R. solani throughout the growing season.

Development of black scurf on potato tubers (cv. Nicola) was compared in plants inoculated with isolates of Rhizoctonia solani of three anastomosis groups (AG2-1, AG3 and AG5) which occur in potato crops in Finland. All isolates induced stem canker lesions but only isolates of AG3 formed efficiently black scurf on progeny tubers. Among the AG2-1 and AG5 isolates tested, only one AG2-1 isolate formed a few sclerotia on 13.5 % of the progeny tubers in one experiment. The data indicate that isolates of AG3 differ from those of AG2-1 and AG5 in having a higher ability to form sclerotia on tubers. Therefore, while AG2-1 and AG5 isolates have a broader host range, AG3 is more efficient in producing black scurf, which provides this anastomosis group with more efficient means of dissemination on seed potatoes. These differences probably explain the predominance of AG3 (98.9 % of isolates) in potato crops in Finland and other northern potato production areas.