Johnson — Further Observations on Powdery Potato-Scab. 167 



disease, arising from a very different fungus, is frequent, especially in 

 calcareous districts, and is known commonly by the name of the scab, the 

 surface of the potato being covered with pustules, which at length become cup- 

 shaped, and are powdered within with an olive-yellow meal, consisting of the 

 spores of a fungus. This also has been partially investigated by Martins, who 

 has illustrated his observations with some characteristic figures." 



In another article, entitled "On a form of Scab in Potatoes" (King's Cliff, 

 Nov. 1847), Berkeley writes, " There are two very different diseases known 

 commonly under the name of scab." (His article deals mainly with what we 

 still call, without understanding its cause, " common scab.") " The first, of 

 which it is not now my intention to treat, was described and figured by 

 Martius [Die Kartoffel-Epidemie, p. 23, tab. 2, figs. 9-13; tab. 3, figs. 36-38), 

 and is characterized by the presence of an olive-green or brownish pulverulent 

 Hypliomycete {Tuburcinia Scabies, Berk., Journal Royal Horticultural Society, 

 London, 1846, vol. i., p. 33, tab. 4, figs. 30, 31), which gives a very peculiar 

 appearance to the pustules, and to which indeed it is not confined, but 

 occasionally forms a stratum a line or more in thickness beneath the greater 

 portion of the cuticle. A few scattered tubers occur now and then affected 

 by this disease, but it is very rarely so prevalent as to draw much attention. 

 The potato crops, however, suffered greatly from its ravages in the Scilly 

 Islands and in Cornwall during the present summer, where it appeared under 

 a very destructive form. Mature si^ecimens were forwarded to me, with the 

 promise at some future period of a supply of tubers in every stage of the 

 disease. I was, however, disappointed in my hope of being enabled to 

 investigate its nature more closely, possibly because the malady, as Martius 

 reports, is several weeks in going through its phases." Again, in the Annals 

 and Magazine of Natural History (7), under "Notices of British Fungi" by 

 Berkeley and Broome, our scab appears as Tubercinia Scabies, Berk. ; " Sporis 

 globosis cavis liic illic laouuosis olivaceis" (Berk., Journ. Hort. Soc, vol. i., t. 4, 

 figs. 30, 31). Bliizosporium Solani, Rab., No. 900. On potatoes: very common: 

 often confounded with the true potato scab. 



" The spores of this species are very curious ; they are composed of minute 

 cells, forming together a hollow globe with one or more lacunae communicating 

 with the external air. A hollow shell with one or two apertures will give a 

 notion of their form. They are generally attached laterally by a delicate 

 thread." 



This interpretation of the scab is repeated by M. C. Cooke in " Micro- 

 scopic Fungi " (p. 231), where the scab is called " Potato Smut." Plowright 

 includes the scab in the Ustilaginese in his work on " Uredineae and Ustila- 

 ginese," but states that he has never seen the organism, and can find no trace 



of it in his specimens from Cooke's Exsiocati. 



2e2 



