By James Woodhall, University of Idaho, Parma and Lina Rodriguez Salamanca, Iowa State University
Rhizoctonia is a ubiquitous fungus, and members of the group contain many important plant pathogens. Rhizoctonia, in its most widely used but artificial sense, is considered a species complex that comprises several groups including the multinucleate Rhizoctonia solani (Thanatephorus cucumeris), binucleate Rhizoctonia species (BNR or Ceratobasidium species), and Waitea circinata (Table 1). Knowing the anastomosis group (AG) present is often essential for an accurate diagnosis and to provide recommendations as AGs can have different host ranges, fungicide sensitivity profiles, growth temperatures, and means of dispersal. They can even specialize in the plant part they colonize. For example, in potatoes, AG8 exclusively infects the roots, while AG3-PT can cause stem cankers and black scurf. Within each AG, there can be multiple subgroups assigned on various critera often unique to each subgroup, such as thiamine autotrophy, the ability to grow at 35°C (such as AG2-2IIIB), host range, and ITS sequence. García et al. (2006) provide an overview of subgroups and their hosts.
Table 1. Current nomenclature and Anastomosis Groups (AGs) of frequently encountered plant pathogenic Rhizoctonia species.
Sharon, M., Kuninaga, S., Hyakumachi, M., and Sneh, B. 2006. The advancing identification and classification of Rhizoctonia spp. using molecular and biotechnological methods compared with the classical anastomosis grouping. Mycoscience 47:299-316.
Sharon, M., Sneh, B., Kuninaga, S., Hyakumachi, M., and Naito, S. 2008. Classification of Rhizoctonia spp. using rDNA-ITS sequence analysis supports the genetic basis of the classical anastomosis grouping. Mycoscience 49, 93–114,
Sneh, B., Burpee, L., and Ogoshi, A. Identification of Rhizoctonia species. (APS press, 1991).
Rhizoctonia is a ubiquitous fungus, and members of the group contain many important plant pathogens. Rhizoctonia, in its most widely used but artificial sense, is considered a species complex that comprises several groups including the multinucleate Rhizoctonia solani (Thanatephorus cucumeris), binucleate Rhizoctonia species (BNR or Ceratobasidium species), and Waitea circinata (Table 1). Knowing the anastomosis group (AG) present is often essential for an accurate diagnosis and to provide recommendations as AGs can have different host ranges, fungicide sensitivity profiles, growth temperatures, and means of dispersal. They can even specialize in the plant part they colonize. For example, in potatoes, AG8 exclusively infects the roots, while AG3-PT can cause stem cankers and black scurf. Within each AG, there can be multiple subgroups assigned on various critera often unique to each subgroup, such as thiamine autotrophy, the ability to grow at 35°C (such as AG2-2IIIB), host range, and ITS sequence. García et al. (2006) provide an overview of subgroups and their hosts.
Table 1. Current nomenclature and Anastomosis Groups (AGs) of frequently encountered plant pathogenic Rhizoctonia species.
| Anastomosis Groups (AGs) | |||
| Multinucleate | |||
|
sensu stricto |
Thanatephorus cucumeris |
Rhizoctonia solani |
AG 1 to 13 (There used to be 14 AGs but AG-BI was moved to AG2) |
|
sensu lato |
Waitea circinata cucumeris |
Rhizoctonia oryzae, Rhizoctonia zeae, Rhizoctonia circinata, Rhizoctonia agrostis |
Many subgroups |
| Binucleate | |||
| sensu lato | Ceratobasidium species | AG A to AG W | |
Isolation
- Water agar amended with an antibiotic can be used for isolation from plant material, and isolates can be subcultured onto potato dextrose agar for longer-term growth and storage. Storage on barley grains at -20 or -80°C is useful for long-term storage while maintaining pathogenicity.
Morphology
- Typically, the hyphae of R. solani are wider than Ceratobasidium and appear more distinctive. It is very easy to confuse Ceratobasidium with other frequently encountered soil-borne fungi. If you are trying to find a BNR species, it is best to be inclusive in your subculturing and sequence more suspect isolates
- AGs were originally assigned based on observing hyphal fusion between reference isolates of known AG and the test isolate. This long and laborious process requires some experience and may not be suitable for routine use in diagnostic labs.
Molecular Identification of AGs
There is enough evidence in the literature for AGs to be routinely assigned using rDNA ITS sequencing, and there are numerous conventional and PCR assays available that can be used for identification.- Many PCR assays are designed to the rDNA ITS region; while this is a multi-copy gene and is great for creating sensitive assays, it is still a conserved gene. Consequently, any assay you deploy in your lab must be thoroughly verified for specificity against isolates from closely related AGs.
- When using rDNA ITS sequence-based identification, it is best quick blast searches are not used alone. Many sequences present on GenBank are misidentified, and constructing a phylogenetic tree from trusted reference (type) sequences representative of multiple AGs is the best way to perform sequence-based identification. Misidentification of AG A and AG K is common due to the relative similarity in the sequences. The sequences used in Sharon et al. (2006, 2008) are particularly useful.
Resources
García, VG, Onco, MAP, and Susan, VR. 2006. Review. Biology and Systematics of the form genus Rhizoctonia. Spanish Journal of Agricultural Research 4: 55–79.Sharon, M., Kuninaga, S., Hyakumachi, M., and Sneh, B. 2006. The advancing identification and classification of Rhizoctonia spp. using molecular and biotechnological methods compared with the classical anastomosis grouping. Mycoscience 47:299-316.
Sharon, M., Sneh, B., Kuninaga, S., Hyakumachi, M., and Naito, S. 2008. Classification of Rhizoctonia spp. using rDNA-ITS sequence analysis supports the genetic basis of the classical anastomosis grouping. Mycoscience 49, 93–114,
Sneh, B., Burpee, L., and Ogoshi, A. Identification of Rhizoctonia species. (APS press, 1991).
