530 Journal of Agricultural Research voi.xx.No. ? 



VARIETAL RESISTANCE TO WILT 



The early infection experiments indicated that a difference in varietal 

 resistance to the Fusarium-wilt probably existed in tobacco, but facilities 

 for carrying out varietal tests under field conditions were not readily 

 obtainable. It was therefore decided that preliminary tests would be 

 carried out on a small scale, using artificially inoculated soil in green- 

 house "flats" (boxes 16 inches by 24 inches and 3 inches deep). Twelve 

 to 14 of these flats were filled with greenhouse soil and sterilized at ioo° C. 

 for two hours. When cooled, each flat was inoculated by mixing into 

 it a sand-cornmeal culture previously referred to, after which the soil in 

 all the' flats was dumped together and again thoroughly mixed to obtain 

 uniform infestation, and the flats were again filled. Twenty plants of 

 each variety used were then transplanted into each flat from the sterilized 

 soil in which they had been grown. Three series of tests were carried out, 

 two out of doors in the summers of 1918 and 1919 and one in the green- 

 house in November, 191 8. In the first two tests no special attempt at 

 artificial wounding was made. In the last series the plants were wounded 

 by pinching off two basal leaves from each plant. Relatively higher in- 

 fection was obtained in this manner. The varieties tested represent 

 practically all the types grown commercially in the United States, and a 

 few others, including two other species, Nicotiana glauca and N. rustica, 

 and in one instance also an Fj of a cross between a White Burley resistant 

 to Thielavia rootrot and Fusarium-wilt and one susceptible to these 

 diseases. The experiments were terminated about one month after 

 transplanting. In taking notes on the results it was found convenient 

 to grade the individual plants into one of four classes: 1, dead; 2, badly 

 diseased; 3, slightly diseased; 4, healthy. 



If a plant was completely wilted and dried it was classed as dead. 

 All remaining plants showing any exterior symptoms of disease were 

 classed as badly diseased. The remainder of the plants were then cut 

 off close to the root system, slit longitudinally, and examined for dis- 

 colored vascular systems. If any discoloration occurred attributable 

 to infection, the plant was listed as slightly diseased, and if none occurred 

 it was placed in the healthy class. In this manner the classification in- 

 cluded the important conditions and yet was not wholly arbitrary. 

 The results of the three tests are shown in Table I. In order to average 

 these data and to arrive at a fair average figure for relative resistance 

 expressed on the percentage basis, a more or less arbitrary formula 

 was established. This method may be briefly described as follows: 

 If a plant remained healthy it was credited with three points; if slightly 

 diseased, 2 points; if badly diseased, 1 point; and if dead it was rated at 

 zero. Twenty seedlings in one flat all healthy would be credited with 

 60 points (20X3), which is the maximum given and corresponds to 100 

 per cent resistance. Twenty seedlings in one flat all dead would receive 



