443 



May 27, 1918 Pox of Swcet Potato 



will be seen to spread downward in the direction of the water-fall — 

 that is, from the highest to the lowest point — but seldom in the opposite 

 direction. 



IS POX A STORAGE TROUBLE? 



Elliott (j) States that— 



A secondary infection by swarm spores in small immature " pits, ' ' causing extensive, 

 blister-like elevations in the skin of stored sweet potatoes, has been observed. 



While there seems no doubt of the observation of these blister-like 

 elevations, it seems very probable that the infection did not occur in 

 storage, but late in the field at digging. When these roots were taken 

 in storage, incubation probably was very slow, and because of unfavorable 

 indoor temperature conditions, infection resulted, not in normal spots, 

 but in blisters. In fact, the above supposition is evidently supported 

 by Elliott (4) himself in the following statement: 



On infected roots kept in the laboratory, in a dry chamber, a hitherto undescribed 

 secondary infection was observed [referring here to the blister infection]. 



It seems safe to state that pox is primarily a field trouble, and not a 

 storage trouble. In fact, growers in the infected districts prefer not to 

 sell the infected crop when freshly dug, but to store it over the winter. 

 In the storage house the pox spots dry, and by the time the roots are 

 ready to be shipped, most of them have fallen out. Similar observations 

 are also recorded by Townsend {14) and Wilcox (i_'^). 



OTHER CROPS SUSCEPTIBLE TO POX 



As will be seen presently, pox attacks not only the sweet potato but 

 other hosts as well. In the summer of 1914 an old Virginia grower 

 stated to the writer that he never plants white (Irish) potatoes {Solanum 

 tuberosum) on the same land where sweet potatoes afifected with pox have 

 grown for " the white potato, too, is subject to the same disease." Upon 

 further inquiry it was found that the same practice was observed by 

 most growers there. Bearing this in mind, in 191 5 the writer planted 

 Irish potatoes side by side with sweet potatoes in a field badly infected 

 with pox. Observations were made from time to time by pulling out 

 growing plants. Unmistakable symptoms of the pox were noticed at a 

 very early stage. At harvesting, about 60 per cent of the tubers were 

 affected with pox. The symptoms on the Irish potato were no different 

 from those of the pox on the mature roots of the sweet potato (Pi. 51, 

 C-E). However, the spots on the Irish potato seemed to be more 

 shallow. Potato growers in Virginia maintain that some varieties of 

 Irish potatoes seem to be more resistant to pox than ethers. The 

 Irish Cobbler seems to be the least resistant. This statement is worthy 

 of further investigation. That pox is a serious disease of the Irish 



