High spatio-temporal resolution imaging reveals brown margins associated with lesion restriction in finger millet blast caused by Magnaporthe oryzae

Abstract Finger millet blast, caused by the fungal pathogen Magnaporthe oryzae, is an important disease affecting finger millet, Eleusine coracana , a cereal grain grown across Africa and South Asia. Blast lesions on susceptible finger millet leaves are often associated with a brown margin surrounding a gray lesion center. However, the progression of finger millet blast disease over time and the potential role of the brown margin in lesion development remain unknown. We developed an innovative high-temporal resolution imaging system to document lesion development from 2 to 7 days post-inoculation. Our imaging system revealed that after an asymptomatic period of three days, gray lesions began to develop on the infected finger millet leaf. We discovered that the brown margin formed as partial segments of brown pigmentation separated from the gray lesion center, which then continued to develop to encircle the gray center, preventing further expansion. Cytological analyses revealed that invasive hyphal colonization was retained within the boundary of the brown margin-associated cells, and reactive oxygen species accumulation preceded brown margin formation. Together, these findings indicate that the brown margin is associated with lesion restriction and suggest that even a susceptible host retains the capacity to limit lesion expansion.