Junes, i9i6 Silver-Scurf of Irish Potato 349 



nation of these lots in June, 191 5, the fungus was found fruiting on both 

 treated and untreated infected tubers, but no infection was found on 

 the untreated clean tubers. 



As the treatments described do not absolutely control silver-scurf and 

 as clean, tubers only escaped infection, it is evident that disease-free seed 

 should be selected in the fall and should be kept from contact with 

 infected tubers in storage. Moreover, in view of the inhibitory effect of 

 very low temperatures on the development of the fungus, the tubers 

 should be stored at the lowest temperature permissible. 



SUMMARY 



A study of silver-scurf of the Irish potato, caused by Spondylocladium 

 atrovirens Harz, shows that, notwithstanding the wide range in spore 

 dimensions, which led certain investigators to believe there might be a 

 large-spore and a small-spore species in this country, there is but one 

 species, as proved by the fact that conidia ranging from 18 to 64/^ were 

 produced by a single spore culture. 



5. atrovirens is negatively heliotropic. This, however, does not mate- 

 rially influence tuber infection in nature. 



Severe drying of the conidia and mycelium in agar culture at room 

 temperature does not kill the fungus. 



S. atrovirens withstands a wide range of temperature. Its growth is 

 inhibited at 2° to 3° C, but it is not killed at — 10°. Its optimum temper- 

 ature is 21° to 27°, maximum 30° C. 



Optimum reaction to media varies with the kind used, neutral to 

 slightly acid reactions being most favorable to the development of the 

 fungus. Five per cent of cane sugar in nutrient agar inhibited sporula- 

 tion. 



The fungus enters the tuber through the lenticels or the epidermal 

 layers between the lenticels. The mycelium invades and disorganizes 

 the epidermal and corky layers, leaving in bad cases only one or two 

 instead of six or more layers, thus apparently accelerating transpiration. 



The disease may be carried from place to place by infected tubers, in 

 which it lives over from one season to another, or to the succeeding crop 

 by the infected tubers which remain in the field over the winter. 



Under favorable moisture and temperature conditions potatoes may 

 become infected throughout the entire storage season. Both old and 

 young tubers are subject to infection. 



Inoculations on living stems, stolons, and roots in the field and labora- 

 tory experiments produced no infection. 



Warm solutions of mercuric chlorid have a more toxic effect on 5. 

 atrovirens than cold solutions. 



