The crown and root rot disease is a significant threat to strawberry production worldwide (Fang et al., 2011). According to the grower at Sangju, South Korea, the level of the crown and root rot in strawberry greenhouse increase in the late production season. Low pH levels (4.5–6) influence the outbreak of this disease (Fang et al., 2011). Three fungal species were recovered from the deceased strawberry crown and were discerned as F. oxyporum species complex, F. solani species complex, and P. cucumerina based on distinct molecular and morphological characteristics. The pathogenicity assay revealed that the isolates of these fungal species reproduced identical disease symptoms. Therefore, this study concluded F. oxyporum species complex, F. solani species complex, and P. cucumerina are responsible for the crown and root rot of strawberry in Sangju, South Korea. F. oxyporum species complex were the most reported fungal species that cause crown and root rot of strawberry, but this is the first report showing F. solani species complex and P. cucumerina as the causative agent of this disease. Proper diagnosis of this new pathogen is crucial because early identification and disease monitoring are the most important critical steps for implementing any disease management program (Riley et al., 2002).
With the progress in molecular analysis and bioinformatics, the crystal explicit knowledge regarding fungi and their association with the host has increased over the last decade (Chung et al., 2020). The multigene phylogenetic approach uses sophisticated and reliable tools for the classification of cryptic fungal taxa and fungi, including plant pathogen. The filamentous fungal genus Fusarium is one of the most destructive plant pathogen comprised of 300 phylogenetically distinct species distributed among ~ 23 species complex (O’Donnell et al., 2015; Dongzhen et al., 2020). Initially, fungal species within the Fusarium genus were identified based on iconic fusiform multiseptate macroconidia. The morphological and morphological phylogenetic studies revealed that they convergently evolved into different lineages of ascomycetes (O’Donnell et al., 2015). So, the use of molecular markers (DNA sequences) is more reliable for identifying closely related species. In this study, we identified F. oxyporum and F. solani species complex based on phylogenetic inference using concatenated sequences of EF-1 α, and rpb2. The sequences of EF-1 α and rpb2 highly recommend for phylogenetic analysis and BLASTn against curated reference sequences in the FUSARIUM-ID and Fusarium MLST database for identification to assess species-level identification within the genus Fusarium (O’Donnell et al., 2015; Šišić et al., 2018). In this study, these loci showed high potential to distinguish the different Fusarium species complex, including the F. oxyporum and F. solani species complex.
Plectosporium was first described in 1995 as the new genus for the species previously known as Fusarium tabacinum (Cephalosporium tabacinum), the anamorph of Plectosphaerella cucumerina (Palm et al., 1995). Some studies recommend using LSU, ITS, EF1α, and RPB2 sequences data to resolve the taxonomy of the Plectosphaerella species (Giraldo & Crous 2019; Zhang et al., 2019). This study also found that the concatenated sequences of these loci are very informative for identifying P. cucumerina.
The morphological characteristics of identified fungal species in this study were comparable with that of the respective reference species and species complex (Tables 3 and 4). This study found that there are some differences in morphological characteristics; for example, macroconidia of the present isolate of F. oxyporum species complex (SJB 593) were smaller than that of reference isolate (CBS 144134), whereas macroconidia of the present isolate of F. solani species complex (SJB222) were larger than that of reference isolate Yadou7-6 (Table 3). The conidia of P. Cucumerina also isolates are larger than that of reference isolate CBS 131739. The slight morphological differences within the same species are explained by the difference in host, geographical location, and cultural condition. Many studies reported that morphological characteristics fungal species are influenced by the host, geographical location, culture media, and cultural condition (Stewart et al., 2006; Weir et al., 2012; Hassan et al. 2018). This study also found that F. oxyporum species grew well on PDA and V8 and F. solani and P. cucumerina on V8 and CMA. So, V8 could be the best choice to culture these species.
Among the fungal species, Fusarium species exhibited greater mycelial growth in warm temperature (Fig. 8). The adaptability of Fusarium species in warm temperature explain its current geographical distribution. Warm and rainfall are the favorable condition for Fusarium species to infect crops (Doohan et al., 2003). In contrast, the favorable condition for P. cucumerina were high humidity and low temperature (20°C–26°C) (Gilardi et al., 2012).
In general, both Fusarium species (F. oxyporum and F. solani) soil in-habiting species capable of causing disease, including wilt, root rot and crown rot of all economically important crops. In South Korea, F. oxyporum has been reported as the causative agent for wilt of many plant species, including prickly lettuce, larch, and chrysanthemum (Kwon et al., 2013; Hassan et al., 2019; Kim & Choi 2020). In contrast to F. oxyporum, F. solani is not a frequently reported fungal species in South Korea. It has been reported as the causative agent wilt of cucumber coastal hog fennel and sweet potato (Han et al., 2012; Yang et al., 2018; Kwon et al., 2020). To the best of our knowledge, F. solani has not been reported as the causative agent of crown and root rot of strawberry in South Korea. Like Fusarium spp., P. cucumerina also soil inhabiting plant pathogen and has been reported on many crops, including alfalfa, cabbage, potato, tomato, and sunflower (Abad et al., 2000; Li et al., 2017). Though having a wide host range, it was reported only on the wild arrowhead in South Korea (Li et al., 2017). To the best of our knowledge, this is the first report of root and crown rot of strawberry caused by P. cucumerina in South Korea.
Evolving of the new pathogen under existing cultural practices and control methods indicates that it is necessary to adopt new cultural practices and control strategies. Strawberry grower in Sangju use river water for irrigation. Inoculum of this disease may come through contaminated irrigated water. Plant debris of affected plant also a crucial source of inoculum. Plant debris was observed during the survey. This plant debris may be mixed with soil in pot mixture and infect the newly propagated plantlet. Proper disposal of affected plant debris is necessary. For developing chemical strategies to control this disease, a representative isolate of identified fungal taxa in this study needs to be tested for estimation of efficacy of traditional fungicides (benomyl, captan, mefenoxamand thiophanate methyl).
In conclusion, the finding of this study indicates essential information to develop sustainable control measures for crown and root rot of strawberry in South Korea.