296 BACTERIA IN RELATION TO PLANT DISEASES. 
be destroyed by sprays of kerosene or arsenates. Several successive crops of these ‘‘trap- 
plants” should be made to come on at short intervals. The first one should be planted 
some weeks in advance of the crop which it is desired to grow and the others at intervals 
of afew days. In this way most of these insects may be destroyed, especially if the growers 
of a whole neighborhood will combine. Squash-plants have been recommended as a trap- 
crop for the Diabroticas. There should be an abundance of these trap-plants, so that for 
some weeks they will offer a continuous feeding ground for the beetles. They are especially 
fond of collecting in the freshly opened flowers of the squash. Hand-picking at sunrise when 
these insects are sluggish and when they often congregate in large numbers under the leaves 
or inside of the flowers, should also be practiced systematically. Of course, in case of a 
disease of which insects are common carriers, much may be done to reduce its prevalence 
by systematically removing diseased plants as soon as 
the first signs appear, so that the opportunity for these 
insects to become contaminated, or for the bacilli to 
spread from these plants in any other way, in reduced 
to a minimum. Diseased plants should be pulled and 
burned at once or stored in some safe place and burned 
when dry. The disease is readily carried from one plant 
to another by insects, and this probably explains its 
appearance on fields not previously planted to cucurbits. 
If the disease has prevailed disastrously on any field the 
cultivation of other crops for some years is urgently 
advised. Cucumber-fruits are sometimes attacked and 
occupied by the bacteria, but it is not known whether 
the disease is transmitted to healthy fields through the 
agency of the seed-trade. The presumption is against 
this, because spores have not been found and because in 
some experiments it has been found that this organism 
is easily killed by dry air. It is too much, however, to 
assert absolutely that spores do not exist or that the 
disease can not be carried on seeds. Further studies are 
necessary. The organism does not grow at blood-tem- 
perature, and no harm is likely to ensue from the con- 
sumption of infected fruits. 
The writer tried Bordeaux mixture without success, 
for this disease as it occurs in cucumbers near Wash- 
ington, and Sturgis reported from Connecticut a similar 
Fig. 96.* want of success in melons. Additional trials are advised. 
Experiments by the writer have demonstrated that 
occasionally the disease may be cut out by removing the inoculated leaves, soon after the 
first appearance of the wilt (p. 279). For field use, however, this method is not practical, 
owing to the fact that the bacillus advances down the vessels of the leaf at the rate of an 
inch or two a day and has usually entered the stem, before the farmer discovers and removes 
the wilting leaf. By the time the primary wilt has advanced so as to cover several square 
inches of the leaf-blade only a small proportion of the plants (cucumbers in my experiment) 
can be saved by removal of the affected leaves. 
The disease is not to be feared in hothouses, if its appearance is recognized promptly, 
and if the insect carriers of infection are destroyed by the proper use of hydrocyanic acid 
gas. Otherwise an entire crop might be lost. Of course, diseased plants should be removed 
promptly and burned. 
*Fic. 96.—Three-months old culture of B. tracheiphilus in 10 cc. litmus milk showing delicate colorless crystals 
obtained by wetting sides of tube and allowing fluid to flow back. These crystals appeared in three cultures of this 
organism and not in three check tubes. Only one experiment, however. Photographed June 28, 1905. x2. 
