18 



ClBCULAR NO. 127, BUREAU OF PLANT INDUSTRY, 



The sclerotial stage was until rec(Mitly thought to be due to another 

 fungus called Phellomyces sderotiopltorus. Black sclerotia, S to 16 

 microns in dianietcn', are formed more often in the surface layer of 

 cells (fig. 4), although, according to Johnson, they may also be pro- 

 duced in the dee))er layers of the periderm and parenchyma. From 

 this obseryation it would seem that the fungus is ah\o to attack any 

 part of the tuber, a fact tending to show its parasitic nature. 



Fig. 3.— Later stages of silvpr scurf infection in the same variety of potato as shown in figure 2. It is a 

 very common occurrence to find such infections as shewn in the lower right-hand tuber. The ctTect of 

 this disease on the potato is clearly shown in the upper tuber. The surfaces are shriveled, due to the 

 loss of moisture and sloughing off of the protective covering. 



The fungus causing silyer scurf has been described as Spondylo- 

 cladium atrovirens by Appel and Laubert. The spores are reported 

 to be "4-8 (Meist 6-9) Querwanden, 7, 8-11, 9; 36-61, 5, ini Mittel 

 IQ-AQ/j.." Clinton found that the species he studied had smaller 

 spores. The WTiter has also found that the sj)orcs are not nearly as 

 large as re])orted by Appel and Laul^ert. Wheth(n- this difl'ert^nce in 

 the size of the spores is suflicient cause for considering the American 

 species different from the European one is not clear at the present 

 time. According to Clinton it has been suggested by Appel that 

 there may be at least two forms, one haying larger s})ores than the 

 other. A study of this matter is in progress. 



U'ir. 1-7 1 



