BACTERIAL WILT OF CUCURBITS. 



31 



besides the many races or pure lines of the organism showing various 

 degrees of relative virulence to any particular host there may be also 

 distinct cucumber and squash strains. 



SOME HOST RELATIONS. 



On the other side of the host-parasite relation the species and 

 varieties of cucurbits themselves differ in susceptibility to the wilt 

 organism as a whole and to its various races. Thus cucumbers con- 

 stitute the most susceptible host species so far tested, and most of 

 the isolations have proved highly virulent to them. Some differences 

 in varietal susceptibility have been observed, but none have shown 

 marked resistance, and apparently little hope of control through 

 resistant varieties is given here. 



At the other extreme are watermelons, the most resistant of our 

 economic cucurbits. No authentic report of natural infection to this 

 host has come to our notice, and our inoculations have given negative 

 results or at most a very slight local wilt immediately around the 

 points of inoculation. 



The squash group stands next to the watermelon in resistance. 

 Unlike the cucumber, most isolations tested on squash have given no 

 infection or only slight local wilting. The few remaining were highly 

 virulent, but slower in developing the disease than when inoculated 

 into cucumber. Much greater differences in varietal resistance were 

 shown "within the squash group than among cucumbers. 



Aside from the differences in resistance inherent in the species and 

 varieties of cucurbits, a further cause of variability in the host reaction 

 to the parasite appears to be directly connected in some way with the 

 growth condition of the host at the time of inoculation. 



It has long been thought by many growers that the spread of wilt 

 and the extent of damage done are in some way connected with 

 weather conditions. In two different papers, W. G. Sackett 1 states 

 that growers seem to think that wilt is worse during wet weather and 

 just after a heavy rain, especially if the sun comes out hot. He says 

 further that if this is true it is probably due to the fact that these con- 

 ditions favor growth of the germs and bring about a more rapid dis- 

 tribution of the bacteria through the plant. 



G. H. Coons 2 thought it probable that hot, dry summers, such as 

 the summer of 1911, might do much to check the spread of the disease 

 by drying out the wounds made by insects before the bacteria could 

 obtain a foothold. He stated further that perhaps the temperature 

 conditions alone are sufficient to check the growth of the disease. 



1 Sackett, W. G. Some bacterial diseases of plants prevalent in Michigan. Mich. Agr. Exp. Sta. Bull. 

 230, p. 217. 1905. 



Some bacterial diseases of plants. Colo. Agr. Exp. Sta. Bui. 138, p. 22-23, 1909. 



' Coons, G. H. Cucumber and muskmelon wilt. In Mich. Farmer, v. 140, no. 1, p. 1-2, illus. 1913. 



