¢ 
BOLL WEEVIL CONTROL BY USE OF POISON. 7 
operators to blame the poison for any form of plant disease. Plant 
injury by soluble arsenic ordinarily is termed “burning,” and this 
word probably describes the appearance of the condition better than 
any other. The plants look very much as if they had been burned 
or scalded by some hot application, especially in cases of severe 
injury. The leaves and terminals droop and the young and more 
tender shoots wilt badly. This is followed very shortly by the 
death and drying of the most seriously injured tissue, causing lighter 
colored spots of dead tissue to appear over the leaves. In cases of 
mild injury this frequently involves only a sort of “shot-hole” ef- 
fect over the leaves, but when the injury is severe the entire leaf is 
killed and falls from the plant in a short time. In still more severe 
cases the entire plant may die soon after the application. 
One fact which should be borne in mind is that plant injury by 
soluble arsenic is generally very erratic and depends to a great ex- 
tent upon the weather conditions prevailing at the time of and 
directly following the application. It will be noted in the specifica- 
tions that a maximum limit of 0.75 per cent of water-soluble arsenic 
oxid is recommended. This does not mean that all material con- 
taining more than this amount of water-soluble arsenic will be in- 
jurious to the plants at every application. In reality, applications of 
calcium arsenate containing as high as 3 or 4 per cent of water- 
soluble arsenic frequently will not burn the plants, but it has been 
tound from a large number of tests that any material running over 
the maximum limit established in these specifications will burn the 
plants seriously when certain weather conditions are experienced. 
The most dangerous calcium arsenates are those averaging about 2 
or 3-per cent in water-soluble arsenic, as it is possible to make two or 
three applications of these before any plant injury is experienced. 
The most dangerous conditions possible for plant burning are 
found during showery days when there are alternate showers and 
bright sunshine. On such a day the plants are moist and the at- 
mospheric conditions ideal for making a satisfactory application of 
poison, but it has been found that if the water-soluble arsenic con- 
tent of the poison used is the least too high it will produce very 
severe Injury. | 
Another point which should be noted is that very serious loss from 
plant burning may result from a degree of injury which appears 
very slight at first glance. In many cases the injury is such that 
only a portion of the leaf surface is burned, comparatively little 
leaf shedding is caused, and the plants appear to recover completely 
in a day or two. In reality, however, a very slight degree of leaf 
burning may assume serious proportions because it causes sufficient 
plant disturbance to produce a heavy shedding of fruit. 
