2. Pathogenesis Biology of Rhizoctonia solani
French mycologist Augustin Pyramus de Candolle originally characterized the basidiomycete Rhizoctonia in 1815
[14].
R. solani is a soil-borne phytopathogenic fungus with a wide range of hosts and a widespread geographic distribution. It is known to cause several essential crop diseases, which negatively impact agriculture and the economy. More than 3.2 million hectares in China have been infected by this fungus, causing yield losses of more than 200 million kg per year
[5]. Appearing in natural circumstances,
R. solani is a semi-saprophytic pathogen that can harm more than 15 families, including maize, soybean, grain crops, horticultural crops, and several other genera
[15][16]. Temperature and soil moisture play important roles in disease development. The optimum temperature for this pathogen is 20–25 °C, and 20% moisture is required for its growth
[17]. Although the fungus
R. solani has a thin and weblike basidiocarp, it is encountered in its anamorphic state as sclerotia and hyphae. The teleomorph is
Thanetophorus cucumeris [18]. The earliest stage of the fungus
R. solani, which is based on Thanatephorus cucumeris, appears on leaves, soil, and diseased sheaths slightly above the ground as a thin, mildew-like growth
[19]. The best method of preventing rice sheath blight is still the use of resistant cultivars
[20]. The resistance factor relies on numerous genes as a quantitative characteristic with a large usable resistance QTL (quantitative trait locus)
[21]. This specific approach to managing the fungus
R. solani does not work well when chemical fungicides like carbendazim and triazole are applied
[22]. There are several cases of Trichoderma being used to manage sheath blight. According to Chen et al.
[22] and de França et al.
[23], volatile secondary metabolites and
Trichoderma asperellum aqueous solution can effectively control rice sheath blight. Regular disease surveillance should begin at the period of panicle differentiation and continue through the heading. Sheath blight infection after heading may result in considerable financial losses
[19].
3. Mycoviruses
In the edible mushroom
Agaricus bisporus (phylum:
Basidiomycota), the first mycoviruses were discovered in 1962
[24]. Since then, the fungal taxa have all been shown to include mycoviruses
[25][26]. To find new, unidentified mycoviruses, next-generation sequencing (NGS) techniques are now being applied
[27]. Although ssRNA and DNA viruses have been found, the most commonly described are mycovirus dsRNA genomes
[26]. According to the International Council for the Taxonomy of Viruses (ICTV), fungal viruses are now grouped into 19 officially recognized families and 1 floating genus that has not yet been assigned to a family. These include the straight dsRNA viruses (
Partitiviridae,
Amalgaviridae,
Totiviridae,
Botybirnavirus,
Chrysoviridae,
Quadriviridae,
Megabirnaviridae, and
Reoviridae) and the linear positive-sense (+) ssRNA families (
Alphaflexiviridae,
Barnaviridae,
Botourmiaviridae, and
Endornaviridae [28][29]. Mycoviruses have the potential to spread within the pathogen population efficiently.
Symptoms of Mycoviruses and Their Interactions with Hosts
Some significant alterations include altered sexual reproduction, aberrant pigmentation, and uneven development. There are two categories of phenotypic changes: those that are helpful to the host of mycoviruses and those that are destructive. Harmful interactions include hypovirulence and debilitation, which have been found in several plant-pathogenic fungi, such as root rot and rot fungi. Advantageous interactions are identified in fewer plant endophytic and pathogenic fungi, where virulence and heat tolerance increase
[30]. The host fungus for mycoviruses is increasingly recognized as a model filamentous fungal organism for virus–virus interaction and viruses’ interactions with their hosts
[31].
4. Transmission Properties of Fungi
To accurately avoid transmission to unfavorable hosts, the mycovirus must be able to establish and spread inside the selected host population
[32]. There are two recognized modes of transmission: vertical transmission via sporulation and transmission through the hypha of fungi and heterokaryons
[33], which results in extensive coverage of the biocontrol agent. This kind of transmission is connected to increased biocontrol mechanisms as opposed to vertical transmission, which is often associated with lower efficiency
[34]. There have been several instances of horizontal transmission in mutualistic symbioses
[35].
Sclerotinia sclerotiorum hypovirulence-associated DNA virus-1 (SsHADV-1) is a new circular ssDNA virus that may spread extracellularly and uses the mycophagous insect
Lycoriella ingénue as a vector for transmission
[36][37].
5. Transmission of Viruses in Rhizoctonia solani
Successful transfection techniques have previously been used for various mycoviruses, including those from the families
Partitiviridae,
Megabirnaviridae,
Totiviridae, and
Reoviridae. This method often necessitates using polyethylene glycol (PEG) 4000
[38]. The transmission of RsPV2/GD-11-purified particles, either horizontally or vertically, has been successful
[39]. Moreover, members of the Endornaviridae family, which do not create viral particles, can effectively transmit horizontally and vertically. For instance, the
alphaendornavirus RsEV1/GD-2 can spread horizontally using its hyphal anastomosis, while a
betaendornavirus identified in
R. solani Ra1 could spread vertically through basidiospores
[11]. Moreover, hyphal anastomosis might transmit the
betapartitiviruses RsPV6/YNBB-111, RsPV7/YNBB-111, and RsPV8/YNBB-111 horizontally
[40][41].
6. Mycoviruses of Rhizoctonia solani
The pathogenic fungus
R. solani contains the first dsRNA element ever reported, being discovered by Butler and Castano
[26]. Pathogenic
R. solani isolates have so far been found to contain about 100 viral infections, consisting of several known families of viruses that contain dsRNA, positive ssRNA, and negative ssRNA, as well as individuals of suggested families and undefined RNA components. On the one hand, certain viruses identified as infecting
R. solani are members of families of mycoviruses that have been extensively investigated, including the
Barnaviridae,
Botourmiaviridae,
Endornaviridae,
Deltaflexiviridae,
Hypoviridae,
Narnaviridae,
Megabirnaviridae, and
Partitiviridae. On the other hand, some are proposed members of the orders
Bunyavirales,
Tymovirales, and
Serpentovirales and belong to or are very similar to families historically recognized as affected plants, such as CMV
[11][42].
R. solani AG-1 IA, a pathogenic fungus isolated from rice crop, has been found to be infected by several viruses, including
R. solani dsRNA virus 1 (RsRV1) in 2013
[43],
R. solani partitivirus 2 (RsPV2) in 2014
[38], and
R. solani RNA virus 2 (RsRV2)
[44]. Also, more recently,
Rhizoctonia solani partitiviruses 3 to 8 (RsPV3 to 8, respectively),
Rhizoctonia solani dsRNA virus 3 (RsRV3)
[37][40][45], and RsEV1 (
Rhizoctonia Solani Endornavirus) have been discovered
[46].
R. solani isolates frequently have many co-infections;
R. solani AG2-2 IV DC17, for example, has been reported to carry an endornavirus, a
megabirnavirus, a
mitovirus, two
flexiviruses, and the three invasively related
mycoalphaviruses [38]. Similarly,
R. solani AG-3PT RS002, which infects potatoes, is home to both an endornavirus and a mitovirus
[47][48].
7. dsRNA Mycoviruses
Most mycoviruses with dsRNA genomes are encapsidated in isometric particles
[49]. Currently, there are eight families that fall under
Amalgaviridae (one genomic segment, length 3.5 kbp),
Megabirnaviridae (two genomic segments, length 7.0–9.0 kbp),
Chrysoviridae (three to seven genomic segments, length 2.4–3.6 kbp),
Partitiviridae (two to three genomic segments, length 1.4–2.3 kbp),
Reoviridae (
Spinareovirinae subfamily, ten to twelve genomic segments
[49]. In addition, the
Botrytis porri RNA virus 1 (BpRV1), a dsRNA virus from the
Botybirnavirus genus, has been described
[50]. The taxonomy of mycoviruses often evolves when new viruses are discovered. New families, such as
Fusariviridae (four genomic segments 1.5–3.6 kbp in length) and
Alternaviridae (one genomic segment 6–7 kbp in length), have also recently been proposed
[51]. The majority of positive-sense (+) ssRNA fungal and plant viruses, such as
Alphaflexiviridae,
Betaflexiviridae,
Gammaflexiviridae,
Closteroviridae, and
Potyviridae, form spherical but not filamentous virions; however, a novel dsRNA virus from
Colletotrichum camelliae identified from tea plants in China was discovered
[49][52]. However,
Beauveria bassiana polymycovirus-1 (BbPmV-1) and
Aspergillus fumigatus tetramycovirus-1 (AfuTmV-1) from the insect pathogen
B. bassiana and the human pathogen
A. fumigatus, respectively, have been reported recently (
Figure A2)
[29]. BbPmV-1 exhibits hypovirulence, which is unusual for mycoviruses, in its host
[53].
7.1. Family Megabirnaviridae
One genus of the
Megabirnaviridae family,
Megabirnavirus, is known to infect fungi
[54]; another similar genus, Phlegivirus, has also been proposed
[27]. Individuals of the family may accept straight bi-segmented dsRNA genomes that are each between 7 and 8.9 kbp long and have a combined length of 16.1 kbp. The isometric particles that contain the dsRNA genomes are packed
[27][55]. For each segment, the only officially recognized species
Rosellinia necatrix megabirnavirus 1 (RnMV1/W779) prototype has two tandems, non-relating ORFs
[56].
Sclerotinia sclerotiorum megabirnavirus 1 (SsMBV1)
[57],
Rosellinia necatrix megabirnavirus 2 (RnMBV2)
[58],
Pleospora megabirnavirus 1 (PMBV1)
[58], and
Entoleuca megabirnavirus 1 (EnMBV1) have also been described
[59]. Viral transmission happens horizontally through anastomosis or vertically through spores that cause the production of new progeny
[55]. Moreover, more dsRNA viruses belonging to the Megabirnaviridae family were found using modern NGS methods
[27]. Two linear and independently encapsidated monocistronic dsRNA segments are accommodated in members of the Partitiviridae family, while an extra satellite or damaged dsRNA segment may be present. Each dsRNA segment is one big ORF encoding a putative RNA-dependent RNA polymerase (RdRp) or coat protein (CP) and is between 1.4 and 2.4 kbp in size
[60].
Alphapartitivirus,
Betapartitivirus,
Deltapartitivirus,
Cryspovirus, and
Gammapartitivirus are the five genera that comprise the family
Partitiviridae [60]. Fungal partitiviruses can spread vertically through spores or horizontally through hyphal fusion
[61].
7.2. Family Partitiviridae
R. solani has been confirmed to include members
of alphapartitivirus and
betapartitivirus genera. The cause of rice diseases such as sheath blight,
Rhizoctonia solani AG-1 IA, was shown to be an
alphapartitivirus known as
Rhizoctonia solani partitivirus
2 (RsPV2). Two segments, each measuring 2020 bps and 1790 bps, are supported by RsPV2/GD-11.
Rhizoctonia solani virus 717 (RsV717) has two genomic parts 2363 and 2206 kbp in length. In addition, the complete coding sequences of the
R. solani AG2-2 LP-identified
R. solani partitiviruses 5 to 7 (RsPV6/BR5, RsPV7/BR6, and RsPV8/BR16) and
R. solani dsRNA virus 2 (RsDSRV2/A) have been characterized using NGS. RsPV6/BR5 is a member of the genus
Betapartitivirus, while RsDSRV2/A, RsPV8/BR16, and RsPV7/BR6 RdRps are members of the genus
Alphapartitivirus [62]. Moreover, a fragment of the
Rhizoctonia solani partitivirus 1 sequence, isolated from
R. solani OA-1, has been reported.
8. Unclassified dsRNA Mycoviruses
A few investigations have shown that
R. solani, a pathogenic fungus, is infected by unclassified dsRNA viruses. RsRV1 was the first
Rhizoctonia solani dsRNA virus identified
[43]. Among the unique chromosomal dsRNAs L1 (25 kbp), L2 (23 kbp), and S1 (1.2 kbp), M1 and M2 dsRNAs were discovered in
R. solani Rhs 1A, and they constitute the first thoroughly documented dsRNA elements in the pathogenic fungus
R. solani [63]. Two putative ORFs are accommodated on the positive strand of M1, being identical to the previously identified
Rhizoctonia solani virus 1 (RsV1). Four more ORFs have been discovered on the negative strand
[64]. M2 has a single main ORF that encodes an RdRp that is related to the penta-functional AROM polypeptide of the shikimate pathway and has an intense relationship with the mycovirus-like
Rhizoctonia solani and barely synthesizes five intermediate steps in the (biochemical) shikimate pathway in filamentous yeast and fungi
[41].
9. ssRNA Mycoviruses
Single-stranded (ss) RNA viruses are also common in
R. solani, in addition to dsRNA viruses
[64]. SsRNA serves as the genetic makeup of the tiniest and most basic viruses
[65]. Based on the polarity of their RNA genomes, ssRNA viruses can be categorized as positive-sense (+) or negative-sense (−) RNA. According to Koonin et al.
[66], (+) ssRNA viruses have a straightforward RNA replication and expression mechanism, whereas ssRNA viruses start replication by encapsulating their transcription and replication machinery within particles (virions)
[67]. The linear monopartite (+) ssRNA genome is present in most known ssRNA fungal viruses
[68], including
Alphaflexiviridae (5.4–9 kbp),
Barnaviridae (4 kbp),
Botourmiaviridae (2.9 kbp),
Deltaflexiviridae (6–8 kbp),
Endornaviridae (14–17.6 kbp),
Gammaflexiviridae (6.8 kbp),
Hypoviridae (9–13 kbp), and
Narnavirid (1.7–2.9 kbp).
Sclerotinia sclerotiorum negative-stranded RNA virus 1 has recently been classified as a member of the family
Mymonaviridae and is the only ssRNA mycovirus the ICTV officially recognizes (
Figure A3)
[69].
9.1. Family Barnaviridae
There is presently just one species and one genus of barnavirus in the family
Barnaviridae [70]. The model species’ monopartite (+) ssRNA genome, the mushroom bacilliform virus, is 4.0 kbp in length. The whole genome contains four ORFs, each coding a protein with a function (P1) that is unknown, a polyprotein with VPg, and the domains protease (P2), a putative CP (P4), and RdRp (P3); furthermore, the
Rhizoctonia solani barnavirus 1 (RsBV1) and barnavirus 1, belonging to the genus
barnavirus, have been identified
[71]. The polyprotein encoded by RsBarV1, which has a length of 3915 bp and three ORFs, codes for a protease, the putative domain VPg, a protein that is unknown, a putative CP, and a relationship with MBV. The absence of the ORF, which is thought to encode a hypothetical protein with an unknown function, suggests that the 5′ terminal sequence of RsBV1 is incomplete.
9.2. Family Benyviridae
Positive-sense ssRNA viruses of plants with rod-shaped particles (virions) are capped and polyadenylated genomes. Their lengths range from 1.3 to 6.7 kb, and they are found in the family
Benyviridae [72]. Four species make up the genus
Benyvirus, and the individuals of this family are linked to cell-to-cell migration
[72][73].
R. solani 42304-9a and
R. solani AG-2.2 LP BR2 were discovered to have two different viruses, known as
Rhizoctonia solani Beny-like virus 1 (RsBenV1) and
Rhizoctonia solani Beny-like virus 3 (RsBenV3), which were both identified and partially sequenced. Only one region of each genome was identified as encoding a putative RdRp associated with
benyviruses and beny-like viruses. RsBenV1/42304-9 shares a close resemblance to a recognized individual of the
Benyviridae family, whereas RsBenV1/BR2 resembles the
Sclerotium rolfsii beny-like virus 1.
9.3. Family Botourmiaviridae
Ten species and four genera make up the family
Botourmiaviridae (
Botoulivirus (2.9 kbp long),
Magoulivirus (2.3 kbp long),
Ourmiavirus (three segments, 0.9 kbp, 1.0 kbp, and 2.8 kbp long), and
Scleroulivirus (one segment, 3 kb long)). Just 59% to 87% of the
Rhizoctonia solani ourmia-like virus 1 (RsOLV1) genome has been described, and an essential investigation revealed similarities to the RdRps of
Ourmiavirus genus members, including the Cassava virus C, Ourmia melon virus, and the Epirus cherry
[71][74]. It is thought that plant viruses of the
Ourmiavirus genus developed by reassorting the genomic segments of viruses that infected both plants and fungi. These viruses are tripartite, with each segment coding for one protein: RdRp, putative CP, and protein movement (MP)
[72]. Nevertheless, the putative CP and MP are not encoded in the RsOLV1 genome
[71][74]. According to recent research, the mono-segmented S. sclerotiorum ourmia-like virus 4 (SsOLV4) was isolated from the fungus
S. sclerotiorum and solely encoded the RdRp protein, which is sufficient for infection, replication, and transmission. Moreover, research on SsOLV4 implies that the family
Botourmiaviridae needs a new genus. As a result, fungal ourmia-like viruses only have one segment at this time. Today, Rhizoctonia magoulivirus 1 (RsOLV1) is the family
Botourmiaviridae, a
Magoulivirus genus and legally recognized species. In addition, the evolutionary phylogenetic tree grouped RsOLV 2–5 and
Agaricus bisporus virus 15 into a potentially new genus within the family
Botourmiaviridae, and even shows a new relationship with the family that is named
Basidionarnaviridae [62]. About 70% similarity exists between the RsOLV2/Rs, RsOLV2, and RsOLV3 RdRp sequences. As a result, they are probably distinct isolates of the same species
[62].
9.4. Family Bromoviridae
Alfamovirus,
Anulavirus,
Bromovirus,
Cucumovirus,
Ilarvirus, and
Oleavirus are six of the family’s current genera. The tripartite linear (+) ssRNA genome of the
Bromoviridae family is around 8 kb long
[75][76]. Putative RdRp 1a and 2a are encoded by RNA1 and RNA2. Members of the genera
Cucumovirus and
Ilarvirus have an extra overlapping ORF. For the individuals of the genera
Cucumovirus,
Anulavirus,
Ilarvirus, and
Bromovirus, the family
Bromoviridae generates particles (virions) that are either spherical or quasi-spherical or bacilliform, like the members of the genera
Alfamovirus,
Ilarvirus, and
Oleavirus [75].
9.5. Tymoviridae, Deltaflexiviridae, and Unidentified Viruses of the Tymovirales Order Are Mycoviruses
The families
Alphaflexiviridae,
Deltaflexiviridae,
Betaflexiviridae, and
Gammaflexiviridae are highlighted by the collective name “
flexiviruses” which belongs to the order
Tymovirales. The filamentous virions of flexiviruses possess a monopartite (+) ssRNA polyadenylated genome that is between 6.5 and 9.5 kb long and codes for a replication-associated polyprotein between 150 and 250 kDa in size
[68]. It is known that the flexiviruses may infect both fungi and plants
[77].
Botrytis virus F (BotV-F), which is related to the family
Gammaflexiviridae and the genus
Mycoflexivirus, was the first mycovirus identified in the order
Tymovirales [78]. Three species belonging to the genus
Deltaflexivirus have been described within the family
Deltaflexiviridae:
Sclerotinia deltaflexivirus 1 (SsDFV1),
soybean-associated deltaflexivirus 1 (SlaMFV1), and
Fusarium deltaflexivirus 1 (FgDFV1)
[68].
Rhizoctonia solani flexivirus 1 (RsFV-1) is the only flexivirus that is fully sequenced and has been found to affect the pathogenic fungus
R. solani [38]. The (+) ssRNA genome of RsFV-1, separate from
R. solani AG2- 2IV/DC17, has 10,621 nts without the poly-A tail. One protein, encoded by RsFV-1, is connected to those of other
Tymovirales members
[38].
Tymoviridae is a family that includes (+) ssRNA viruses between 6.0 and 7.5 kb in size
[79]. Forty-one officially identified species are in the family
Tymoviridae, including three genera:
Marafivirus,
Maculavirus, and
Tymovirus [80].
9.6. Family Endornaviridae
A family of viruses known as
Endornaviridae has RNA genomes that are unencapsidated, and their size ranges from 8.7 to 16.6 kb. Just one ORF in the family codes for a polyprotein
[81]. The polyprotein has conserved RdRp domain motifs at the C-terminus and an RNA helicase domain at the N-terminus
[82].
Endornaviruses are chronic infections of plants, fungi, and oomycetes that do not manifest symptoms in their hosts
[82][83]. In fungal hosts, members of
Endornaviridae are transmitted vertically through sporulation and horizontally through anastomosis
[84], while in plant hosts, they are spread via vertical transmission through pollen and ova since they lack a putative MP and cannot move from cell to cell. An endornavirus identified from
R. solani AG-3PT strain RS002 (RsEV-RS002) was partially characterized and tentatively entitled
Rhizoctonia solani endornavirus.
9.7. Family Hypoviridae
One identified species, Hypovirus, and four others with capsidless, mono-segmented (+) ssRNA genomes with a size ranging from 12.7 to 9.2 Kbp make up the family
Hypoviridae [85]. One or two ORFs each have a genome that encodes the domains RdRp and Hel
[86]. Additional domain motifs, such as papain-like protease (PRO), glucosyltransferase (UGT), and permuted papain-fold peptidase of dsRNA viruses and eukaryotes (PPPDE), are also present
[87][88]. The capacity of hypoviruses to reduce the fungal host virulence (hypovirulence) of the chestnut blight pathogenic fungus
Cryphonectria parasitica initially sparked interest in them. Via hyphal anastomosis, the members of the hypovirus can spread to virulent isolates horizontally
[88]. a complete
R. solani hypovirus genome has not yet been found; nevertheless, the NGS technique was previously used to identify whole ORFs of
Rhizoctonia solani hypovirus 1 (RsHV1) and partial ORFs of
Rhizoctonia solani hypoviruses 2 and 3 (RsHV2 and 3, respectively)
[62]. With two ORFs each, genomic sequences of RsHV2 and 3 are 9 and 5 kbp long. Whereas one RsHV3 ORFs encodes a helicase-conserved domain motif, both RsHV2 ORFs produce hypovirus-related proteins without any putative conserved features. In the family
Hypoviridae, a new genus,
Megahypovirus, was also suggested to fit the huge genomes of RsHV1 and SsHV2, as well as RsHV2, RsHV3, and
Agaricus bisporus virus 2
[62]. Also, three fusariviruses were found to be homologous to the members of the proposed family,
Fusariviridae.
Rhizoctonia solani fusarivirus 1, 2, and 3 (RsFV1, 2, and 3, respectively) were described
[89]. The 11 kb long RsFV1 genomic segment contains four putative ORFs. ORF1 encodes a viral helicase. ORF2 and 4 were short ORFs and no significant homology with other proteins. The largest ORF3 codes for a protein with putative RdRp and Hel domains. The genomic architecture of the RsFV2 is identical; however, the incomplete genomic sequence of RsFV3 codes for putative RdRp and Hel domains
[62].
9.8. Mitoviridae and Narnaviridae Families
The simplest known viruses are those belonging to the
Narnaviridae and
Mitoviridae families. Their linear (+) ssRNA genomes, which range in size from 1.7 to 3.6 kb, include a single ORF that encodes a putative RdRp and lacks a capsid.
Mitoviridae and
Narnaviridae each have one genus,
Mitovirus (five species) and
Narnavirus (two species), respectively
[90]. According to Nerva et al.
[91], all known members of the genus Mitovirus infect filamentous plants, and a fungal
Narnavirus has also been discovered
[50]. Although mitoviruses and narnaviruses do not produce viral particles, they are linked to lipid-membrane-bound vesicles with genomes between 2.3 and 3.1 kb in length
[37][92]. In contrast to members of the narnavirus family, which are known to replicate in the cytosol, mitoviruses reproduce in the host cell’s mitochondria. Since the first Mitovirus was found in
C. parasitica, many mitoviruses have been found in phytopathogenic fungi
[37]. The pathogenic fungus
R. solani does not include any narnaviruses or complete mitovirus genome sequences. Forty incomplete mitovirus genome sequences associated with
R. solani have been identified
[27][63][93].
9.9. Family Togaviridae
The Eastern equine encephalitis virus (EEEV), the Western equine encephalitis virus (WEEV), the Venezuelan equine encephalitis virus (VEEV), the Sindbis virus (SINV), the Ross River virus (RRV), the Semliki Forest virus (SFV), and the Chikun virus (SFV) are all members of the
Togaviridae family, which includes 31 species in total
[94]. Arboviruses called alphaviruses alternately infect vertebrate hosts and are spread by insect vectors
[95]. Small enveloped (+) ssRNA viruses with a genome encoding structural and non-structural proteins are found in members of the
Togaviridae family and range in size from 10 to 2 Kbp
[96]. The viral particles comprise surface glycoproteins, a lipid bilayer, and a nucleocapsid core
[96].
R. solani is not infected by any members of the genus
Alphavirus. However, recently, partial genomic sequences belonging to the
Togaviridae family, including
Rhizoctonia solani alphavirus-like 1, 2, and 3, were found in
R. solani AG-2 LPin (RsALV1/BR15, RsALV2/BR14, and RsALV3/BR8)
[62]. The RsAVL2 partial ORF encodes a putative RdRp, whereas the RsAVL1 and RsAVL3 partial ORFs include both putative viral helicase and RdRp domains
[61].
9.10. Order Serpentovirales
Aspiviridae, formerly known as
Ophioviridae, is a family of adaptable filamentous viruses that belong to the order
Serpentovirales and are known to infect plants
[97]. Currently, the family
Aspiviridae has seven species, including the genus
Ophiovirus. The (−) ssRNA genomes of the family
Aspirividae range in size from 11.3 to 12.5 kb and are divided into three to seven segments
[97].
Rhizoctonia solani negative-stranded RNA virus 1-3 (RsNSRV1-3) is a recently identified ophiovirus-related incomplete viral genome sequence that infects soil-borne
R. solani strains. The lettuce ring necrosis ophiovirus and other members of the
Aspiviridae family have RNA1s that encode big ORFs with considerable similarities to the L-protein domain. The
Fusarium poae negative-stranded RNA virus1 (FpNSV1), isolated from the pathogenic fungus
Fusarium poae, has also been proposed as a new family,
Betamycoserpentoviridae, in the order
Serpentovirales [71].
9.11. Order Bunyavirales
The Arenaviridae, Cruliviridae, Hantaviridae, Fimoviridae, Leishbuviridae, Mypoviridae, Nairoviridae, Phasmaviridae, Peribunyaviridae, Phenuiviridae, Tospoviridae, and Wupedeviridae families are considered herein. Many (ssRNA) fungal viruses linked to bi- and tri-segmented (ssRNA) viruses, like phenuiviruses and peribunyaviruses, were found in plant-pathogenic fungi after metatranscriptomics analysis
[71]. For instance, the first segmented (ssRNA) virus discovered to infect fungus is the Lentinula edodes negative-stranded RNA virus 2 (LeNSRV2), which also infects Lentinula edodes
[42]. In contrast, more viruses belonging to the order Bunyavirales were found in fungi connected to the ascomycete Entoleuca spp. and the marine organism
Holothuria polii [59].
10. Effects of Viral Infection in Rhizoctonia solani
Mycovirus infections are frequently asymptomatic (cryptic), although research focuses on possible hypovirulence, a feature that might emerge in the setting of long-term biological control of fungi. The best example is
Cryphonectria hypovirus 1 (CHV1), which was successfully used in Europe to eradicate the plant disease
Cryphonectria parasitica, the cause of chestnut blight
[32]. The term “hypovirulence” was coined due to this discovery, which completely changed the field of fungus biological management
[98]. The “La France” illness of Agaricus biporus caused by the “La France” isometric virus (LIV) and other mycoviruses may also have more deadly consequences. These fungi infections are caused by the OMSV and OMIV (oyster mushroom spherical and isometric viruses)
[93]. To study how mycoviruses affect their host, it is essential to create an isogenic line that is virus-free, either by curing the virus-infected individual or transferring the mycovirus into a virus-free strain
[29].