Pethybridge — The Verticillium Disease of the Potato. 65 



quite close to the point of attachment of the rhizome, to discolouration of the 

 walls of other cells also. (See fig. 7, Plate III.) This browning of the vascular 

 ring could be seen with the naked eye, extending in some cases for quite a 

 considerable distance towards the rose (distal) end of the tuber, and, with the 

 microscope, it was still to be seen in sections of the vascular tissue made at 

 the extreme rose end. 



Transverse sections of the stalks of affected plants, made both in their 

 over- and underground portions, and examined with the microscope, showed 

 that the wood vessels were more or less thoroughly choked with branched, 

 septate fungus mycelium which, in a rare instance or two, bore single-celled 

 oval conidia within the cavities of the vessels. Similar mycelium was found 

 in abundance in the wood vessels of the leaves, roots, and rhizomes of affected 

 plants. From the rhizomes the mycelium was definitely traced into the 

 wood vessels of the new tubers, and in one case, which will be described in 

 detail later, it was followed by means of hand sections to a distance of four 

 centimetres from the heel end of the tuber. 



When cut portions of the affected plants (including tubers) were kept 

 moist for a few days in covered dishes, the fungus grew out from the wood 

 vessels and formed aerial, verticillately branched conidiophores, on the tip of 

 each of which a glistening, spherical globule, containing a number of conidia, 

 was borne. (See figs. 5 and 6, Plate III.) 



When portions of affected stalks were allowed to remain for about twelve 

 days under such conditions, they began to rot ; and microscopical examina- 

 tion showed that the mycelium had spread from the vessels to the surrounding 

 tissues, was turning black, and assuming a resting condition. (See fig. 4, 

 Plate III.) 



The fungus, which infection experiments have proved to be the cause 

 of the disease, was identified as Verticillium albo-atrum, a species first 

 described by Eeinke and Berthold 1 in 1879, and assigned by them as the 

 cause of a potato disease, which they regarded as Curl (Krauselkrankheit). 



The appearances described above are quite characteristic of the Verti- 

 cillium disease. A study of it, however, over several seasons has shown 

 that its external symptoms are subject to considerable variation in intensity. 

 For instance, the curling or rolling of the leaflets has been found to be not 

 an absolutely constant feature of the disease, and thus the terms " Curl " and 

 " Leaf Poll " are to be avoided in speaking of this disease, the more so since 

 they have been used in the past in a somewhat indiscriminate way, as will be 

 seen from the discussion in a later section of this paper. 



1 Reinke, J., and G. Berthold, Die Zersetzung der Kartoffel durch Pilze. 

 (I)rifcter Abschnitt. Die Krauselkrankheit der Kartoffel.) Berlin, 1879, p. 67. 



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