Characteristics of anastomosis groups and subgroups of Rhizoctonia solani and binucleate Rhizoctonia spp

The genus concept of Rhizoctonia spp. was established by de Candolle (1815) (Sneh et al., 1998). However, the lack of specific characters led to the classification of a mixture of unrelated fungi as Rhizoctonia spp. (Parmeter and Whitney, 1970; Moore, 1987). Ogoshi (1975) enhanced the specificity of the genus concept for Rhizoctonia by elevating the following characteristics of R. solani to the genus level. Based on this revised genus concept, species of Rhizoctonia can be differentiated by mycelia color, number of nuclei per young vegetative hyphal cell and the morphology of their teleomorph. The teleomorph of Rhizoctonia spp. belongs to the sub-division Basidiomycota, class Hymenomycetes.


Introduction
The genus concept of Rhizoctonia spp. was established by de Candolle (1815) (Sneh et al., 1998).However, the lack of specific characters led to the classification of a mixture of unrelated fungi as Rhizoctonia spp.(Parmeter and Whitney, 1970;Moore, 1987).Ogoshi (1975) enhanced the specificity of the genus concept for Rhizoctonia by elevating the following characteristics of R. solani to the genus level.Based on this revised genus concept, species of Rhizoctonia can be differentiated by mycelia color, number of nuclei per young vegetative hyphal cell and the morphology of their teleomorph.The teleomorph of Rhizoctonia spp.belongs to the sub-division Basidiomycota, class Hymenomycetes.
The anamorphs of Rhizoctonia are heterogeneous.Moore (1987) placed the anamorphs of Thanatephorus spp. in Moniliopsis.She reserved the genus Rhizoctonia for anamorph of ustomycetous fungi which have septa with simple pores.Moniliopsis species have smooth, broad hyphae with brown walls, multinucleate cells, dolipore septa with perforate parenthesomes and teleomorphs in the genera Thanatephorus and Waitea.Of the binucleate Rhizoctonia spp., the anamorphs of the R. repens group (teleomorph Tulasnella) were assigned to the new genus Epulorhiza.Anamorph of Ceratobasidium was assigned to the new genus Ceratorhiza (Moore, 1987).Moore's system is taxonomically correct and justified.At present, the concept of genus Rhizoctonia has become clear from these taxonomical studies at the molecular level (Gonzalez et al., 2001).However, many researchers (Sneh et al., 1998) in the world still retain the name Rhizoctonia for Moore's Moniliopsis spp., Ceratorhiza spp.and Epulorhiza spp.. Hence, I used the name of Rhizoctonia in this study.
Anastomosis reactions between hyphae of paired isolates of R. solani consist of several types; such as perfect fusion, imperfect fusion, contact fusion and no reaction (Matsumoto et al., 1932).At present, four categories of anastomosis (C3 to C0) defined by Carling et al. (1996) have been accepted by many researchers.These are useful for a better understanding of the genetic diversity of R. solani populations, because of the background genetically supported by vegetative or somatic compatibility (VC or SC) of confronted isolates (MacNish et al, 1997).Each of categories is as follows: C3: walls fuse; membranes fuse, accompanied with protoplasm connection; anastomosis point frequently is not obvious; diameter of anastomosis point is equal or nearly equal hyphal diameter; anastomosing cells and adjacent cells may die, but generally do not.This category occurs for the same anastomosis group, same vegetative compatibility population (VCP) and the same isolate.
C2: wall connection is obvious, but membrane contact is uncertain; anastomosing and adjacent cells always die.This category occurs in same AG, but not between different VCPs.
C1: wall contact between hyphae is apparent, but both wall penetration and membranemembrane contact do not occur; occasionally one or both anastomosing cells and adjacent cells die.This category occurs between different AGs or in the same AG.C0: no reaction.This category occurs between different AGs.
In general, hyphal fusion occurs at a high frequency (50%≥) within members of the same AG, with the exception of non-self-anastomosing isolates (Hyakumachi and Ui, 1988).On the other hand, hyphal fusion among members of different AGs occurs at either a low frequency (≤30%) or no fusion occurs.Rhizoctonia isolates giving C3 to C1 reactions in anastomosing test have been taken to be the same AG.
To date, isolates of multinucleate R. solani have been assigned to 13 anastomosis groups (AG-1 to AG-13), some of which include several subgroups and isolates of R. zeae and R. oryzae have been assigned to WAG-Z and WAG-O, respectively (Sneh et al., 1998;Carling et al., 1999Carling et al., , 2002c)).Isolates of binucleate Rhizoctonia spp. with Ceratobasidium teleomorphs have been reported.A system developed in Japan (Ogoshi et al.,1979(Ogoshi et al., , 1983 a,b; a,b;Sneh et al., 1998;Hyakumachi et al., 2005) includes 21 anastomosis groups designated AG-A to AG-U, in which at present AG-J and AG-M still are in question as members of binucleate Rhizoctonia.Another system developed in the USA (Burpee et al., 1980a) includes 7 anastomosis groups designed as CAG-1 to CAG -7.CAG-1 corresponds to AG-D, CAG-2 to AG-A, CAG-3 and CAG-6 to AG-E, CAG-4 to AG-F, CAG-5 to AG-R, and CAG-7 to AG-S (Sneh et al., 1998;Ogoshi et al., 1983a).At present, the anastomosis system based on AG-A through AG-U used in this review paper is widely accepted by many researchers.Some homogenous groups of isolates of R. solani are well known as bridging isolates (AG-BI) that anastomose with members of different AGs (Carling, 1998).In general, there is no contradiction in the conventional anastomosis grouping system by taking anastomosis frequency into consideration.However, two exceptional cases where anastomosis frequency mismatched with morphological, physiological and pathogenic characteristics have been reported from tobacco (Nicoletti et al., 1999) and soybean (Naito and Kanematsu, 1994).These demonstrate the limitations of using hyphal anastomosis as the sole criteria for characterization and identification of closely related fungi.In addition, it is not easy to determine the subgroup of isolates within the same AG because no differences occur in their anastomosis reaction.Thus, in order to determine AGs or subgroups in R. solani, genetic analysis using molecular approaches that employ multiple genetic loci is needed.
Isolates of R. solani that exhibits DNA base sequence homology and affinities for hyphal anastomosis may represent a diverging evolutionary unit (Kuninaga and Yokosawa, 1980).This hypothesis is supported by analysis of restriction fragment length polymorphisms (RFLPs) and the sequences with in ribosomal RNA genes (rDNA) among different anastomosis groups of R. solani (Vilgalys and Gonzalez, 1990;Gonzalez, et al., 2001;Carling et al., 2002b).
As mentioned above, many AGs and subgroups of R. solani and binucleate Rhizoctonia spp.have been reported as causal of agents Rhizoctonia diseases on a wide range of host species.However, little is known about the Rhizoctonia diseases and the anastomosis groups and subgroups of their causal fungi on vegetables, ornamentals and food crops in the Asian tropics especially the southern parts of China.

Characteristics of anastomosis groups and subgroups of Rhizoctonia solani and binucleate Rhizoctonia spp.
Disease symptoms and host range of each AG and its subgroups are summarized as follows.In this review, the book by Sneh et al., 1998 entitled "Identification of Rhizoctonia Species" provided a substitute for the reference before 1998.
Note: This group has a tendency to attack aerial parts of the plants.Basidiospore infection of rice has been reported, but sclerotia are more important as an infection source.The optimum growth temperature is higher than those of AG-1 IB. (Sneh et al., 1998;Naito, 2004;Yang et al., 2005b).Symptoms: sheath blight, leaf blight, foliar blight, web-blight, root rot, damping-off, head rot, and bottom rot.