196 THE POTATO 



"Badly affected plants will be found to have the 

 root hairs and rootlets rotted away and often the 

 larger roots appear sickly. Cross sections of the 

 main root often appear brownish in the region of 

 the vascular bundles or woody part. Under the 

 microscope thin sections of such roots and of the 

 lower part of the stem will show the delicate fila- 

 ments of the fungus which pass upward through the 

 water-conducting tubes of the plant and eventually 

 clog them to such an extent that the flow of sap is 

 greatly obstructed. This is what causes the wilting 

 and eventual drying of the foliage of the plant. 



"The fungus also passes into the tuber-bearing 

 stems under ground and frequently enters the stem 

 end of the tuber for some distance. In bad cases 

 the stem end of the tuber may be rotted away and 

 the presence of the fungus deeper in is indicated 

 by the browning of the vascular ring shown in* a 

 cross section of the tuber. The fungus may also 

 enter the tuber from the soil through any bruise, 

 crack, or other break in the skin. Attacks of 

 insect larvae upon the tubers are often followed by 

 this disease through the wounds which the ' worms ' 

 produce. 



"Under conditions of plenty of moisture and 

 high temperature, this disease makes its most rapid 

 progress and may reach its culmination at about 

 the time when the tubers are ordinarily half to two 

 thirds grown. When a plant once shows the infec- 

 tion to any marked degree, all further growth 

 ceases. The plants seem to stand still and event- 

 ually wilt down entirely or else struggle along in a 

 dwarfed and sickly condition for some time. 



"A common source of infection in newly planted 

 fields is through the use of tubers for seed that 

 already contain the fungus. Another common 



