922 Rural School Leaflet 



decayed spot is produced at this point. If the spores of the fungus 

 come in contact with the potatoes at digging time, subsequent infection 

 will take place. When the vines are infected it is wise to wait until after 

 they are entirely dead before digging. Infection before or at the time of 

 digging is responsible for much of the rot that occurs in cellars later. 



/ 



A tuber showing the dry rot caused by the jutigus that produced the downy 



mildew of the leaves 



Weather conditions largely determine whether this disease will be 

 destructive in any year. Common observation has shown this to be so 

 true that many people believe that rainy weather during the summer is 

 the cause of the disease. You can see, however, that the disease would 

 not occur in rainy seasons if the fungus were absent; nor would it be 

 destructive should the season be dry, even though the fungus were present. 

 Rainy periods during the growing season are favorable for the develop- 

 ment of parasitic fungi, as for higher plants, and it is during such seasons 

 that fungous diseases are most common. 



Control. — From our study of the life history of the parasite it is c\'idcnt 

 that there are two methods open for the control of this disease; namely, 

 to keep the fungus causing it from getting a start, and to protect healthy 

 vines from infection. Since the disease starts from affected tubers, it 

 would seem that we might treat such tubers in some way so that the myce- 

 lium of the fungus would be killed, or we might exclude those tubers alto- 

 gether. Treatment of seed tubers to destroy this fungus has not been 

 very successful, because a method severe enough to kill the mycelium has 



