In using microorganisms to control wheat sheath blight, most of the reported biocontrol strains were bacteria. Nanjing Agricultural University of China reported the effects of using Bacillus subtilis B3 to control R. cerealis and the optimization scheme of fermentation conditions (Zhang et al., 1995). Jiangsu Academy of Agricultural Sciences of China has been dedicated to the screening of biocontrol bacteria of R. cerealis starting from the 1990s. The academy reported the biocontrol value of nearly 30 strains and performing a brief analysis of the biocontrol mechanism of some strains, of which Paenibacillus polymyxa and Pseudomonas fluorescens were identified (Shi et al., 1996; Lin et al., 2003; Chen et al. 2008). B. subtilis have been studied the most amongst the biocontrol bacteria of R. cerealis. The strain B. subtilis LF58 applied to wheat seeds could effectively reduce the incidence of wheat sheath blight infection under controlled greenhouse conditions Innocenti et al. 2003. Wheat endophytic bacteria B. subtilis T10 could significantly reduce the incidence and severity of R. cerealis by activating peroxidase, polyphenol oxidase and β-1,3-glucanas, which are the defence enzymes in wheat leaves Zhang et al. 2014. Except for B. subtilis, Bacillus cereus was also reported to control R. cerealis. One endophytic fungus B. cereus isolated from wheat could colonize the root system of wheat in a certain time and control the disease by inhibiting the hypha growth of R. cerealis Wang and Li 2005. B. cereus strain B3-7, which had good environmental adaptability, could persistently colonize the wheat roots, effectively reduce the severity of wheat sheath blight and increase the yield of infected wheat under field conditions Huang et al. 2014.
Few reports could be found on the use of fungi, which mainly belong to genus Trichoderma, to control wheat sheath blight. In dual culture experiments, Clonostachys rosea, Trichoderma atroviride, Trichoderma harzianum and Trichoderma longibrachiatum could inhibit the mycelial growth of R. cerealis through matrix competition Innocenti, Roberti, Montanari and Zakrisson 2003. Trichoderma viride LTR-2 is a strain with biocontrol potential to R. cerealis and the β-1,4-glucanase gene from Bacillus megaterium strain Ap25 was recombined into strain LTR-2 chromosome DNA by using REMI technology in 1 d. Compared with the original strain, the recombinant L-10 against R. cerealis showed an increase in the inhibition rate by 27.0% and the control effect of greenhouse increased by 26.7% Li et al. 2013. The mechanism of biocontrol fungi to control pathogenic microorganisms includes matrix competition, mycoparasitism and antibiosis Kusari, Kusari, Spiteller and Kayser 2013. Competition and mycoparasitism mainly depend on the living beneficial fungi and use the spores or hyphae with infectivity to inhibit the growth of pathogenic fungi. This biological fungicide, which uses the living propagators of fungi, is often affected by many natural conditions in the actual agricultural production and the effect is not significant Duke et al. 2010. In the process of antibiosis, antagonistic and pathogenic fungi do not contact directly but inhibit or even kill pathogenic fungi, mainly via the secondary metabolites secreted by antagonists. Moreover, the secondary metabolites of microorganisms could be industrially produced by scale-up fermentation technology. The potential for further development of biological fungicides could be obviously increased if biocontrol strains with antibiosis against pathogenic fungi could be found.
Endophytic fungi are present in every plant through long-term co-evolution with host plants; they not only do not cause plant diseases but also promote plant growth, improve plant stress resistance and produce some secondary metabolites to resist and inhibit the invasion of pathogens Zheng et al. 2016. Therefore, starting from endophytic fungi is an important method to search for biocontrol fungi with antibiosis. In the present study, five strains with antibiosis against R. cerealis were selected from 163 cornel endophytic fungi with biocontrol potential to their host. Amongst them, T. assiutensis strain R-03 had the strongest antibiosis. During the process of culture, this strain grew slowly at a growth rate of only 1/4–1/3 of that of R. cerealis. However, even under such adverse circumstances, a 7 mm-inhibition zone was produced (Fig. 1). By contrast, R. cerealis had no effect on the growth of strain R-03. Meanwhile, when the colony radius of strain R-03 no longer changed over time, a circle of secretion around the edge of its colony was observed. Moreover, with the accumulation of the secretion, the inhibition zone tended to broaden. Therefore, T. assiutensis strain R-03 could continuously produce secondary metabolites with inhibitory effect on R. cerealis.
The genus Talaromyces is a sexual type of Penicillium found widely in terrestrial and marine climates. The reported fungi of Talaromyces are mainly derived from soil, plants, sponges and food Fang and Shi 2016; Halo et al. 2020; Ramos et al. 2015; Yamashita et al. 2019. A large number of studies showed that the fungi of this genus could produce various secondary metabolites, such as tetraenolides, diphenyl ether derivatives, anthraquinones, terpenes, indoles alkaloids, macrolides, organic acids and cyclic peptides Fu et al. 2016. Many of these secondary metabolites exhibit good activities, such as antibacterial, antitumor, insecticidal and enzyme inhibitors, thereby showing good application prospects in food processing, agricultural production, pharmaceutical development and other fields Aït Hamza et al. 2017; Ginting et al. 2013; Li et al. 2011; Ramos, Prata-Sena, Castro-Carvalho, Dethoup, Buttachon, Kijjoa and Rocha 2015; Yilmaz et al. 2016. Meanwhile, the fungi of this genus could also enhance plant resistance to stress and promote plant growth Halo, Al-Yahyai and Al-Sadi 2020; Sahu et al. 2019. Therefore, researchers pay increasing attention to the development and utilization of this fungal resource.
As for T. assiutensis, a strain isolated and identified from olive nursery substrates infested with root-knot nematodes in Morocco could kill all Meloidogyne javanica juveniles during predation tests in vitro, the most efficient nematophagous species against M. javanica compared with other recovered nematophagous fungi Aït Hamza, Lakhtar, Tazi, Moukhli, Fossati-Gaschignard, Miché, Roussos, Ferji, El Mousadik, Mateille and Boubaker 2017. In the investigation on fungi from mangrove of the South China Sea, a fungus T. assiutensis JTY2 was obtained from the leaves of Ceriops tagal. The ethyl acetate extract of a solid rice fermentation of the fungus exhibited antimicrobial and anti-inflammatory activities. Bioassay-guided fractionation of the bioactive extract led to the isolation of four compounds with inhibitory activities against NO production induced by lipopolysaccharide in mouse macrophage RAW 264.7 cells in vitro and one compound with broad spectrum antibacterial activity against six terrestrial pathogenic bacteria Cai et al. 2019. No studies are available on the application of fungus T. assiutensis in the control of fungal diseases at present. Therefore, the T. assiutensis strain R-03 with antibiosis obtained in the present study could be further evaluated and selected for suppressing R. cerealis in pot and field experiments.
In the present study, some of the 12 strains that showed good inhibitory activity against R. cerealis, contained three functional genes, whilst some contained two, only one and even none. This finding indicated no direct correlation between the antifungal activity and the three target genes, which was consistent with the results from previous reports Kampapongsa and Kaewkla 2016; Zhao et al. 2011. The absence of three target genes in the three antibiotic strains belonging to the genus Talaromyces revealed that active metabolites, such as aminoglycoside, may be produced by other biosynthetic pathways Huang et al. 2005; Kampapongsa and Kaewkla 2016. The exploration of biocontrol genes in antagonistic strains is one of the next research objectives.