10 CIRCULAR 692, U. S. DEPARTMENT OF AGRICULTURE 
ResuLts ON FARMERS FIELDs 
The occurrence of bacterial wilt in spots on many rotated fields suggested that 
the rotations were not equally effective in all areas of the same field. In order to 
study this phase of the problem, maps were made of wilt distribution during 1938 
and 1940. Permanent plots, each 50 by 50 feet and containing about 260 tobacco 
plants, were set up, and wilt counts were made about August l each year. Data 
from one field planted in tobacco in 1938, corn in 1939, and tobacco in 1940 are 
presented in figure 4. The well-defined pattern of wilt distribution was 
100 
HB io3ss 
90 |— 
80 
o 
~ 70| ZZ. 
W 
a | 
~ 60 
WILT SEVERITY 
p 
{e) 
NNN 
~ IAQ MMA AMT 
: Y 
20° V > ] J Y 
PLOT NO. 
Figure 4.—Distribution of wilt in 1938 and 1940 on the same field. 
approximately the same in both years. Similar results were obtained on 
two other fields, one rotated to crabgrass and the other to corn. The entire 
area of each field had been in rotations of various types for 20 or perhaps 30 
years. The rotations, however, had not been uniformly effective. In some areas, 
as shown in plots 1, 2, 5, 6, 9, and 10 of figure 4, considerable control had been 
obtained, but in other areas of the same field, as shown in plots 4, 8, and 12, wilt 
was severe. Therefore the effectiveness of rotations appeared to be dependent on 
favorable interaction with the soil environment, a condition not under positive 
control. 
EFFECT OF SUSCEPTIBLE WEEDS 
Susceptible weeds growing in the fields and on their borders during the rotation 
period have been considered by several investigators (3, 5, 9, and others) a possible 
cause of the erratic results obtained from crop rotation. A study of this phase of 
the problem was conducted on six plots in which the wilt control from rotations of 
weedy corn and weed-free corn was compared. 
The weedy corn was grown on three plots as follows: Four plants, one each of © 
ragweed (Ambrosia artemisiaefolia L.\, horseweed (Hrigeron canadensis L.), 
cocklebur (Xanthium sp.), and jimsonweed (Datura stramonium L.), were trans- 
planted into each 109 linear feet of row and cultivated along with the corn crop. 
Nearly all the jimsonweed and part of the ragweed were killed by the disease each 
vear. In addition to the weeds transplanted into the corn rows, a border of native 
weeds 5 feet wide was left on the 2 sides of each plot. 
The weed-free corn was grown on three plots by thoroughly roguing the weeds 
from the plots and borders at frequent intervals. 
Comparisons of wilt control were made by planting the rotated plots to tobacco 
in pairs, one weedy and one weed-free, during four separate seasons. The data 
are summarized in table 3. 
