Pathogenesis of Thielaviopsis basicola on a Susceptible and a Resistant Cultivar of Burley Tobacco

Stages in pathogenesis were examined on cultivars of burley tobaccc that are either susceptible (Burley 21 x Kentucky 10 [B21x10]) or completely resistant (Tennessee 90 [TN90]) to black root rot, caused by Thielaviopsis basicola. The initial interaction of T. basicola hyphae with host tissue was examined microscopically on roots grown in vitro and in soil under greenhouse conditions. T. basicola penetrated root hairs and epidermal cells of both cultivars within 24 h of inoculation. Epidermal cells were the most common sites of penetration, and infection of these cells often was characterized by extensive bell-shaped collars around the penetration hyphae. Collars also were observed in cortical cells during colonization. Amber discoloration of infected cells was apparent on both cultivars within 72 h of inoculation ; however, the reaction of cortical cells of TN90 was very restricted compared to a more diffuse amber region on B21x10. Hyphae advanced from the necrotic regions into asymptomatic cells on B21x10, but hyphae were limited to discolored cells on TN90. Sporulation was prolific in and on roots of B21x10, but was rare on TN90. A computer-driven image analysis program was used to collect quantitative data on lesion development following inoculation of aeroponically grown roots with endoconidia of T. basicola. Lesions developed on both cultivars, but lesion number, lesion size, and secondary inoculum production were severely limited on TN90 compared to B21x10. Because of limited lesion expansion, absence of secondary infections, and continued root growth, TN90 root systems outgrew the effects of the initial inoculation ; whereas, the root systems of B21x10 became severely diseased.