Plant Desiccation and Root Rot in Rosemary: Insight into Macrophomina phaseolina, Ceratobasidium sp. and Fusarium falciforme Roles in Co-Infection

Rosmarinus officinalis (rosemary) is one of the most cultivated Lamiaceae species because of its medicinal, aromatic and ornamental properties. In the late summer of 2023, symptoms of desiccation were observed in a rosemary field located in southern Italy and the isolation from necrotic roots revealed three different fungal species: Ceratobasidium sp., Fusarium falciforme and Macrophomina phaseolina. All the isolates were morphologically and molecularly identified and Koch’s postulates assays were performed to establish the pathogenicity and the effect of each fungus on the disease incidence and severity. A dual plate method was used to investigate the interaction between Ceratobasidium sp., F. falciforme and M. phaseolina, mixing them in pairs and all together. Similarly, in vivo inoculations with the pathogens in combination were used to verify the role of each isolate in the interaction with the others and the contribution in plant disease expression. The results allowed to identify M. phaseolina and F. falciforme as causal agents of desiccation and root rot of rosemary, with M. phaseolina being the most aggressive between the two. Conversely, Ceratobasidium sp. behavior was influenced by the interaction with the other pathogens and the plant. Notably, this is the first report of M. phaseolina and F. falciforme on rosemary plants worldwide. This disease, caused by multiple pathogens, highlighted the crucial role of the combination of traditional in vivo assays and the use of molecular methods to achieve an accurate diagnosis.