444 Journal of Agricultural Research voi. xiii, No. 9 



potato there seems no doubt. Dr. T. F. Manns,^ of the Delaware Station, 

 writes as follows : 



The disease (pox) has proven very severe this year on white potatoes in lower 

 Delaware and Maryland, some specimens being worse than any specimens of scab I 

 believe I have ever seen. 



Elliott (5, 4), too, has recorded the Irish potato as a host to pox. There 

 seems no doubt that the disease on the Irish potato is far more wide- 

 spread than has heretofore been recognized ; it is very probable also that 

 it has been mistaken and confused with other troubles. Morse and 

 Shapovalov (lo) and more recently Ramsey (12) in their work on the 

 disease caused by Rhizoctonia sp., have noticed a pitting disease of the 

 Irish potato that has been attributed to that fungus. No doubt Rhizoc- 

 tonia sp. is abundant in these pits, but from the illustrations given by 

 Morse and Shapovalov {10) it is evident that they were dealing with the 

 pox, and that the trouble caused by species of Rhizoctonia was merely 

 secondary. 



The writer and Elliott (4) have already stated that the pox spots on 

 the Irish potato are of a shallow type; however, under Maine conditions, 

 it is very probable that Rhizoctonia sp. merely enters as a result of the 

 injury caused by Cysiospora batata, and that having once penetrated, it 

 is capable of working in farther, thus deepening the pit. Prof. Ramsey 

 was kind enough to send the writer slides of his so-called "Rhizoctonia 

 pits." In every case these slides were sections of cracked "pits." A 

 careful examination showed a few remaining cysts irregularly scattered 

 in the tissue of the "pit" area. Furthermore, the largest quantity of 

 filaments of Rhizoctonia sp. are found in the center, at each side of the 

 crack of the "pit," a place from which the invading plasmodium mi- 

 grates back to the soil. It seems very probable that the growth of 

 Rhizoctonia sp. in the "pits" is limited by the secretion of a toxin which 

 the Plasmodium of C. batata leaves in the occupied cells before migrating. 



Pox on the Irish potato has so far been found to attack the tubers only, 

 and not the roots and rootlets, as it does with the sweet potato. Infec- 

 tion apparently takes place at a lenticel, as Ramsey (12) also found. 



The turnip (Brassica rapa) is also susceptible to pox. In the summers 

 of 1 91 6 and 191 7 the writer sowed turnip seed in soil infected Avith 

 Cysiospora batata, on which sweet potatoes had been badly diseased. A 

 large percentage of the turnips showed unmistakable pox infection. 

 Here, however, the spots were more superficial than on the Irish potato. 

 It is now suspected that the beet {Beta vulgaris) and tomato (Lycopersicon 

 esculentum) are also subject to the attack of this disease. Further 

 studies on these two hosts are now in progress. 



' Letter dated November 15, 1916. 



