88 



PROTECTION OF PLANTS, 1921-22 



11) is from Lachaine's material. Happily the disease according to present indi- 

 cations does not appear to be serious on potatoes in this country although more 

 work must be done on it to determine this authoritatively. 



Symptoms 



The first signs of the disease are patches of whitish mycehum on the outside" 

 of the potato stem at, or just above, ground level. If the weather is humid pr6- 

 fuse mj^celial growth occurs with the later development of external sclerotia. 

 At first these are whitish, turning black and finally falhng to the ground. At 

 the same time the mycelium gradually penetrates the inner tissues reaching 

 to the pith. In the pith rapid growth takes place with the formation of internal 

 sclerotia. These are well shown in Fig. 11. If moist conditions prevail, the 

 stem will by this time have wilted and in many cases have fallen over, collapsing 

 at the affected part. On the other hand dry weather will tend to check external 

 development and only internal sclerotia may be found. The outer cortical 

 tissues will be killed and discoloration will be prominent on the stem but it will 

 not fall over. Nor may wilting occur but in all cases there will be a yellowing of 

 the leaves. 



Lachaine informs me that the symptoms were first noticed in New Bruns- 

 wick on August 24th, 1921, at a point about half an inch above ground on the 

 stems. A blackish zone extended from this point upward for 3 to 5 inches giving 

 somewhat the appearance of "black-leg." By rubbing, the cortical tissues were 



easily removed and on September 

 16th (3 weeks later) the outer 

 necrotic tissues were dried out 

 while the leaves were j'ellowed. 

 Above and below the necrotic area 

 the stem tissues were still green. 

 On splitting open the affected 

 stems abundant sclerotia were 

 found in the place of the pith. 



Life History of the Organism 



Sclerotia remain dormant in 

 the soil until early summer when 

 they may germinate, giving rise 

 to apothecia bearing ascospores. 

 The ascopores are discharged and 

 infect the older leaves of the stem 

 in the lowest axils. 



The sclerotia maj' also live over 

 winter in the soil and germinate 



Fig. 9 — Sclerotial disease of potato. Stems 

 opened to expose sclerotia. Cortical tissues 

 disintegrated. 



by mycelium which can infect the plants. 



