12 BULLETIN 562, U. S. DEPARTMENT OF AGRICULTURE. 



the first three years half of one of the plats would come into tobacco 

 •each year, while the other half would be kept in other crops for an- 

 other series of years. This arrangement served to furnish light as 

 to the number of years of cropping to immune crops that is re- 

 quired to bring the wilt under control. The general arrangement 

 of the various plats and the system of cropping on each are shown 

 in detail in figure 1. Cotton was not originally included in the 

 tests, and the intention was to use rotation plat 1 for other purposes ; 

 but beginning with 1913 half of this plat was cropped to cotton 

 each year and the other half used as a check plat, being cropped to 

 tobacco even 7 year. After it was observed in 1912 that the peanuts 

 on plat C were seriously affected with wilt, a portion of this plat 

 was used in succeeding seasons for making certain tests on the sus- 

 ceptibility to wilt of varieties of tomatoes. The acre plat shown 

 in the figure was used for the tobacco variety tests already dis- 

 cussed. A line of tile cylinders used in the tests with chemicals and 

 disinfectants occupied the eastern edge of plat A. The results of 

 the cropping tests are expressed in percentages, which were arrived 

 at by making counts of the numbers of healthy and diseased plants 

 on each plat at the time of maturity. These results are brought 

 together in Table III. 



Table III. — Results of 5-year cropping and rotation tests for the control of 

 tobacco wilt at Creedmoor, N. C. in 1916. 



Designation of plat. 



Number of plants. 



Health v 



Wilted. 



Percent- 

 age of 

 wilt. 



Plat B (sweet potatoes continuously) 927 



Plat C (peanut? continuously) * 475 



Plat D (corn continuously) 1,146 



Plat E (red clover and mixed grasses continuously) | 1, 106 



Plat F (wheat and cowpeas continuously) < 1, 010 



Rota! ion plat 4-A 871 



Plat 1-B (tobacco continuously) 1G5 



249 



Gi9 

 44 

 57 

 65 

 107 

 717 



21.2 



5< . 7 



■6.1 



4.9 



6 



10.9 



S1.3 



' A portion of plat C was cropped 1o tomatoes in 1912, 1914, and 1915. 



Table III shows that on plat 1-B, cropped to tobacco each year, 

 81 per cent of the plants showed wilt in 191G. Figure % i gives a good 

 idea of the extent of the wilt on this plat. A large proportion 

 of the diseased plants on plat 4-A were found in a poorly drained 

 corner on the northern edge. Barring this corner, there was only 

 about 5 per cent of wilt on this plat, which had been rotated in corn, 

 wheat, cowpeas. grass, and clover for five years. It will be noted 

 also that on plats D, E, and F, which had been cropped to corn, 

 clover and grass, and wheat and cowpeas. respectively, for five years, 

 there was only 4 to 6 per cent of wilt, so that the loss was 



