62 PROTECTION OF PLANTS, 1915-16 



Leaf Roll. The leaflets and leaves usually roll upwards on their mid- 

 ribs, often forming a nearly tubular shape. The growth is lessened and 

 the leaves are often discoloured, and it will be found when the plant is pulled 

 up that the mother tuber is entirely intact and in a good state of pres- 

 ervation. The symptoms are similar to other diseases as Blackleg, Rhizoc- 

 tonia etc., but the other diseases invariably show the characteristic symp- 

 toms of that disease, whereas the well preserved tuber seems to be character- 

 istic of the Leaf Roll. 



The last mentioned three diseases are only controllable by eradication 

 or removing from the field of all diseased or discoloured plants. There 

 are three important diseases of the tuber, namely. Common Scab, Powdery 

 Scab, Canker or Black Wart. The first two to the uninitiated are somewhat 

 similar, and to the expert hard to diagnose by the naked eye. 



Common Scab. The scabs or lesions are usually shallow and composed 

 of corky material. The scabs are usually large and irregular in shape, 

 and in bad cases the scab becomes one mass and covers the area entirely, 

 making separate scabs indistinguishable. 



Powdery Scab. The scabs are usually small, roundish, the outline 

 being bordered by the upraised outer epidermal layer of the tuber, so that 

 small pits are formed, which at maturity are filled with a brownish, or olive 

 green powder or dust. Each scab, even in a bad cluster, is almost distinct, 

 that is, a point outline of each scab may be traced. 



For the benefit of those who do not know Powdery Scab, a comparison 

 of the two scabs may not be out of place. 



The scabs of Powdery Scab are usually isolated, circular and smaller 

 than Common Scab. The edge of the scab is usually surrounded by an 

 upraised layer of the epidermis of the potato giving the appearance of a 

 crater or cup. The pits are usually deeper and filled with brownish powder 

 while the Common Scab is shallow and filled with corky material. In 

 the early stage Powdery Scab somewhat resembles a half pea and can easily 

 be rubbed away from the tuber. 



Canker. At present not known in Canada or the States although 

 present in Newfoundland. Canker deforms the potato into an irregular, 

 warty, and at first greenish body. In the early stage the eyes first appear 

 grayish, then turn brown and finally black. 



The common scab may be controlled by the treatment of the seed. 

 Powdery Scab and Canker when once introduced are not controllable, 

 except by rotation and not planting on soil that has been infected for at 

 least seven years. 



