Potato is affected by many pests and pathogens, including the fungus Rhizoctonia, which causes severe qualitative and quantitative economic losses to potato production. Several anastomosis groups (AGs) and subgroups of Rhizoctonia solani and binucleate Rhizoctonia (BNR) are involved in causing different disease symptoms mainly on potato tubers (e.g. black scurf, elephant hide, corky cracks), but also on stems (stem canker), stolons (stolon canker) and roots.  Specific AGs or subgroups may occur individually or in combination on or in symptomatic potato tissue.  Potato rhizoctoniasis refers to all the diseases on potatoes caused by Rhizoctonia. Research on potato rhizoctoniasis in South Africa were mainly done at the University of Pretoria in collaboration with the ARC and funded by the Potatoes South Africa and the National Research Foundation.  The main outputs from the research are described below.

Surveys of AGs of R. solani associated with potatoes in South Africa showed most isolates belonged to AG 3-PT and it was widely distributed throughout all potato growing regions in South Africa (Truter & Wehner, 2004; Muzhinji et al. 2015). Other AGs, AG 2-2IIIB; AG 4HG-I; AG 4HG-III; and AG 5 were also found associated with potato diseases albeit at lower frequencies, and were found in distinct localities. In addition to R. solani, BNR AG A and AG R were also found associated with potato diseases (Muzhinji et al. 2015). Pathogenicity results showed that AG 3-PT isolates significantly caused more black scurf on potato tubers than other AGs, but can also cause corky cracks, similar than growth cracking and elephant hide symptoms (Muzhinji et al. 2014a). Recently, R. solani AG 2-2IIIB was shown to also cause elephant hide symptoms (Gush et al. 2019). BNR isolates of AG A and AG R were able to cause stem canker on potato stems and black scurf on potato tubers. Overall, AG 3-PT, AG 5 and AG R were the only AGs that formed black scurf on progeny tubers and should be an important target for control of both soil- and seed tuber-borne inoculum and prevent long distance dispersal of the pathogen on seed tubers. AG 2-2IIIB, AG 4 HG-I and AG 4 HG-III (Muzhinji et al. 2014b) were more aggressive on stolon and stem canker, indicating that some AGs are specialised in infecting specific parts of the potato plant. Control strategies for non-tuber infecting AGs should therefore focus on soil-borne inoculum. Generally, pathogenicity results showed inter and intra AG variability, indicating that pathogenicity is isolate-dependent rather than AG-dependent.

BlackScurf1.pngBlackScurf2-TN.pngBlackScurf3-TN.png

Black scurf of potato tubers (Truter, 2005).  Potato stem canker (Truter, 2005).  Elephant hide of potato (Muzhinji & Van der Waals, 2017)

The population genetic diversity and structure of R. solani AG 3-PT was investigated using microsatellite analysis (Muzhinji et al. 2016). There was evidence of high intra-population genetic diversity, low levels of clonality, and population differentiation and recombination among the populations. The AG 3-PT populations were geographically differentiated especially for the most distant populations (e.g. Limpopo vs. Western Cape). High differentiation of populations observed in this study may be because long distance dispersal of the pathogen via infected seed tubers is minimal due to long geographic distances between potato growing regions in South Africa.  AG 3-PT populations in South Africa are genetically diverse and poses a mixed reproductive system of clonality and recombination.

Rhizoctonia disease epidemics on potato crops caused by AG 3-PT can be initiated by soil- and/or tuber-borne inoculum. The relative importance of soil- and seed tuber-borne inoculum of R. solani AG 3-PT on potato disease development and the evolution of a R. solani AG 3-PT experimental field population over the growing season were evaluated and investigated. Two distinct sets of genetically-marked isolates as revealed by PCR-RFLP analysis were used as seed tuber-borne and soil-borne inocula in a mark-release-capture experiment (Muzhinji et al. 2017). Seed tuber-borne was found to be the predominant and relatively most important genotype in inciting potato diseases, although the synergistic effect of the two types of inoculum was evident. The proportion of isolates with genotypes that differed from the inoculants increased over the growing season and across growing seasons indicating the possibility of gradual evolution of the pathogen in the field. Therefore efforts to reduce primary inoculum should focus on reducing pathogen population on seed tubers to prevent introduction of different genotypes into the field.

Contact: Dr Mariette Truter

References

Gush, S., Muzhinji, N., Truter, M & van der Waals, J.E. 2019. First report of Rhizoctonia solani AG 2-2IIIB causing elephant hide on potato tubers in South Africa.  Plant Disease (in press).

Muzhinji, N., Woodhall, J.W., Truter, M. & Van der Waals, J.E. 2014a. Elephant hide and growth cracking on potato tubers caused by Rhizoctonia solani AG 3-PT in South Africa.  Plant Disease 98(4): 570. http://dx.doi.org/10.1094/PDIS-08-13-0815-PDN.

Muzhinji, N., Woodhall, J.W., Truter, M. & Van der Waals, J.E. 2014b.  First report of Rhizoctonia solani AG 4HG-III causing potato stem canker in South Africa.  Plant Disease 98(6): 853. http://dx.doi.org/10.1094/PDIS-11-13-1131-PDN.

Muzhinji, N., Truter, M., Woodhall, J.W. & Van der Waals, J.E. 2015. Anastomosis groups and pathogenicity of Rhizoctonia solani and binucleate Rhizoctonia from potatoes in South Africa.  Plant Disease 99: 1790-1802. http://dx.doi.org/10.1094/PDIS-02-15-0236-RE.

Muzhinji, N., Woodhall, J.W., Truter, M., van der Waals, J.E. 2016. Population genetic structure of Rhizoctonia solani AG 3-PT from potatoes in South Africa. Fungal Biology 120: 701-710. http://dx.doi.org/10.1016/j.funbio.2016.02.009.

Muzhinji, N., Woodhall, J.W., Truter, M., van der Waals, J.E. 2017. Relative contribution of seed tuber- and soil-borne inoculum to potato disease development and changes in the population genetic structure of Rhizoctonia solani AG 3-PT under field conditions in South Africa. Plant Disease 102: 60-66. http://dx.doi.org/10.1094/PDIS-03-17-0329-RE.

Muzhinji, N., Woodhall, J.W., Truter, M., van der Waals, J.E. 2017. Relative contribution of seed tuber- and soil-borne inoculum to potato disease development and changes in the population genetic structure of Rhizoctonia solani AG 3-PT under field conditions in South Africa. Plant Disease (in press).

Muzhinji, N. & Van der Waals, J.E. 2017. Seed tuber-borne inoculum of Rhizoctonia significantly contributes to Rhizoctonia disease epidemics on potato and pathogen population genetic changes. Chips March/April: 22-26.

Truter, M. & Wehner, F.C. 2004.  Anastomosis grouping of Rhizoctonia solani associated with black scurf and stem canker of potato in South Africa.  Plant Disease 88(1): 83.  http://dx.doi.org/10.1094/PDIS.2004.88.1.83B.

Truter, M. 2005. Etiology and alternative control of potato rhizoctoniasis in South Africa.  MSc thesis, University of Pretoria, Pretoria.

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