Insect Pests and Fungoid Diseases. 433 



present in the soil. Seed potatoes lifted before they arc fully grown are 

 seldom infected ; the fungus appears to pass down to the tubers at a late 

 stage in their growth. 



Treatment : Spraying is of no use. As indicated above, lifting tubers 

 for seed before they are mature is probably the best prevention. 



The haulm and~tubers of diseased plants should be collected and burnt. 



POTATO SCAB (Oospora scabies). This fungus is very wide-spread ; 

 it attacks the tubers whilst they are quite small, forming rough scabby 

 patches on the surface, and these increase in size with the growth of the 

 tuber, generally remaining more or less isolated, but sometimes extending 

 to such an extent that when the tuber is full-grown its whole surface is 

 covered with scab. 



Although the market value of the potatoes is depreciated when much 

 scab is present, they are really none the worse for cooking purposes. 



Potato Tubers infected with Scab (reduced to half size). 



Treatment : When land has produced scabbed potatoes no further crop 

 should be grown there for at least three years ; neither should beet, turnips, 

 carrots, and cabbages be planted there, as they also are attacked by the 

 fungus and thus help to perpetuate it. Green manuring is said to have a 

 good effect in clearing the ground of the fungus, whilst lime favours its 

 development and should not be used. Acid manures (superphosphates and 

 dissolved bones) are the most suitable for use on infected ground. On 

 small areas infected by the fungus an attack may be warded off by sprink- 

 ling flowers of sulphur in the trenches before planting the yotatoes. 



Only clean seed should be used when possible, but if the use of scabby 

 seed cannot be avoided it may be disinfected by the following methods : 



(a) For small quantities : 



1. Soak the seed for two hours in a solution of J pint of formalin 

 to 15 gallons of water. 



2. Soak the seed for one and a-half hours in a solution of 1 oz. of 

 corrosive sublimate to 8 or 9 gallons of water. 



3. Roll the seed in sulphur, using 300 Ibs. sulphur to the acre ; any 

 sulphur which does not adhere to the seed is sown in the rows. 



(b) For large quantities : 



Place the seed in bushel crates or shallow slatted trays in an air- 

 tight room. Allow 23 ozs. of permanganate of potash and 3 pints of 



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