Identification and fungicide sensitivity of Microdochium chrysopogonis (Ascomycota, Amphisphaeriaceae), a new species causing tar spot of Chrysopogon zizanioides in southern China

Abstract

Vetiver grass (Chrysopogon zizanioides) has received extensive attention in recent years due to its diverse applications in soil and water conservation, heavy metal remediation, as well as essential oil and phenolic acids extraction. In 2019, the emergence of tar spot disease on C. zizanioides was documented in Zhanjiang, Guangdong Province, China. Initially, the disease manifested as black ascomata embedded within leaf tissue, either scattered or clustered on leaf surfaces. Subsequently, these ascomata became surrounded by fisheye lesions, characterised by brown, elliptical, necrotic haloes, which eventually coalesced, resulting in leaf withering. Koch’s postulates demonstrated that the fungus isolated from these lesions was the causal agent. Microscopic examination showed that the pathogen morphologically belonged to Microdochium. The phylogenetic tree inferred from the combined ITS, LSU, tub2 and rpb2 sequences revealed the three isolates including GDMCC 3.683, LNU-196 and LNU-197 to be a novel species of Microdochium. Combining the results of phylogenetic, pathogenicity and morphological analyses, we propose a new species named M. chrysopogonis as the causal agent of C. zizanioides in southern China. The optimum growth temperature for M. chrysopogonis was determined to be 30 °C. The in vitro fungicide sensitivity of M. chrysopogonis was determined using a mycelial growth assay. Four demethylation-inhibiting (DMI) fungicides, including difenoconazole, flusilazole, propiconazole and tebuconazole and one methyl benzimidazole carbamate (MBC) fungicide, carbendazim, were effective against M. chrysopogonis, with mean 50% effective concentration (EC50) values of 0.077, 0.011, 0.004, 0.024 and 0.007 μg/ml, respectively. These findings provide essential references for the precise diagnosis and effective management of M. chrysopogonis.