To investigate the routes of U. virens infection and analyze the infection effects on rice flower development, we performed semi-thin sectioning of smut balls. The cross- section showed the stigma with two branches were enclosed by hyphae (Figure 4(a)). High magnification showed hyphae could not breach the epidermal cells of the stigma (Figure 4(b)), even the stigma were densely
covered by hyphse. In transverse section of smut balls, mycelia compactly packed the stigma and deformed the papillae. Hyphae entered the intercellular spaces of stig- ma via the thin-walled cells of papillae, they only breach- ed at the sites of the plumose part of the stigma, but
could not reach the vascular bundles (Figure 4(c)).