Trichocladium solani sp. nov.—A New Pathogen on Potato Tubers Causing Yellow Rot

A new species, Trichocladium solani, was isolated from potato (Solanum tuberosum L.) tubers from Russia. The species has no observed teleomorph and is characterized morphologically by non-specific Acremonium-like conidia on single phialides and chains of swollen chlamydospores. Phylogenetic analysis placed the new species in a monophyletic clade inside the Trichocladium lineage with a high level of support from a multi-locus analysis of three gene regions: ITS, tub2, and rpb2. ITS is found to be insufficient for species delimitation and is not recommended for identification purposes in screening studies. T. solani is pathogenic to potato tubers and causes lesions that look similar to symptoms of Fusarium dry rot infection but with yellowish or greenish tint in the necrotized area. The disease has been named “yellow rot of potato tubers”.


Introduction
Pathogenic fungi are prominent agents in ecosystems, involved in the control of populations of plants in natural ecosystems [1] or causing potentially significant damage to agricultural ecosystems (www.fao.org (accessed on 20 August 2022)), and they require control measures for crops' protection. For effective control, it is widely accepted that the identification of a causative agent is one of the first and most important steps to ensure appropriate crop protection actions are implemented [2,3]. Fungal identification is known to be a demanding task. Hyphomycetes often show little difference in their morphology, producing similar propagules or chlamydospores, and sometimes do not produce teleomorphs or anamorphs, or neither, appearing sterile. Phylogenetic studies using DNA sequencing provide vital clues to what phenotypic features might be informative and serve as barcodes for identification purposes. Most identification is performed using ITS sequence data which is a well-established barcode for the identification of most fungi [4][5][6]. However, when observing ambiguous, poorly featured isolates from crop-related material, it is crucial to carry out a careful and thorough investigation as novel pathogens emerge continuously [7,8] and their timely identification and monitoring are at the base of any prevention measures.
Chaetomiaceae is a diverse family of fungi with more or less well-developed distinctive perithecial ascomata with a hairy surface developing terminal and/or lateral hairs of various forms. Their anamorphs vary significantly but are sometimes supplemented with nonspecific Acremonium-like conidiation with single phialides producing chained conidia [9][10][11]. Some species are known to be thermophilic [11][12][13], some are able to decompose cellulose [14], and some are promising agents of biocontrol of other pathogenic conditions of sampled tubers from different storages were 6-10 • C and 85-90% humidity. Infected potato tubers were thoroughly washed and then submerged into a sanitizing solution of 2% sodium hypochlorite for the removal of surface contamination before being air-dried. The tubers were then sliced across the damaged areas with a sterile blade to remove the necrotized tissue. A slice of living infected potato tissue under the necrosis was removed and placed on plates with potato dextrose agar (PDA) [31] in sterile conditions. In order to collect the air sample, opened PDA plates were exposed for one hour inside the sorting compartment of a potato storage facility, closed, and then cultured and purified from contamination using standard culture methods. All the isolates used in this study are listed in Table 1, including the obtained sequence data generated in [13]. DNA isolation, sequencing, and phylogeny-Genomic DNA was extracted from fungal mycelium grown on PDA after 6 days of growth. The mycelium was placed into 2.0 mL sterile reinforced microtubes with caps (SSI, USA) along with 700 µL of CTAB buffer (1.4 M NaCl, 0.1 M Tris-HCl, 20 mM EDTA, and 2% hexadecyltrimethylammonium bromide) and ground with zirconium oxide beads (Bertin Instuments, Montigny-le-Bretonneux, France) in the following composition: 1 × 5 mm; 2 × 2 mm; 3 × 0.1 mm (per tube) using the Precellys ® Evolution homogenizer (Bertin Instruments, Montigny-le-Bretonneux, France) in two cycles of 8000 rpm for 10 s with 5 s resting time in between. Tubes then were incubated at 65 • C for 1 h with intensive shaking every 20 min. After incubation, 500 µL of cold chloroform was added to the tubes that were centrifuged at 16,249× g for 10 min, and the supernatants were transferred to clean 1.5 mL microcentrifuge tubes. Isopropanol (400 µL) and 70 µL of 5 M potassium acetate (pH 4.6) were added to each supernatant; then, the tubes were shaken carefully and centrifuged at 16,249× g for 10 min at room temperature. The supernatants were then discarded, 150 µL of 70% ethanol (v/v) was added, and the tubes were centrifuged at the same gravity force for 5 min twice. Pellets were air-dried at room temperature and resuspended in 50 µL of deionized water. The resulting DNA solutions were diluted to a concentration of~50 ng/µL. DNA concentration was determined using a spectrophotometer NanoDrop 2000 (Thermo Scientific, Waltham, MA, USA) by measuring absorbance at 260 nm.
The cycle conditions for the amplification of internal transcribed spacer regions (ITS) included cycles of 95 • C/3 min (initial denaturation) followed by 30x cycles of 94 • C/30 s, 55 • C/30 s, 72 • C/45 s, and ending with 72 • C/8 min (final extension). The cycle conditions of the amplification of partial tub2 gene using T1/TUB4Rd were identical to the ITS protocol, apart from the annealing cycle of 60 • C/30 s. The protocol for the amplification of the partial rpb2 gene using rpb2-5F2/rpb2AM-7R included the initial denaturation Target PCR products were run in a 1.5% agarose gel stained with ethidium bromide (0.5 µg/mL) in 0.5× Trisborate EDTA (TBE) buffer at a constant voltage of 85V for about 1 h and visualized under ultraviolet (UV) light. PCR products were extracted from agarose gels and cleaned using Cleanup Mini Kit (Evrogen Co, Moscow, Russia), then sequenced using the BigDye ® Terminator v.3.1 Cycle Sequencing Kit (Applied Biosystems, San Francisco, CA, USA) and the Applied Biosystems 3730 xl automated sequencer (Applied Biosystems, USA). Each fragment was sequenced in both directions using the same primers described above. Consensus sequences for each locus were assembled using GENEIOUS PRIME 2022.2 software and MEGA X [36]. The obtained sequences were aligned with publicly available sequences from [13] using MEGA X software. Phylogenetic analyses were based on Bayesian inference (BI) and maximum likelihood (ML). ML analysis was performed with MEGA X and IQ-TREE web server [37]. For BI, BEAST the best evolutionary model for each locus was determined using MRMODELTEST 2.0 [38] and IQ-TREE [39]. Obtained trees were viewed in FIGTREE 1.1.2 [40] and subsequently visually prepared and edited in ADOBE ® ILLUSTRATOR ® 22.3.0.
Morphological descriptions are based on cultures grown on OA and PDA. Microscopic observations were performed in lactic acid mounts that were gently heated to remove air bubbles or on thin agar blocks with the mycelium that were cut from the Petri plate to the size of the cover glass and rinsed with a detergent solution to remove excessive conidia, if necessary. Alternatively, transparent adhesive tape was used for the preparation of the mount [45]. At least 35 measurements were made for all morphologically informative features. The measurements include the extreme values given in parentheses and, in between, the 95% confidence interval of 30 individual measurements using the methodology described in [10].
Whereas a common practice is to describe culture characteristics with colors using A mycological colour chart by R.W. Rayner [46], the color pallets can vary depending on the condition of the paper print or scanned material and the publication itself is not always available for some research groups. Therefore, additional color measurements were developed and implemented to provide more constant data that can be reproduced regardless of the availability of a specific color chart. In order to create the reference color pallet plates with grown culture, they were photographed with SpyderCHECKR 24 charts (Datacolor, USA). The photos were then white-balanced and light-balanced on the color checker using ADOBE ® PHOTOSHOP ® 2022 23.1.1.202 software with the curves tool. Images of colonies from different photos were then cut with the selection tool and moved to a new PNG file containing the cut colonies' pictures on a transparent background. Using PALETTE WIZZARD 1.4, the color palette was extracted from the PNG file; then, the color palette was brought down to the 9 most characteristic color tones. The obtained palette was refined and color IDs were added with CYOTEK PALETTE EDITOR 1.7.0.411. Rayner's color chart [46] was still used as a general guide for textual descriptions of colors, but we note that with the standard HEX color codes provided, the textual descriptions of the colors of colonies in the current study are not definitive and serve an indicative purpose.
Pathogenicity tests-Healthy hydroponically grown potato tubers, uniform in size, were washed and surface-sterilized in 0.5% sodium hypochlorite solution for 10 min, then rinsed in distilled water, air-dried, and peeled with a sterile blade (or left unpeeled in cases when a tuber was known to be clean coming from the sterile growing environment) and placed into sterile wet chambers. To determine the pathogenic status of the isolates, the tubers were wounded with a needle (sterile water for control or inoculated with a piece of mycelium taken from a margin of a 5-day-old colony grown on PDA). Three tubers were used for each strain for each of the two temperature conditions of 25 • C and 18 • C (to simulate storage conditions) for 25 days. For each isolate, a small infected-tissue sample was taken from the margin of the internal necrotic region with a sterile scalpel, surface-sterilized in 0.6% sodium hypochlorite for 15 s, rinsed twice in sterile distilled water, and blotted dry on sterile filter paper. The tissue pieces were then plated onto PDA Petri plates and incubated in the dark at 25 • C for 7 days. Following incubation, hyphal tips from the margins of actively growing isolates were removed with a sterile probe and plated onto one-fourth-strength PDA to generate pure cultures.

Phylogeny
The concatenated alignment included 59 isolates representing 46 taxa. The alignment contained 1999 characters (including gaps) and was composed of three partitions: 852 characters for rpb2, 582 characters for tub2, and 559 characters for ITS. Of those, 1202 (60.13%) characters were constant, 701 were parsimony-informative, and 96 were parsimony-uninformative. For the Bayesian inference, the TIM + F+I + G4 model was selected as optimal for all partitions based on the results of the MrModeltest and IQ-tree tests. The results of ML and BI analyses of concatenated ITS-tub2-rpb2 showed similar results that were used to build a phylogenetic tree (Figure 1). Phylogenetic analyses demonstrated that all studied isolates formed a monophyletic clade within the Trichocladium lineage. The clade is sister to Trichocladium sp. CBS 351.77, T. crispatum, and T. atharcticum but is separated with a high level of support (ML-BS 99%; MP-BS = 100%; PP = 1.00).
results of ML and BI analyses of concatenated ITS-tub2-rpb2 showed similar results that were used to build a phylogenetic tree (Figure 1). Phylogenetic analyses demonstrated that all studied isolates formed a monophyletic clade within the Trichocladium lineage. The clade is sister to Trichocladium sp. CBS 351.77, T. crispatum, and T. atharcticum but is separated with a high level of support (ML-BS 99%; MP-BS = 100%; PP = 1.00). A significant level of similarity in all the studied isolates of T. solani was observed: 1825 (99.4%) characters were constant, with no parsimony-informative sites. No difference was found among ITS sequences whatsoever. Among tub2 sequences, four transitions were determined at sites 31, 59, 62, and 122, and one transversion at site 129. Among rpb2 sequences, all the isolates, apart from VKM F-4915, were identical, and VKM F-4915 had five transitions from G to A in sites 412, 429, 474, 484, and 490, and one deletion of A in site 454. Inside the clade, the best-fit model for Bayesian interference was determined to be JK. The best branching support for the mL tree built according to the model was 40% and 52% within the clade for overall alignment, with several internal branches showing near-zero (<0.0005) support. The obtained data suggest that the T. solani clade is monophyletic.
Among the used genes, both rpb2 and tub2 demonstrated efficiency in the delimitation of T. solani from similar species having 320 and 294 parsimony-informative sites, respectively, and high resolution of the branches. The ITS region, on the contrary, had limited resolution and could not resolve T. solani clade from similar species with a satisfying level of support.

Pathogenicity towards Potato Tubers
All studied isolates showed pathogenicity to potato tubers in tests developing symptoms similar to those found in the tubers from which they were isolated, with darkened olivaceous areas of the infected tissue. There was no statistically significant difference in aggressiveness among isolates with damaged areas reaching 6-12 mm in diameter at 18 • C after 25 days and 15mm in diameter at 25-30 • C after 7 d (Figure 2). In all the experiments, the same cultures as inoculated, checked by morphology and sequencing, were isolated from infected potato tissues, confirming Koch's postulates.
concatenated alignment including the partial rpb2 gene region, partial tub2 gene region, and IT region. The confidence values are indicated at the branches. The scale bar shows the expected num ber of changes per site.
A significant level of similarity in all the studied isolates of T. solani was observed 1825 (99.4%) characters were constant, with no parsimony-informative sites. No differenc was found among ITS sequences whatsoever. Among tub2 sequences, four transition were determined at sites 31, 59, 62, and 122, and one transversion at site 129. Among rpb sequences, all the isolates, apart from VKM F-4915, were identical, and VKM F-4915 had five transitions from G to A in sites 412, 429, 474, 484, and 490, and one deletion of A i site 454. Inside the clade, the best-fit model for Bayesian interference was determined t be JK. The best branching support for the mL tree built according to the model was 40% and 52% within the clade for overall alignment, with several internal branches showin near-zero (<0.0005) support. The obtained data suggest that the T. solani clade is mono phyletic.
Among the used genes, both rpb2 and tub2 demonstrated efficiency in the delimita tion of T. solani from similar species having 320 and 294 parsimony-informative sites, re spectively, and high resolution of the branches. The ITS region, on the contrary, had lim ited resolution and could not resolve T. solani clade from similar species with a satisfyin level of support.

Pathogenicity towards Potato Tubers
All studied isolates showed pathogenicity to potato tubers in tests developing symp toms similar to those found in the tubers from which they were isolated, with darkened olivaceous areas of the infected tissue. There was no statistically significant difference i aggressiveness among isolates with damaged areas reaching 6-12 mm in diameter at 1 °C after 25 days and 15mm in diameter at 25-30 °C after 7 d (Figure 2). In all the experi ments, the same cultures as inoculated, checked by morphology and sequencing, wer isolated from infected potato tissues, confirming Koch's postulates. A supplementary experiment of inoculation of the whole tubers as described in [47] with both isolate VKM F-4915 and Pseudomonas sp. (culture provided "As Is" by the laboratory of molecular analyses of microorganisms, RUDN) caused a synergic effect resulting in the almost total destruction of a tuber's tissues in the same time period of two weeks at 25 C. Etymology: solani (Latin) refers to the first known host plant (Solanum tuberosum L.) from which isolates were obtained.
Ecology: All studied isolates appear to be more or less thermophilic and able to utilize cellulose as a source of carbon. Observed isolates are pathogenic to potato tubers causing yellow rot during storage.   Notes: Descriptions of colonies are given for 7-day-old cultures grown on standard media. However, more saturated shades of yellow can be observed if grown on rich media with abundant nutrients or at a later stage of growth. Regardless of the medium, colonies can become thin floccose to adpressed with age and develop dark olivaceous-green soluble pigment.
Ecology: All studied isolates appear to be more or less thermophilic and able to utilize cellulose as a source of carbon. Observed isolates are pathogenic to potato tubers causing yellow rot during storage. Notes: Descriptions of colonies are given for 7-day-old cultures grown on standard media. However, more saturated shades of yellow can be observed if grown on rich media with abundant nutrients or at a later stage of growth. Regardless of the medium, colonies can become thin floccose to adpressed with age and develop dark olivaceous-green soluble pigment.
Ecology: All studied isolates appear to be more or less thermophilic and able to utilize cellulose as a source of carbon. Observed isolates are pathogenic to potato tubers causing yellow rot during storage.

Relations and Features of the Species
Phylogenetically, Trichocladium solani is shown to be close to T. crispatum (=Chaetomium crispatum) and T. antarcticum (=Thielavia antarctica) and falls into subclade 2 defined in [13] as containing only sexual species that produce ostiolate or non-ostiolate ascomata. The new species appear to have neither a teleomorph nor typical Trichocladium-like conidiation similar to T. amorphum X. Wei Wang & Houbraken, described in [13]. Numerous attempts were undertaken to induce sporulation among all of the studied isolates. All the methods described in the methodical part of the current study: UV-A light for 12 h described causing sporulation in fungi in [41,42]. Heat-shock treatment of 42 • C for 2h, mentioned by [43], or growth on Hutchinson medium [44] with different cellulose substrates as a source of carbon for 6.5 months have not triggered the development of a teleomorph or typical Trichocladiumlike solitary, 1-to 2-celled, obovate, hyaline or dark monoblastic conidia. None have been observed in situ in the source potato tubers. To the best of our knowledge, nonspecific, Acremonium-like phialidic conidia are the only way of sporulation for the species and the absence of a teleomorph appears to be intrinsic. This feature, along with the phylogenetic placing, contradicts the connection suggested in [13] between the morphological features and phylogenetic subclades of the genus.
Trichocladium solani also appears to be closely related to Trichocladium sp. CBS 351.77, although the isolate was clearly placed apart from the species' clade. CBS 351.77, collected by W. Gams in 1946 and identified by S. J. Hughes [48] as an unknown four-spored Chaetomium, has been reviewed in [13]. It was noted that the isolate was sterile, and the authors were unsure if it belonged to any of the species discussed in that study. Trichocladium sp. CBS 351.77 may be one of the close relatives or, perhaps, an ancestor form. However, we too are hesitant to place the isolate inside the T. solani until more data regarding the species' geographical distribution and molecular diversity are collected in further studies.

Ecology
All known isolates of the species are related to potato tubers: the first isolate (VKM F-4903) for this study was obtained from an air sample from the sorting room of the potato storage facility. All other isolates are from infected potato tubers. In our experiments, all the studied isolates were pathogenic towards potato tubers damaging their tissue.
During the growth on the Hutchinson medium with different cellulose substrates, all the studied isolates of T. solani showed the ability to decompose cellulose. Among the sources of cellulose used in the study, the combination of oak and maple shavings appeared to be the most favorable for the isolates followed by ashless pulp, cellophane, and straw. Sulphated (kraft) cellulose and pine wood allowed the least abundant development of the mycelium. The ability to decompose cellulose has been listed as one of key pathogenicity factors for many plant-pathogenic fungi [49]. It is possible that the same enzymes used in the cellulose decomposition play a role in the lithic infection of the tuber cell wall. It is worth pointing out that the ability to utilize cellulose as a source of carbon is not uncommon among the Chaetomiaceae family [12,50]; however, the pathogenicity to living plants has not been usually reported for the group.
Trichocladium solani appears to be highly pathogenic towards potato tubers: the observed source tubers have usually suffered substantial damage for more than 50% of the mass from the infection that is normally accompanied by bacterial and nematode agents ( Figure 6); although, in our experience, only 10-15% of the surface is where the damage is noticeable from the outside view. The infection tends to spread inwards causing extensive cavities inside a tuber. This feature can be attributed to the fact that the studied isolates tend to increase pathogenicity at higher temperatures in the experiments. Under the lab conditions, colder temperature caused a slow development of the infection whereas warmer temperatures noticeably improved the rate at which the tissue was colonized. We hypothesize that in the environment of potato storage, the rot spreads inwards along with bacteria, causing a local temperature rise which is better sustained in the microclimate inside the tubers' cavities. Our current understanding of the infection suggests that it will likely manifest in storages with inefficient cooling conditions or ventilation or when stored tubers are not evenly distributed which leads to the development of clusters with increased local temperature and humidity [51]. A supplementary experiment with co-inoculation with a culture of Pseudomonas sp. hints that bacterial infections of tubers might significantly increase the rate of the rot on potato tubers in storages. bacteria, causing a local temperature rise which is better sustained in the microclimate inside the tubers' cavities. Our current understanding of the infection suggests that it will likely manifest in storages with inefficient cooling conditions or ventilation or when stored tubers are not evenly distributed which leads to the development of clusters with increased local temperature and humidity [51]. A supplementary experiment with co-inoculation with a culture of Pseudomonas sp. hints that bacterial infections of tubers might significantly increase the rate of the rot on potato tubers in storages.

Monitoring of the Infection
The control of any infection starts with monitoring and identification. However, Trichocladium solani might prove challenging to reveal in routine storage surveys: infected potato tubers present symptoms that carry a strong resemblance to potato dry rot caused by fungi of the genus Fusarium [52,53]. The similarity of the two can most prominently be observed when comparing yellow rot symptoms caused by T. solani ( Figure 6) with those of Fusarium dry rot presented by Wharton et al. [54]. In our experience, T. solani infection on tubers in their natural unwashed state in storage can be highly challenging to distinguish from the Fusarium dry rot, even by a trained specialist. Some thoroughly washed tubers in the late stages of the infection can have slight features that can point out the T. solani infection, such as the color of the overgrowing mycelium or shade of the necrosis. The difference is seen only in dissection where the mycelium beneath the necrosis and the infected tissues exhibit a yellowish, greenish, or olivaceous tint (Figure 6), in contrast to the mainly reddish or whitish tints associated with Fusarium infections [52], which, however, may also exhibit yellow tints thanks to the coloration of spores [55]. It is necessary to point out that differences in coloration of the lesions are mostly seen during the late stages of the infection. Therefore, symptomatic differentiation of the infections should not be carried out visually, as a final diagnosis and molecular identification or culture isolation are necessary to determine the etiologic agent of the infection. The new infection has been named "Yellow rot of the potato tubers".
We would like to stress that the identification of the species is impossible by the ITS region, a popular barcode choice among various fungi. The internal transcribed spacer regions are insufficient and show little variability inside the Chaetomiaceae family [5,10,13], with T. asperum and Humicola grisea ITS sequences being identical [5]. Our obtained sequences of T. solani ITS regions also showed high similarity (99.65%, 99.82% and 99.47%, respectively) to T. asperum, T. crispatum and H. grisea. This fact adds to the

Monitoring of the Infection
The control of any infection starts with monitoring and identification. However, Trichocladium solani might prove challenging to reveal in routine storage surveys: infected potato tubers present symptoms that carry a strong resemblance to potato dry rot caused by fungi of the genus Fusarium [52,53]. The similarity of the two can most prominently be observed when comparing yellow rot symptoms caused by T. solani ( Figure 6) with those of Fusarium dry rot presented by Wharton et al. [54]. In our experience, T. solani infection on tubers in their natural unwashed state in storage can be highly challenging to distinguish from the Fusarium dry rot, even by a trained specialist. Some thoroughly washed tubers in the late stages of the infection can have slight features that can point out the T. solani infection, such as the color of the overgrowing mycelium or shade of the necrosis. The difference is seen only in dissection where the mycelium beneath the necrosis and the infected tissues exhibit a yellowish, greenish, or olivaceous tint (Figure 6), in contrast to the mainly reddish or whitish tints associated with Fusarium infections [52], which, however, may also exhibit yellow tints thanks to the coloration of spores [55]. It is necessary to point out that differences in coloration of the lesions are mostly seen during the late stages of the infection. Therefore, symptomatic differentiation of the infections should not be carried out visually, as a final diagnosis and molecular identification or culture isolation are necessary to determine the etiologic agent of the infection. The new infection has been named "Yellow rot of the potato tubers".
We would like to stress that the identification of the species is impossible by the ITS region, a popular barcode choice among various fungi. The internal transcribed spacer regions are insufficient and show little variability inside the Chaetomiaceae family [5,10,13], with T. asperum and Humicola grisea ITS sequences being identical [5]. Our obtained sequences of T. solani ITS regions also showed high similarity (99.65%, 99.82% and 99.47%, respectively) to T. asperum, T. crispatum and H. grisea. This fact adds to the challenge of identifying the yellow rot in tubers when use of highly specific genetic markers is unavailable.
Due to the absence of the more specific sequences of tub2 and rpb2 regions in databases before the work of Wang et al. [13] and the rare usage of molecular sequencing in potato monitoring surveys, it is difficult to determine the world distribution of T. solani and the spread of the yellow rot. However, to the best of our knowledge, no other Trichocladium species is pathogenic to, or closely associated with, potato-related substrates. Accepting the fact that the following observation is a purely hypothetical extrapolation, it can be noted that mentions of supposed Trichocladium can be encountered in some previous literature. Before the work of Wang at al [13], ITS sequences of T. solani were aligning with T. asperum MF782819.1 and H. grisea KU705826.1/T. griseum MN547382.1 via the BLAST algorithm, and after the mentioned paper, with T. crispatum/Chaetomium crispatum MH861360.1. Maintaining reasonable moderation in the conclusions that can be drawn out from this fact, we can observe that 'Trichocladium asperum' is mentioned from sclerotia of Rhizoctonia solani from a potato filed in Denmark in 1979 [56]; from eggplant (Solanum melongena L.) fruits in Saudi Arabia in 1985 [57]; from the soil after three continuous years of potato cultivation in Poland in 2002 [58]; then, again, in Poland in 2004 from the potato plant's rhizosphere [59]; from potato tubers in France in 2010 [60]; and then from potato tubers in storage in Latvia in 2015 [61]. 'Trichocladium sp.' is mentioned on potato tubers in Poland in 1987 [62], and from soil after a long-term period of the cultivation of Solanum tuberosum in Gansu province in China in 2017 [63]. This observation is in no way proving anything about the true geographic range of T. solani on potato tubers in the world. However, it hints that the real distribution of the species is likely not limited to Russia where isolates were initially found. The development of a reliable and specific test system for T. solani and extensive research is required to monitor and control the spread of yellow rot in storages around the world.

Conclusions
Trichocladium solani, the newly described species, is a pathogen of potato tubers. It was described in those from Russia; however, circumstantial evidence suggests a wider distribution of the species is likely to be confirmed in the following studies. The new species is highly pathogenic to potato tubers and causes yellow rot of potato tubers. As a result of this cellulolytic activity, the fungus appears to cause significant damage to the tuber during storage. The pathogen can be tricky to distinguish from a dry rot caused by Fusarium spp. and produces few distinct morphological features: it has no observed teleomorph and, morphologically, is characterized by nonspecific Acremonium-like conidia on single phialides and chains of swollen chlamydospores. Internal transcribed spacer regions (ITS) were found to be insufficient for species delimitation and cannot be used to distinguish the species from other Chaetomiaceae in screening studies. The development of a species-specific test system and extensive potato survey is the next necessary step in the assessment of the world distribution of the pathogen and the development of control measures towards the infection.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/jof8111160/s1, File S1: Concatenated alignment file used for phylogeny analysis; File S2: alignment of rpb2; File S3: alignment of tub2; File S4: ITS alignment.  Data Availability Statement: Newly generated ITS, TUB, and RPB1 sequences are deposited in GenBank under accession numbers specified in Table 1. All alignments are available provided as Supplementary Files.