CONTROL OF BACTERIAL WILT 9 
TABLE 2.—Wilt on tobacco grown on large plots in 1939, after 3 years of another crop 
; Tobacco Acre yield of tobacco 
Crop grown for 3 years before tobacco plants 
2 secs 5 : ; wilted 
August 1 Weight Value 
4 Percent Pounds Dollars 
BNO IDACCO Metter ence reagan sea ae naneR aa Sat Stale Me re me SL etch geen OSS 1 100 0 0 
ENGST EU CE Sec ceietere vasa ee a Me hehe te eh CRE SSCP at aS | 84 | 298 48. 78 
CARE Of 2 RSIS sO i a en eC 91 | 340 55. 28 
SIWCC GOT at O ees ere eae es i Ce ee aes | 75 473 78. 09 
FUE GIG ONO ne eeeeetene eps ee area aria Panerai ks ane NUE Reenter sey 59 733 120. 51 
(CCS ao Ga ae UL Ree UA | 41 809 129. 52 
1 Approximately 800 plants per plot. 
2 Sudan grass (Sorghum vulgare var. sudanense (Piper) Hitche.) grown in the summer of 1936 and redtop 
(Agrostis alba L.) seeded in the fall of that year. 
Wilt increased about 100 percent on all rotated plots later in the season. 
For example, tobacco grown after the redtop and corn rotations had 59 and 41 
percent wilted plants on August 1, 90 and 74 percent by September 1, and by 
October 1 nearly all the plants were dead on both plots. It was apparent that 
the increased yields resulted not from elimination of the disease but from a delay 
in its development until the greater part of the crop was harvested. 
EXPERIMENT ON SINGLE-ROwW PLots 
The foregoing experiments were conducted on plots protected from reinfesta- 
tion. Marked differences were obtained from crop rotation on unprotected plots. 
In 1936 several varieties of velvetbean were grown in rows 4 feet apart to tesi 
their susceptibility to wilt by natural infection, and a row of tobacco was planted 
in the middle as a control. All the tobacco was killed by wilt, but the velvet- 
beans remained healthy. In the fall of 19386 and the spring of 1987 the entire 
area was plowed and harrowed flat. Tobacco was planted and cultivated in the 
usual way without any effort to prevent the spread of wilt from row to row. 
The differences were striking in the earlier part of the season, as shown in figure 3, 
photographed July 21, 1937. By August 15, however, all plants were badly dis- 
eased. Similar results were obtained with rotations of crabgrass, cowpeas, and 
immune weeds during several different seasons, demonstrating that the effects of 
rotation persist in the presence of maximum opportunity for recontamination. 
aS 
* * 
Freurer 3.—Results with rotation on single-row plots: The middle row was planted 
in tobacco in 1936; rows on the left and right were planted in velvetbeans 
in 1936. Photographed July 21, 1937. 
