HOW TO INCREASE THE POTATO CROP BY SPRAYING. 



15 



FACTORS INFLUENCING LATE-BLIGHT. 



Tlie origin oj the infection.. — 

 Late-blight does not occur iii 

 the soil. It overwmters iii 

 stored tubers. Potatoes with 

 more or less dry rot 18) 



are planted frequently, and 

 some of them give rise to weak 

 sprouts. The fungus grows up 

 these shoots and produces spores 

 on the above - ground parts, 

 which are carried to adjacent 

 plants and start centers of infec- 

 tion. Such first cases of late- 

 blight often are present in the 

 fields two weeks or more before 

 the disease becomes epidemic. 



In preparing potatoes for 

 planting it is important to re- 

 ject seed potatoes affected with 

 late-blight dry rot, as infected 

 tubers often decay in the 

 ground without germinating, 

 but there is little hope of avoidhig.tho disease by this precaution, 

 smce under favorable weather conditions blight is carried manyiailes 

 in a short time. No method of treating seed potatoes to kill late 

 blight has been found practical )lc. Thorough spraying with Bor- 

 deaux mixture is the only preventive. 



TemiJerature. — Late-blight spreads most rapidly when the daily 

 mean temperature is 72° to 74° F. with abundant moisture. Weather 

 which in the North is designated as "warm and muggy" is therefore 

 favorable to the rapid development of late-blight, whereas the hot sum- 

 mer weather of the Southern 

 and Central States checks it 

 completely. 



Moisture. — In dr}^ weath- 

 er the production of spores 

 is checked, and the disease 

 ceases to spread because 

 the spores can not germi- 

 nate except in the pres- 

 ence of moisture. Dry 

 fohage can not be infected. 

 FIG. i8.-Latc.hiight tnber-rot. It is only wlien drops of 



Fig. 17.— Potato late-blight. 



