Soil Temperature as a Factor in the Frenching of Tobacco (nicotiana Tabacum L.).

During a study of the effect of soil temperature on the development of a tobacco mosaic, frenching was observed on the plants at the higher soil temperatures. Since this is apparently the first time, JONES (3), that soil temperature has been recognized as a factor in inducing frenching of tobacco, Nicotiana Tabacum L., it seemed important that the technique be developed as a possible means of discovering the direct cause of this peculiar physiological malady. Tobacco plants grown at the high soil temperature, 350 C., always became frenched, characterized by chlorotic, narrow leaves with wavy leaf margins. WOLF (19) expertly describes the symptoms which were also observed in this investigation: >'. . . the internodes remain short, apical dominance is lost, and an unusually large number of leaves appear, oftentimes two to three hundred in a single plant. Such plants have the appearance of a rosette or witches' broom, and therefore the severe form of frenching may properly be called 'polyphylly'...." WOLF (19) has reviewed the reports and investigations of frenching up to 1935. Apparently the malady is of physiological origin since it has been impossible to isolate an organism or inoculate plants by using virus technique. Among the causes suggested, emphasis is placed upon field conditions, particularly a subsoil of hard-pan nature which prevents root penetration and also results in poor drainage of surface moisture. Frenching has also occurred as an aftermath of heavy rainfall. Investigations conducted with various forms of nitrogen and calcium compounds have given results so inconsistent that lack or excess of these elements forms no basis as a primary cause. Neither does soil reaction offer a good approach to an answer, although beneficial effects have been noted in soils with low pH values and when acids have been applied to soils producing frenched plants. The soil toxin theory has been investigated, but, here again, the experiments led to inconclusive results. When portions of frenched tobacco plants, containing a growing point, were grafted on to healthy tobacco plants the later growth of the graft lost its frenching habit (17, 18). Such experiments not only indicate the noninfectiousness of the causal agent, but also eliminate the possibility of a genetic character being involved. McMuRTREY (5) and SPENCER (11) have demonstrated that symptoms 1 Published as Contribution No. 655 of the Massachusetts Agricultural Experiment Station.