Rubellin D Production, Symptom Development and Fungal Biomass Accumulation of Three Different Ramularia collo-cygni Strains in Two Barley Cultivars

Ramularia collo-cygni is the causal agent of Ramularia leaf spot (RLS) in barley, representing an increasing threat to farmers globally. The development of RLS symptoms has been linked to fungal biosynthesis of the non-host-specific phytotoxin rubellin D. However, further investigation is necessary to understand the interaction of fungal development and host susceptibility in relation to rubellin D. In this study, three R. collo-cygni strains (SC19, RccPG4 and NZ62) with differing rubellin D in vitro production were artificially inoculated onto leaves of two barley cultivars (Fairing and Planet) known to have differing susceptibility to RLS. Infection development was monitored at three time points (7, 15 and 21 days post-inoculation) by evaluating symptoms, chlorophyll content, fungal biomass accumulation (using quantitative PCR) and secondary metabolite biosynthesis (via LC–MS/MS). Overall, strains SC19 and RccPG4 exhibited higher virulence in both barley cultivars, accompanied by increased rubellin D production. Significant correlations between symptom expression and rubellin D biosynthesis were observed in Fairing, the most susceptible cultivar. No correlations between fungal biomass accumulation and the other variables (i.e., disease symptoms and rubellin D) were detected. The results suggest that rubellin D may act as a virulence factor, contributing to symptom development, particularly in the more susceptible barley cultivars. In addition, R. collo-cygni demonstrated the ability to produce a broad range of secondary metabolites, commonly associated with other pathogenic fungi, which may contribute to pathogenicity and warrant further investigation to better elucidate the infection process.