60 BULLETIN 727, U. S. DEPARTMENT' OF AGRICULTURE. 



dermis was covered with the spray. These results indicate that an 

 actual coating of spray material prevents infection through the upper 

 epidermis, but that infection may readily occur through the unpro- 

 tected lower epidermis. 



In practice, it is impossible to coat the lower surfaces of the leaves, 

 and a spray is therefore only one-half efficient as a protection. Fur- 

 thermore, the rapid growth of runners constantly exposes unpro- 

 tected tissue. 



For the partial protection afforded, growers of melons, field cucum- 

 bers, and seed crops can probably spray with profit. The fruits in 

 these cases are longer exposed to infection and the economic diffi- 

 culties are negligible. But in the cucumber-pickle industry, where 

 the crop is grown in widely scattered small patches, usually among 

 the less prosperous farmers, the practical difficulties in any spraying 

 program become insurmountable. 



SEED DISINFECTION AND CROP ROTATION. 



Since it has been shown that the disease overwinters in the soil, it 

 goes without saying that a rotation of crops should be practiced and 

 that clean soil should be considered a prime requisite. The site chosen 

 should be well removed from any field in which the disease was pres- 

 ent the preceding year. 



Given a clean soil, the problem of disease prevention depends 

 largely upon the validity of the seed-carriage hypothesis previously 

 presented. If this hypothesis is correct, prevention depends upon 

 the development of a harmless, effective, and practicable seed treat- 

 ment or upon the availability of disease-free seed. Regarding the 

 availability of disease-free seed, anthracnose seems to be prevalent 

 in eastern seed farms, but not in Colorado. Although the disease was 

 reported from that State in 1917, Colorado-grown seed should be 

 comparatively free from anthracnose. 



Concerning the effect of seed treatment upon germination, much 

 remains to be done. Injury has resulted from hot water and formal- 

 dehyde. Carsner found that hot water, 52° C. for 10 minutes, was 

 harmless. Field tests in 1917 indicate the safety of all of the treat- 

 ments except that of formaldehyde in Table VII. The 4 per cent 

 formaldehye 1 for 5 minutes caused great damage and the 2 per cent 

 solution for 10 minutes caused rolling of the cotyledons. Mercuric 

 chlorid, 1 to 1,000, for 5 minutes appears to be harmless except that 

 in some cases a very slight retardation after germination can be 

 detected. Copper sulphate is somewhat objectionable because of 

 the bluish stain left on the seed. 



While the 1917 experimental field tests at Madison were inconclu- 

 sive because of the late appearance of disease in the control field, it 



i 40 per cent, solution. 



