Pathogene~is of Thielaviopsis basicola on a Susceptible and h"'Resistant Cultivar of Burley Tobacco

Hood, M. E., and Shew, H. D. 1996. Pathogenesis of Thielaviopsis basi cola on a susceptible and a resistant cultivar of burley tobacco. Phytopa thology 86:38-44. cells of TN90 was very restricted compared to a more diffuse amber region on B2IxIO. Hyphae advanced from the necrotic regions into asymptomatic cells on B2IxIO, but hyphae were limited to discolored cells on TN90. Sporulation was prolific in and on roots of B2IxIO, 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 1: basicola. Lesions developed on both cultivars, but lesion number, lesion size, and secondary inoculum production were severely limited on TN90 compared to B2IxIO. Becavse 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 B2lxlO became severely diseased. Stages in pathogenesis were examined on cultivars of burley tobacco that are either susceptible (Burley 21 x Kentucky 10 [B2IxI0]) or completely resistant (Tennessee 90 [TN90]) to black root rot, caused by Thielaviopsis basicola. The initial interaction of 7: basicola hyphae with host tissue was examined microscopically on roots grown in vitro and in soil under greenhouse conditions. 7: 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 Additional keywords: complete resistance, histopathology, partial resis. tance. all hosts, colonization of root cortical tissue and subsequent sporulation by 7: basicola were reported. Although chemical (6,11,25) and biological controls of black root rot (9,27) have been reported, genetic resistance is the primary means of disease management in burley tobacco production (3,25,26). Two types of genetic resistance are used for control of black root rot: low to moderate partial resistance from N. tabacum and single-gene resistance derived from N. debneyi Domin (3,30). N. debneyi-derived resistance was first implemented in burley tobacco production in the 1950's (3). The resistance of N. debneyi is considered complete (3) and appears to be durable because no isolate of 7: basicola has been identified with the ability to cause disease on tobacco cultivars carrying this gene for resistance (24). In work described herein, interactions of 7: basicola with burley tobacco cultivars that are either susceptible or resistant to black root rot were examined using quantitative and histological methods. Significant differences were observed in stages of pathogenesis on these cultivars, and the role of th~e differ~ces in disease development is discussed. Thielaviopsis basicola (Berk. and Broome) Ferraris (synanamorph Chalara elegans Nag Raj & Kendrick) is a soilborne, plant pathogenic fungus commonly found in cultivated and noncultivated soils (31). 7: basicola parasitizes a wide range of hosts (16) and is reported to have significant saprophytic ability (6). Important agricultural hosts include cotton, beans, carrots, pansies, peanuts, and tobacco. On tobacco (Nicotiana tabacum L.), 7: basicola causes black root rot and is found in all temperate growing regions of the world (22). Major symptoms of black root rot include dark cortical lesions that often result in root pruning, foliar stunting, and significant loss of yield (11). Development of 7: basicola during penetration and colonization of several hosts has been examined histologically. Susceptible tobacco genotypes are penetrated by threadlike hyphae followed by the development of spear-shaped swellings inside the cell (10). Growth of intracellular hyphae in tobacco occurs as characteristic crescent-shaped cells. Similar observations were made on bean (2,18) and citrus (28). Formation of appressoria was reported on cotton (13), carrots (20), peanuts (8), and chicory (19), whereas direct penetration without appressoria was observed on beans (2) and holly (29). Penetration of cotton and chicory occurs through root hairs and root epidermal tissue (8,12,19), whereas Conant (4) reported infection of tobacco roots primarily through wounds. The deposition of papillae onto the inner surface of host cell walls has been associated with 7: basicola penetration of peanuts (8) and red clover (14). Ultrastructural examination of infections on red clover revealed that the host plasma membrane remained intact for more than 24 h following penetration (14). On MATERIALS AND METHODS Host cultivars, pathogen maintenance, and inoculum production. Two cultivars of burley tobacco, Burley 21 x Kentucky 10 (B21xlO) and Tennessee 90 (TN90) (F. W. Rickard Seeds, Winchester, KY), were used in all experiments. Although B21xlO has a low level of partial resistance to black root rot (25,30), in current burley tobacco production B21xl0 is among the most susceptible cultivars planted a;nd suffers significant disease in 7: basicola-infested fields (25). Therefore, B21xlO was considered a susceptible control in this study. In contrast, TN90 has complete, single-gene resistance to black root rot derived from N. debneyi (3,25,30). Corresponding author: H. D. Shew; E-mail address: [email protected] Publication no. P-1996-1120-02R @ 1996 The American Phytopathological Society