REPORT OF THE SOCIETY 93 



Symptoms 



In early stages of the disease the plant shows rolled, slightly yellowed foliage. 

 The rolling can be differentiated from that in "Leafroll" because the leaves are 

 not stiff and inclined to brittleness as in the latter disease. If the attack is 

 slight rolling and yellowing of the leaves combined with a dwarfing of the plant 

 may be the only symptoms until late in the season, when such plants die down 

 prematurely. If the invading organism develops more rapidly the lowest leaves 

 yellow and fall and there is a tendency to "rosette-top" quite similar to that in 

 the Rhizoctonia disease. Given optimum conditions for the fungus the 

 vines lose their lower leaves and wilt. In very severe cases a lesion may be 

 obvious just below ground level but, as a rule, it is only on cutting through the 

 stem that one can see the browned condition of the vascular tissues. Some 

 times only one or two haulms in a hill are affected but generally the whole plant 

 shows the symptoms. 



An inspection of the root system proves that root tips and feeding roots are 

 rotted off and larger roots when sectioned are seen to have browned vascular 

 tissues. Lesions occur on the stolons owing to attacks from the outside. The 

 organism can grow from the affected stem or stolon into the vascular tissues of 

 the tuber and here it gives rise to "net-necrosis". The necrosis of the vascular 

 tissues of the tubers varies from slight, when it only penetrates a quarter of an 

 inch from the stem, to severe when it may extend from the stem-end over half 

 way through the tuber. In the latter case tuber rot may occur if moisture and 

 temperature conditions during storage are suitable. In the former such tubers 

 used for seed develop "spindling" sprouts. 



Sources of Infection 



There are two sources of infection: — (1) the soil and (2) diseased seed- 

 tubers. In the first case wilting does not usually occur until late in the season 

 if it occurs at all, and this is the condition generally here. In the second case, 

 either the plants are so spindling that they cannot grow or wilting may .occur 

 earlier in the season when tubers are about half grown. 



Resistant varieties 



Little is yet known of varieties actually resistant but it is important to 

 notice that early potatoes escape the disease. Thus Irish Cobbler tubers are 

 fairly mature by the time Fusariose becomes prevalent while Green Mountain 



tubers are just forming. 



Control 



1. If the soil is known to be infested crop rotation must be practised and if 

 the infestation is severe a long rotation of five or more years is necessary. 



2. Tubers should be selected for soundness. All germinated tubers show- 



