BACTERIAL WILT OF CUCURBITS. 



33 



coupled with prolonged temperatures at or above the maximum for 

 growth of the wilt bacteria will reduce the percentage of infection and 

 rate of progress of the disease, and in extreme cases the wilt may be 

 even practically stamped out. However, under ordinary field condi- 

 tions in the Northern and Middle States, the limiting factors appear 

 to be (a) the presence of wilt-carrying beetles and (&) a vigorous, 

 succulent growth condition in the host. 



Certain data relative to the incubation period of the wilt organism 

 have been compiled from our inoculation experiments and field records. 

 Among 295 sets of greenhouse inoculations on cucumber seedlings, 

 in each case using two or more plants 6 to 18 inches high, the great 

 majority showed signs of wilt in 4 to 6 days after needle-prick inocu- 

 lation, wilting of the first leaf in 5 to 8 days, and complete wilting of 



FIRST SIGNS OFWILT 

 WILT OF FIRST LEAF 

 WHOLE PLANT WILTED 



>v?^. 





NUMBER OF DAYS 



30 



40 



=* 



,J< 



Fig. 7.— Diagram showing the progress of wilt in 295 inoculation tests on cucumbers (mostly the Arlington 

 White Spine variety), with two or more plants in each test and in all using 103 different isolations of 

 Bacillus tracheiphilus. 



the plant in 11 to 15 days (fig. 7). Comparatively few of these 

 cucumber plants took more than three weeks for complete wilting, 

 and out of the whole 295 experiments only 6 exhibited any cases of 

 recovery after once showing signs of the disease. Upon reference 

 to the notes it was found that our 5 least virulent isolations (En 15, 

 R 235, R 311-a, R 317, and En 160) had been used for inoculation in 

 these 6 experiments. These 5 isolations were all low in virulence, 

 but, with the exception of R 3 17, they usually caused complete wilting 

 of a plant in the end. 



In 51 inoculation tests on squash seedlings 6 to 12 inches high 

 most of the plants showed incipient wilt in 5 to 8 days, wilting of the 

 first leaf in 6 to 13 days, and complete wilting, where it occurred at 



