A histological comparison of fungal colonization in tomato seedlings susceptible or resistant to Fusarium crown and root rot disease

Root colonization of tomato cultivars susceptible or resistant to Fusarium crown and root rot disease, caused by the pathogen Fusarium oxysporum f.sp. radicis-lycopersici Jarvis & Shoemaker, was studied histologically. In seedlings of susceptible cultivars ('Ohio MR13', 'Bonny Best', and 'Vendor') held at 22 °C, direct penetration of epidermal cells occurred by 24 h after inoculation and colonization of suberized hypodermal cells and adjacent intercellular spaces by 72 h. The cortex was colonized between 72 and 96 h after inoculation and the stele was commonly colonized by 120 to 144 h. Colonization of the cortex and stele was associated with the breakdown of parenchymatous cell walls and middle lamellae near fungal hyphae. In cultivars resistant by a single dominant gene ('CR6', 'Larma', and 'B82-865') colonization was similar to that in susceptible cultivars until 72 h after inoculation. By this time, papillae were abundant within hypodermal cells. Successful colonization of hypodermal sites was associated with the incorporation of phenolic or lignin-like materials and suberin within cell walls of the underlying cortex. These cortical wall modifications were paralleled by the deposition of electron-opaque material into cortical cell walls and middle lamellae and the production of finely granular bands around the peripheries of colonized intercellular spaces. Phenolic-containing structural defensive barriers (i.e., papillae and modified cortical cell walls) appear to be important in limiting fungal colonization in cultivars possessing single dominant gene resistance to this disease.