234 



SCIENTIFIC AGRICULTURE 



March. 1922. 



Diseases of the Potato 



By B. T. Diekson. 

 Professor of Botaiiv. MacHldiiald College. 



{Co)i{inued.) 



Group 5. 



Disease Caused by an Ascomycete. 



Wilt and Stem-Rot or Stalk Disease. 



Sclerotial diseases of the jx-tato have 

 been reported from many countries within 

 recent years. That known as "stalk dis- 

 ease" caused by Sclerotinia sclerotiorum is 

 common in the West of Ireland and in the 

 northern damper parts of England and 

 Scotland. Growers believe that it is the 

 most serious potato disease in these dis- 

 tricts next to Late Blight. A similar di- 

 sease caused by the same fungus attacks 

 tomato, artichoke, sunflower, bean, squash, 

 cucumber, carrot and turnips (Cotton, 

 A.D.) in Great Britain. Bisby has report- 

 ed a sclerotial disease of sunflower from 

 Manitoba which is common also in Quebec 

 and elsewhere. MacAlpine found a scle- 

 rotial disease of potato in Australia, which 

 he attributed to S. sclerotiorum, Carpen- 

 ter one in Hawaii caused by S. Rolfsii, and 

 Pole Evans reports one from South Africa. 

 During the summer of 1921 Mr. 0. W. La- 

 chaine found 10 per cent infection of a 4 

 acre field of potatoes in Restigouche Coun- 

 t}', New Brunswick, and four other fields 

 of 3 or 4 acres had from 1 per cent to 2 

 per cent infection. The photograph (Fig. 

 11) is from Lachaine's material. Happily 

 the disease according to present indications 

 does not appear to be serious on potatoes 

 in this country although more work must 

 be done on it to determine this authorit- 

 atively. 



Symptoms. 



The first signs of the disease are patches 

 of whitish mycelium on the outside of the 

 potato s'em at, or just above, ground level. 

 If the weather is humid profuse mycelial 

 growth occurs with the later development 

 of external sclerotia. At first these are 

 whitish, turning black and finally falling 

 to the ground. At the same time the my- 

 celium gradually penetrates the inner tis- 

 sues reaching to the pith. In the pith ra- 

 pid growth takes place with the formation 

 of internal selerotia. These are well 

 sliown in Fig. 11. If mois* conditions pre- 



vail tlie stem will by this time have wilted 

 and in many cases have fallen over, col- 

 lapsing at the affected part. On the other 

 hand dry weather will tend to check ex- 

 ternal development and only internal scle- 

 rotia 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 

 aU cases there will be a yellowing of the 

 leaves. 



Lachaine informs me that the sj^mptoms 

 were first noticed in New Brunswick on 

 August 24th, 1921, at a point about half 

 an inch above ground on tlie stems. A 

 blackish zone extended from this point 

 upward for 3 to 5 inches giving somewhat 

 the appearance of "black-leg." By rub- 

 bing, the cortical tissues were ea.sily re- 

 moved and on September 16tli (3 weeks 



Fig. 11. — Sclerotical disease of potato. Stems 



opened to expose sclerotia. Cortical tissues 



disintegrated. 



la*erl tlie outer ncci-otic tis.sues were d.ied 

 out while the leaves were yellowed. Above 

 and below the necrotic area the stem tis- 

 sues were still green. On splitting open 

 the affected .stems abundant sclerotia were 

 found in the place of the pith. 



