(a) An uninfected epidermal cell in the leaf sheath of sorghum. When uninfected cells are treated with a 0.85M solution of potassium nitrate, containing the vital dye Neutral Red, the plasmalemma (P) and tonoplast (T) both contract, and Neutral Red accumulates inside the vacuole (V). Bar = 10µm.

(b) In susceptible cultivars of sorghum infection vesicles (Ve) have formed below appressoria (A) by 42 hours after inoculation. Note that both the plasmalemma (P) and tonoplast (T) have contracted as in the uninfected cell (a), indicating that the cell is still living. Bar = 10µm.

(c) Primary hyphae (PH) then branch out from the infection vesicle (Ve) and colonise adjacent cells. Bar = 10µm.

(d) As the primary hyphae (PH) advance from cell to cell, the previously infected cells gradually lose their viability, start to senesence and eventually die. As a consequence, the most recently colonised cell (1) is intact whilst earlier infected cells are dead (5) or dying (2 & 4). Uninfected cells (3) remain viable with both the plasmalemma (P) and tonoplast (T) able to contract, unlike dying cells (4) where only the tonoplast remains able to contract, and dead cells (5) where neither can. Bar = 10µm.

(e) Approximately 66 hours after inoculation narrow secondary hyphae (SH) start to branch out from primary hyphae (PH) within epidermal cells and radiate into adjacent epidermal and mesophyll cells. Bar = 10µm.

(f) By 90 hours after inoculation secondary hyphae (arrows) are proliferating throughout the leaf sheath. The orange pigment (O) spread throughout the cells is thought to represent the host defense response, although it has little apparent effect on fungal colonisation of tissue. Bar = 10µm.

(g) By 114 hours after inoculation, numerous acervuli (Ac) are visible protruding through the leaf cuticle. These acervuli contain sterile hairs or setae (S) and conidiophores (C) which produce the fungal spores. Bar = 10µm.

(a) In resistant sorghum cultivars, papillae (Pa) form beneath appressoria (A) during penetration of the leaf cuticle. By 36 hours after inoculation, pigmented inclusions (In) have also accumulated around the site of penetration and the papillae become heavily pigmented.

(b) In most cases the fungus is able to penetrate the epidermal cell wall and by 42 hours after inoculation has formed an infection vesicle (Ve) and primary hyphae (PH). However, the infection vesicle and primary hyphae become impregnated with host defense compounds and stained red. The red pigment also accumulate inside the appressoria.

(c) By 66 hours after inoculation the primary hyphae (PH) and the cell wall (CW) are intensely stained by the pigmented compounds. Thesecompounds kill both the cell and fungus inhibiting further fungal development.

(d) In the leaves the host defense response is restricted to infected cells and those immediately surrounding them; equivalent to 3-4 cells. These groups of pigmented cells make up the pigmented spots which can be seen on the leaf with the naked eye.