212 DISEASES OF TROPICAL PLANTS CH . 



out of reach of children and away from animals. 

 Because of its action on metals it must be mixed in a 

 wooden vessel. This vessel, and in fact everything 

 with which the solution comes in contact, must be 

 thoroughly cleaned or destroyed. The formalin 

 should be used at the rate of a half-pound (a half- 

 pint) in fifteen gallons of water. The tubers should be 

 soaked as above for two hours, then spread out and 

 allowed to dry. This treatment has some advantages, in 

 that it can be mixed in any kind of a vessel and is not 

 poisonous to handle. However, it is quite irritating to 

 the nose and to the skin. 



Black Scab. This disease is known in Australia, and 

 is due to the fungus Oedomyces leproides, Trub. The 

 young shoots become brown and wrinkled ; the tubers 

 wrinkled and warty. The organism lives in the soil, 

 and is thought to gain entrance to the tubers through 

 the eyes. It also attacks the beets and many other 

 root crops. The crops should be rotated, and only clean 

 potatoes used for seed. 



Fusarium Wilt (Fusarium oxysporum, Schlecht). 

 This disease is also very widespread, but appears to be 

 very much more severe in some places than others. It 

 may appear upon the plants at any age, but the earlier 

 it appears the more severe it will be. The diseased 

 leaves are a much lighter green in colour than the 

 normal leaves, and eventually the plant withers and 

 falls. Examination of the roots shows that many 

 are dead and covered with a white or pinkish mould. 

 Sections of the stem show brownish discoloration in the 

 fibre-vascular bundles, and sections of tubers show dis- 

 colorations just below the surface. 



This disease also appears in the stored potatoes, 

 causing what is known as dry rot, in which the tubers 

 shrivel, beginning at the stem end ; the inside becomes 

 brown, and the tuber light in weight. This, dry rot is 

 frequently accompanied by a secondary soft rot which 

 is due to bacteria. Use clean healthy tubers, and rotate 

 the crops. 



