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 a few 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 

 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. 



*Fio. 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. 



Fig. 96.' 



