powDERv SCAB OF potatoe;s. 93 



they are formed. These amebse may continuously divide and 

 increase in number as the cell in which they are located increases 

 in size until they occupy the greater part of its area. 



After the contents of the whole cell are exhausted the ameb-^ 

 fuse together into a larger body called the plasmodium. This 

 is the first stage of spore formation. Following this, certain 

 other changes take place untifl finally the plasmodium is con- 

 verted into a loose, spongy, yellowish or brownish body usually 

 spherical to ovoid in shape and varying in size but averaging 

 about 1-500 of an inch in diameter. These are the so-called 

 '■'spore balls" of the organism. They are made up of aggregar.os 

 of many small, spherical spores somewhat less than 1-5000 of 

 an inch in diameter and it is the masses of these spore ba'.ls 

 which constitute the powdery substance found in the pustules 

 on the diseased potatoes. 



Effect of the Disease upon the Host. 



The early stages of powdery scab on young tubers appear 

 in the form of small pimples with a slight discoloration of the 

 surface. When cut open the infected areas appear purplish.'^ 

 These minute pustules may occur in patches or scattered over 

 the surface of the tubers. As they enlarge they become raised 

 and break through the epidermis which stands up and curls 

 back around the spots in a characteristic manner. 



If the surfaces of the diseased potatoes have not been ex- 

 posed to too much friction, specimens may be collected fre- 

 quently in the storehouses in the winter which show distinct, 

 raised, blister-like, dark coilored spots, usnally not more than 

 one-sixteenth to one-eighth inch in diameter, in no way resem- 

 bling the common type of scab. These are shown indistinctly 

 on the surface' of the apparently decayed tuber illustrated 

 in Figure 47. If the top of one of these blisters or pus- 

 tules is carefully removed by means of a needle or pen- 

 knife the interior will be found to be filled with dead tissue 

 and a dark colored or brownish powder, frequently having 

 a slight olive tinge. Usually, however, when affected tubers 

 come under observation they have been subjected to more or 

 less friction which has not only removed the top of the pustule 

 but most of the powder as well, leaving rather flat and slightly 

 raised spots. After one acquires a slight familiarity with the 



* O'sbom I. c. 328. 



