' SCAB ' 709 



partially consumed ; the starch, however, remains untouched, 

 and the crumbling substance within the tuber so characteristic 

 of the disease consists of starch-grains mixed with the unused 

 residue of the cell-walls and protoplasm of the tissues. 



In many instances the fungus is accompanied by bacteria, and 

 the appearance of the disease is then considerably altered, so 

 much so that the ' dry-rot ' may become a ' wet-rot.' 



In order to prevent the disease from spreading, clamps or 

 ' pies ' should be opened and all infected tubers taken out and 

 destroyed by fire, 



(3) Brown spots, which do not increase on keeping, and which 

 are irregularly distributed in the substance of the otherwise 

 sound tuber, are frequently observable. No parasitic organism 

 is ever present in such spots, and the cause is unknown. 



Ex. 286. — Look over stored potato tubers in winter or early spring and 

 examine any white moulds which may be present upon them. 



Transfer the moulds into water, or into strong alcohol for a few seconds, 

 and then into weaker solutions, as described in Ex. 284. 



Fusarium Solani is commonly met with. 



When found, notice the character of the interior of the tuber, whether hard 

 or soft, wet or dry. 



Transfer with a knife-point Fusarium spots to the cut surfaces and uninjured 

 surfaces of potato tubers. Place the latter on a plate under a bell-jar and 

 keep slightly moist. Examine the growth day by day for a fortnight. 



Ex. 286a. — Cut sections of pieces of the exuberant tissue of Potato-wart 

 disease and make drawings of the sporocysts of Synchytrium endobioticum. 

 Note the colour and thickness of the wall and the spores within. 



(4) ' Scab.' — This term is applied to various irregular forms of 

 rusty rough excrescences on the tubers of the potato. At the 

 points where the ' scabs ' exist there is an abnormal production 

 of cork-tissue which generally commences from the lenticels of 

 the periderm. In certain cases the malady is confined to the 

 surface, while in others it penetrates some distance into the 

 substance of the tuber. 



Rusty scabbed areas on the potato tuber may be produced by 

 various agents. 



(i) One form in which small irregular corky patches are 



