434 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[V 



of the disease is now generally known. At present the following 

 main types of injury are recognized for the potato: (1) black 

 speck scab or sclerotial stage, (2) Rhizoctonia scab, (3) Rhizoc- 

 tonia rot, (4) stem lesions and root rot, (5) rosette and leaf 

 roll, and (6) little potato and aerial potato. 



Black speck is a form of the disease most widely distrib- 

 uted and in itself scarcely merits consideration as a " dis- 

 ease' ' at all, since the sclerotia are superficial on the tuber, 

 and it is merely the appearance of the potato which is affected. 

 The sclerotia may lead to other types of disease which are 

 more serious. The black specks show up most clearly when the 

 potatoes are wet and it is only at this time that they present 

 the appearance of being black, for, as indicated later, the nor- 

 mal color of the sclerotia is deep brown. It was this form of 

 the disease which first gave evidence of the wide distribution 

 of the fungus in America (Duggar and Stewart, '01), and it 

 has been shown to exist in practically all potato-producing sec- 

 tions of the United States and Canada. It occurs throughout 

 Europe, especially on the later varieties of potatoes. It is also 

 reported from India, Africa, and Australia, so that it may be 

 assumed to be world-wide in its distribution on this host. It is 

 safe to say that this is the only form of the disease which does 

 not result directly in serious injury and loss to the crop. In the 

 United States, especially from Ohio westward, other forms of 

 the potato disease assume a seriousness nowhere else attained. 

 If all sucli forms of the disease mentioned below occur in the 

 Atlantic states they are of little consequence. They are, more- 

 over, far less frequent in Europe, India, and Australia. 



The Rhizoctonia scab is believed to occur as a result of the 

 penetration of hyphae during the early stages of sclerotial 

 development, and occasionally it may be induced by a late 

 growth of new hyphae from old sclerotia. The writer has had 

 an opportunity of examining only casually this form of the dis- 

 ease. It is one of the types doubtless seen by Kiihn. Accord- 

 ing to McAlpine ('11), when this disease occurs, practically 

 every part of the tuber is affected, no normal skin remaining. 

 In severe cases the scab areas may be thrown into folds or 

 puckers and these rub off easily in the form of "cork dust." 



