RECENT INVESTIGATIONS ON TOBACCO ROOT ROT IN CANADA

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Abstract
These investigations involved the direct microscopic examination of root systems of tobacco plants showing typical macroscopic evidence of infection by Thielaviopsis basicola (Berk.) Ferraris, supplemented by isolations from both diseased and healthy roots and infection experiments with the isolates obtained. Intensive microscopic examination of lesions on approximately 1600 roots affected with typical black root revealed the presence of many different organisms which were found to occur singly, in various combinations with one another and more especially in frequent association with T. basicola. In addition to T. basicola the organisms observed included the following: the so-called phycomycetous type of "mycorrhizal" fungus, representatives of the genus Rhizoctonia, including R. Solani as well as several endophytic forms of the type familiar in orchids, different members of the genus Pythium, and nematodes. In general, members of the Fungi Imperfecti appeared to be conspicuously absent. Microscopic evidence of parasitism on the part of each of the above-mentioned organisms was revealed by necrosis involving single cells, groups of cells or entire rootlets. The phycomycetous "mycorrhizal" fungus resembling in many respects the fungus reported as being almost universally present in the roots of strawberry, immediately upon penetration produces characteristic coils of mycelium in the outer cortical cells, ramifies deeper, and spreads producing first arbuscules and then vesicles in the deeper cortical tissues. Previous investigators (11, 15, 20, 24) have reported this or a closely related form of the fungus only in the roots of host plants. In the present paper it is reported for the first time completely invading the stems and leaves of moss plants, also the thallus of liverworts found growing in muck soil obtained from tobacco seedbeds. Daily microscopic examination of tobacco seedlings developing in muck known to contain the phycomycetous "mycorrhizal" fungus revealed the presence of the latter organism in 30% of the roots as early as five days after germination and in all of them 10 days after germination.Isolation from 206 typically diseased roots has consistently yielded T. basicola as well as representatives of 21 genera of fungi, also bacteria and nematodes. The fungi isolated most frequently in association with T. basicola included representatives of the genera Pythium, Rhizoctonia and Fusarium. Preliminary infection experiments have demonstrated that one Rhizoctonia of the "Solani" type and three endophytes from the same genus, four forms of Pythium as well as T. basicola possess parasitic capability in the roots of tobacco, whereas two other forms of Pythium, three of Rhizoctonia and seven distinct members of the Fungi Imperfecti showed no capability of parasitism of the same host under identical conditions.

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