Figure 4.

Cross-sections of roots from susceptible (YZ—a,b) and resistant (BM13H—c,d) G. arboreum seedlings 10 days after inoculation with BRR spores in sterile culture. (a,b) Abundant spores emerge from the surface of YZ roots (arrowheads in a), the cortex (co) is very heavily colonized, and black fungal hyphae commonly penetrate through the endodermis (en) and colonize the xylem (xy). b = enlarged area outlined in a. (c,d) There is less fungal colonization of BM13H roots and penetration through the endodermis is rare. co = cortex, en = endodermis, xy = xylem poles (2 of 5 indicated) Bar in a for a, c, d = 0.2 mm, bar in c = 50 mm.