WART DISEASE OF THE POTATO. 7 



During the summer the plasmoclium rounds up, forming a smooth, yel- 

 lowish wail about itself. Later the contents of these zoosporangia 

 break up itito numerous zoospores, which escape through a hole in 

 the wall and attack healthy potato tissue. 



"As the tuber ripens the parasite replaces the summer sporangia 

 by resting ones, which carry the disease through the winter and serve 

 to propagate it in the spring * * *. The resting sporangia, 

 30-70 fx in diameter, are very numerous in diseased tubers and are 

 easily recognizable with a pocket lens. Under the microscope the 

 wall is seen to be not smooth, but ridged or angular. These brown 

 ridges or bands form part of a kind of epispore which arises as the 

 sporangium ripens, and seems to be formed f i-om the residual con- 

 tents of the host cell when not also from its cell wall as well * * *. 

 The ^epispore is thus deposited from without as a third layer on the 

 thickening wall of the sporangium. If this more or less artificial 

 epispore is ignored, then one may speak of the spore wall as 

 smooth * * *. As a rule there is only one resting sporangium in 

 a host cell; occasionally there are two." It is exceedingly difficult to 

 germinate these resting spores artificially. Professor Johnson suc- 

 ceeded in bringing about germination b}' placing them in potato juice. 

 He writes: "At last the potato juice, exercising possibly a chemo- 

 tactic influence, gave success; and sporangia with split walls and 

 escaping zoospores were found. These showed the same sluggish 

 movements observed in the sporangia of certain other Ch3'tridians 

 disturbed during their resting period. Each sporangium contains 

 hundreds of more or less pear-shaped uniciliate zoospores. The zoo- 

 spores measure from 1.5 to 2.4 /^ in diameter. The bod}^ is actively 

 anueboid, while the cilium is comparatively passive." (T. Johnson, 

 1909.) 



Infection takes place usually at the "eyes" of the tuber through the 

 zoospores of either the summer or resting sporangia which are found 

 infesting the surrounding soil. It is also believed by Johnson and 

 others that infection may take place through the internal passage of 

 Plasmodia from diseased seed passing through the stolons arising there- 

 from, and so into the newly-formed tubers. 



MEANS OF EXCLUSION. 



At the present time the United States has no legislation that will 

 prevent the importation of such a trouble. No quarantine is main- 

 tained against plant diseases, nor is the Secretary of Agriculture 

 authorized to inspect or reject infected potatoes, seeds, or nursery 

 stock of any description. 



[c:ir. 52] 



