230 L. R. JONES 



under the dry heat of 1916 these same cabbage fields were swept 

 by the yellows disease as if by fire. 



Every phytopathologist can parallel such experiences. Tem- 

 perature and moisture are, of course, the obvious variable fac- 

 tors. If, however, we are inclined to consider the problem 

 simple we need but turn to Livingston's (3) recent attempt to 

 work out the formulae involved in the physiological temperature 

 indices for the study of plant growth. Livingston was thinking 

 primarily of the higher plants only. The pathologist has to 

 consider simultaneously the relation of these variable factors to 

 the two organisms host and para.site. If we were to accept 

 Livingston's conclusions as applicable to parasite as well as 

 host and then try to work out the complex correlation curves we 

 might well despair at the outset, or at least wait for the phys- 

 iologist to blaze the way. Fortunately the terms seem simpler 

 for some of the problems which are of most immediate interest 

 to the phytopathologist. Thus, for at least two groups of seed- 

 hng or root-invading parasites, certain smuts and Fusariums, 

 shght variations in soil temperature at critical periods seem to be 

 the deciding factors in possible parasitism. While problems 

 involving root parasites are in general more difficult to define 

 than those with aerial parts, in this particular type the advan- 

 tages are with the former, since light and transpiration are 

 largely ehminated as variables with roots, and moisture is more 

 easily controllable than with parts exposed to the air. 



Fusarium species. Oilman's (4) observations, at first in 

 Wisconsin fields and later continued in greenhouse experiments 

 with the soil organism Fusarium conglutinans, seem to show con- 

 clusively that its abihty to induce the cabbage yellows disease 

 is conditioned upon a soil temperature of 17° or above, and 

 that it is powerless as a parasite at lower soil temperatures. 



Tisdale (5), working in our greenhouses, has since sho\vn 

 similar hmitations to hold for another Fusarium root disease, 

 flax wilt, {Fusarium lini). Thus, in his experiments using badly 

 infected soil, the flax developed normally when the soil tem- 

 perature was held continuously below 15°C., but if the tem- 

 perature rose for even one day above 16°C., infection occurred 

 and the wilt followed. 



