210 DISEASES OF TROPICAL PLANTS OH. 



twisted, sometimes becoming rosette in character. Small 

 hard knots of the mycelia, known as sclerotia, are 

 frequently formed on the surface of the tubers. The 

 disease is not especially severe, and can be controlled by 

 the method recommended for scab. 



Bacterial Blight. This is a very widely distributed 

 disease, which also attacks the potato and the egg plant. 

 It is very destructive in warm climates. It causes the 

 wilting of a part or the whole of the plant, which soon 

 dries and drops its foliage. It is due to Bacillus 

 solanacearum, E. F. Smith, minute organisms which live 

 in the fibro-vascular bundles of the plant. A section 

 across the stem shows these bundles to be very much 

 discoloured. The disease also attacks the tubers, causing 

 the brown or black rot, which is easily distinguished in 

 all the fibrous parts. It is readily carried by many 

 kinds of insects, especially leaf-cutters, which should 

 be destroyed so far as possible. Nothing but clean, 

 healthy seed should be planted, and if the disease is 

 not abundant the unhealthy plants should be destroyed. 



Black Leg or Potato Stem Rot. This disease is 

 due to Bacillus phytophthorus, Appel, and has proved 

 very destructive in Western Australia. The leaves wilt, 

 turn yellow, and gradually shrivel from below upwards. 

 The stem gradually becomes black and rotten, and the 

 disease penetrates to the new tubers. Since the 

 organism will attack many root crops, and will persist 

 in the soil for some time, it is rather difficult to control. 

 Old diseased plants should be burned ; tuber and root 

 crops should not be cultivated on infested land for 

 at least two years after the disease is discovered ; lime 

 and nitrogenous manures should not be used. 



A bacterial disease which is very similar and may 

 prove the same is very common in the eastern part of 

 North America, where it is also known as black leg. 

 The diseased plants are light green, sometimes yellowish 

 and dwarfed. The branches and leaves grow upward, 

 forming a more or less compact top. At or below the 

 surface of the ground the stems show a black dis- 



