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Mycoviruses are widespread in all major taxonomic groups of filamentous fungi. Previous research has indicated that mycoviruses are associated with the phytopathogenic fungus Botryosphaeria dothidea. In this study, three distinct double-stranded RNA viruses were detected in B. dothidea strain YCLYY11 isolated from a leaf spot of longan (Dimocarpus longana). The results of BLAST analysis revealed that the predicted amino acid sequences of those viruses were similar to those of Botryosphaeria dothidea chrysovirus 1, Botryosphaeria dothidea partitivirus 1, and an apparent novel victorivirus. Sequencing and analysis of the complete genome of the novel victorivirus indicated it is 5218 bp in length and contains two open reading frames (ORFs) that overlap at the tetranucleotide AUGA. BLASTp analysis of the proteins encoded by ORF1 and ORF2 showed that they were most similar to the coat protein and RNA-dependent RNA polymerase of Sphaeropsis sapinea RNA virus 2 (81.37% and 74.09% identical, respectively). A phylogenetic tree showed that the novel virus clustered together with victoriviruses and was separate from members of the other four genera of the family Totiviridae. Based on its genome structure and the results of phylogenetic analysis, we propose that this novel victorivirus should be named "Botryosphaeria dothidea victorivirus 3". This is also the first report of these three mycoviruses coinfecting a strain of B. dothidea.

In this study, a novel positive-sense single-stranded RNA (+ssRNA) mycovirus, tentatively named Colletotrichum fructicola RNA virus 1 (CfRV1), was identified in the phytopathogenic fungus Colletotrichum fructicola. CfRV1 has seven genomic components, encoding seven proteins from open reading frames (ORFs) flanked by highly conserved untranslated regions (UTRs). Proteins encoded by ORFs 1, 2, 3, 5, and 6 are more similar to the putative RNA-dependent RNA polymerase (RdRp), hypothetical protein (P2), methyltransferase, and two hypothetical proteins of Hadaka virus 1 (HadV1), a capsidless 10- or 11-segmented +ssRNA virus, while proteins encoded by ORFs 4 and 7 showed no detectable similarity to any known proteins. Notably, proteins encoded by ORFs 1 to 3 also share considerably high similarity with the corresponding proteins of polymycoviruses. Phylogenetic analysis conducted based on the amino acid sequence of CfRV1 RdRp and related viruses placed CfRV1 and HadV1 together in the same clade, close to polymycoviruses and astroviruses. CfRV1-infected C. fructicola strains demonstrate a moderately attenuated growth rate and virulence compared to uninfected isolates. CfRV1 is capsidless and potentially encapsulated in vesicles inside fungal cells, as revealed by transmission electron microscopy. CfRV1 and HadV1 are +ssRNA mycoviruses closely related to polymycoviruses and astroviruses, represent a new linkage between +ssRNA viruses and the intermediate double-stranded RNA (dsRNA) polymycoviruses, and expand our understanding of virus diversity, taxonomy, evolution, and biological traits. IMPORTANCE A scenario proposing that dsRNA viruses evolved from +ssRNA viruses is still considered controversial due to intergroup knowledge gaps in virus diversity. Recently, polymycoviruses and hadakaviruses were found as intermediate dsRNA and +ssRNA stages, respectively, between +ssRNA and dsRNA viruses. Here, we identified a novel +ssRNA mycovirus, Colletotrichum fructicola RNA virus 1 (CfRV1), isolated from Colletotrichum fructicola in China. CfRV1 is phylogenetically related to the 10- or 11-segmented Hadaka virus 1 (HadV1) but consists of only seven genomic segments encoding two novel proteins. CfRV1 is naked and may be encapsulated in vesicles inside fungal cells, representing a potential novel lifestyle for multisegmented RNA viruses. CfRV1 and HadV1 are intermediate +ssRNA mycoviruses in the linkage between +ssRNA viruses and the intermediate dsRNA polymycoviruses and expand our understanding of virus diversity, taxonomy, and evolution.

Trifoliate orange (Poncirus trifoliata L) is a thorny tree of the Rue family, which is extensively used as citrus rootstock in China. In January 2021, several leaf yellowing, declining, and wilting citrus seedlings grafted on trifoliate orange rootstock with rotted main roots were observed in orchards located in Wuhan city, Hubei, China. In old orchards, the incidence of diseased roots was approximately 90%. Diseased roots from seven plants were collected and cut into small pieces (0.2 to 0.5 cm). These pieces were then surface-sterilized using 0.1% mercury bichloride for 3 min, 75% ethanol for 3 min, rinsed with sterile distilled water for several times, and then placed on potato dextrose agar (PDA) supplemented with 0.05% lactic acid (v/v), and incubated at at 25±2°C in dark. Fifty-threesingle-conidium isolates with morphological characteristics similar to Fusarium spp. were obtained (Leslie and Summerell 2006), which displayed two kinds of colony morphology. Thirty isolates showed white to orange-white abundant aerial mycelium in rings and acquired a yellow to orange pigmentation, tweenty-three isolates showed white to pink, fluffy aerial mycelium in rings and acquired an orange to red pigmentation. Isolate WG-1 and HrmY-9 from each group were used for future identification. The average colony growth rate of WG-1 and HrmY-9 on PDA was 0.95±0.06 and 0.69±0.11 mm/day, n=4, respectively. WG-1 produced numerous oval, unicellular microconidia without septa, 4.03-9.87×1.01-5.13 µm, n=80 and very few macroconidia with two to four septa, narrowed at both ends, 11.08-22.64×1.67-4.91 µm, n=30. HrmY-9 produced numerous curved macroconidia with three to four septa, 18.03-37.33×2.16-7.8 µm, n=80, microconidia were unicellular, oval, and 5.33-16.19×1.74-6.51 µm, n=50. Sequences of internal transcribed spacer (ITS), translation elongation factor 1-alpha (EF-1α), and DNA-directed RNA polymerase largest subunit (RPB1) genes were amplified with the primers ITS1/ITS4, EF1a-F/EF1a-R, and RPB1-F5/RPB1-R8, respectively (White et al. 1990, O'Donnell et al. 1998, O'Donnell et al. 2010), sequenced and deposited in GenBank. Sequences of isolate WG-1 (GenBank accession No. ON045437, ON063232 and ON089664) and HrmY-9 (GenBank accession No. ON045438, ON063233 and ON089665) were 100% identical with the corresponding sequences of Fusarium oxysporum (OM876904, JF430180, and MT568959) and F. solani (MT605584, MK617767, and MT305110), respectively. Based on above results, WG-1 and HrmY-9 was identified as F. oxysporum and F. solani, respectively. Pathogenicity test were performed on healthy one-year-old trifoliate orange seedlings by dipping their injured roots into conidial suspension (50 ml, 1×106 conidia/mL) for 1 h and the rest of conidial suspension was added to the pot after replanting to make sure the inoculum was in contact with the roots. Roots of control plants were inoculated with sterilized water. All experiments were repeated twice. All plants were cultured at 26°C under a 16-h light/dark cycle. Typical symptoms developed on most of inoculated seedlings two months post inoculation. No disease symptoms appeared on control plants. Same colonies were reisolated from the inoculated roots, confirming Koch's postulates. To our knowledge, this is the first report of F. oxysporum and F. solani causing root rot on trifoliate orange rootstock in China. The identification of F. oxysporum and F. solani as the causal agents of the observed root rot on trifoliate orange rootstock is critical to the prevention and control of this disease in the future.