Symptomatic Evidence for Differential Root Invasion by Fusarium Crown and Root Rot Pathogens Between Common Tomato Lycopersicon esculentum and Its Varieties

The pathogenic isolates (Kin2001a, Kin2001b and Kin2003) of Fusarium oxysporum f. sp. radicis-lycopersici were obtained from hydroponically cultured seedlings of pear tomato (Lycopersicon esculentum var. pyriforme) infected at different times and their pathogenicity examined in an in vitro assay system on cotyledonal seedlings of pear tomato, cherry tomato (L. esculentum var. cerasiforme) and common tomato (L. esculentum). With the in vitro assay, infection and subsequent disease progress could be microscopically observed. Pear and cherry tomatoes suppressed invasion by all isolates at the junctions of epidermal cells along the root, comparable with the resistant cultivars of common tomato. The pathogen entered pear and cherry tomatoes at the tips of lateral roots and tap roots, in contrast to infection of susceptible cultivars of common tomato. In Kin2003-inoculated roots, the top of the lateral rootlets first became discoloured, followed by the cortical parenchyma, central xylem vessel and finally the crown. This dark-brown discolouration expanded rapidly and severe rot developed in the discoloured regions. In contrast, the dark-brown discolouration in Kin2001b-infected roots expanded into the cortical parenchyma cells abutting the originally infected lateral rootlets and at a much slower rate. Kin2001a was in a new group that entered via the cortical cleavage formed by the emergence of lateral rootlets, in addition to the tips of taproots and lateral roots. In this in vitro assay system, the Japanese pathogenic isolates collected from different districts of Japan were characterized and classified by the mode of host invasion. Of 13 isolates, four were placed with Kin2003, six with Kin2001a and three with Kin2001b.