16 



It has been shown that soil temperature has a very decided influ- 

 ence upon the severity of the black root-rot disease of to- 

 bacco {IS). Experiments of a similar nature were made with 

 brown root-rot soil taken from typically diseased soils in the 

 Connecticut Valley (fig. 9). The experiments along this line 

 have not been extensive, but it seems apparent that the optimum 

 temperature for the development of brown root rot lies between 20° 

 and 26° C, and that above 27° the number and extent of the lesions 

 are much reduced, very few, if any, occurring above 31° C. Every- 

 thing considered, brown root rot is favored by higher temperatures 

 than black root rot, and it is doubtful whether sufficiently high soil 

 temperatures to inhibit the disease materially in the field ever occur 

 in the Connecticut Valley. 



Two series of experiments conducted with varying degrees of 

 moisture in brown root-rot soil were not convincing, but indicated 

 that a fairly constant percentage of soil moisture within limits for 

 normal plant growth did not markedly influence the extent of the 

 damage from the disease. Dry weather apparently increases the 

 injurious action of root rot under field conditions, however, since 

 the ratio between functioning roots and the soil moisture may be 

 greatly reduced within a comparatively short time. 



On the other hand, new roots comparatively free from disease 

 may develop in relatively dry soil and will function for some time 

 following rains and permit a partial and often a very marked re- 

 covery of growth, which is not evident in diseased soils remaining 

 continually moist during seasons of excessive rainfall. 



INFLUENCE OF DRYING AND AERATION 



Small quantities of soil have been shipped for several years from 

 a number of different brown root-rot fields in the Connecticut Val- 

 ley to the Wisconsin laboratories. Frequently portions of these soils 

 not used during the winter months immediately following for 

 greenhouse experiments were used the subsequent fall, or one year 

 after having been taken from the field and stored under cover, 

 where they gradually dried out. These stored soils generally seemed 

 to have lost all or a greater part of the ability to produce brown 

 root rot. 



Experiments were consequently undertaken with two different 

 brown root-rot soils, one being from the Helen Crafts farm, 

 Whately, Mass., where rotation experiments were conducted, the 

 other soil of a very sandy type on which a crop failure due to brown 

 root rot occurred the previous season. 



Various methods of drying were resorted to, and the results of 

 different experiments naturally varied with the rate of drying or 

 aeration, depending upon the thickness of the layer of soil used 

 and the atmospheric conditions prevailing. Early experiments 

 showed that soil spread in a relatively thin layer and allowed to 

 dry for two weeks with occasional stirring showed a remarkable de- 

 crease in brown root rot, as measured by plant growth and the num- 

 ber of lesions on the roots. This effect was somewhat more marked 

 in the sandy soil than in the heavier soil, which had greater water- 

 holding capacity. The increase in growth was estimated to be as 

 much as 1,000 per cent in the sandy soil one month after planting. 



