32 BULLETIN 828, U. S. DEPARTMENT OF AGRICULTURE. 



Reviewing our own work and observations, these assumptions 

 apparently have considerable truth in them, but they do not appear 

 to express the whole truth. Among the enormous number of inocu- 

 lations made during the past four seasons, several common factors 

 have been observed running through them all. In our greenhouse 

 experiments during the winter months very little or no difference in 

 the percentage of infection or the rate of progress of the disease has 

 been observed between plants sprayed with tap water immediately 

 preceding inoculation and other plants of the same age and vigor 

 inoculated when dry. The same infection relations also have held 

 between greenhouse inoculations made in sunlight and those made 

 on cloudy days or under shade. 



On the other hand, striking differences are often shown among 

 plants in varying states of vigor or age. After inoculation, badly 

 stunted or old plants have in general shown a very much lower per- 

 centage of infection and slower progress of the disease than young 

 vigorously growing plants of the same variety. These facts have 

 repeatedly been observed during the progress of our studies, and for 

 this reason great care has been exercised in selecting plants for viru- 

 lence tests. 



From these observations, together with the beetle and wilt curves 

 and the meteorological data obtained in field experiments (figs. 2 to 

 4), it appears that within rather wide limits weather conditions have 

 very little direct effect on the percentage of infection. However, a 

 rapid, sappy growth of the vines favors infection and spread of the 

 disease through the plant; whereas senility in the host, whether due to 

 normal maturity or to unfavorable conditions such as drought or lack 

 of plant food, furnishes conditions unfavorable to infection and spread 

 of the bacteria within the plant. Thus, by inducing rapid succulent 

 growth of the vines rainy weather may indirectly raise the percentage 

 of infection and increase the reproduction of the bacteria inside of the 

 plant. If, then, following such a rainy period the sun suddenly comes 

 out hot, any tendency to wilt will obviously become evident at once. 

 Indeed, under these circumstances, the excessive transpiration from 

 too rapidly growing tissues often causes transitory wilting where no 

 parasitic organisms are present. On the other hand, after a long 

 period of drought the prevalence of wilt often decreases, not directly 

 because of the weather conditions but because the vines, have hard- 

 ened up and no longer favor infection. Furthermore, periods of 

 drought often come at midseason when senility is normally approach- 

 ing, and the weather conditions serve only to hasten and intensify 

 the natural process of ripening. A decrease in number of cucumber 

 beetles between broods often comes simultaneously with these mid- 

 season drought conditions and this again reduces the number of infec- 

 tions at their source. Undoubtedly an extremely dry atmosphere 



