230 DISEASES OF TROPICAL PLANTS 



CH. 



and which may eventually cause the death of the plant. 

 In infected areas the pods which come in contact with 

 the soil or with decaying vegetable matter undergo a 

 rot which penetrates their entire thickness. 



Spraying cannot be used successfully in combating 

 this disease. However, by careful selecting of seed, 

 cleaning the old fields, and burning the rubbish, and 

 rotation of crops, the disease can be held in check. 



Cercosporium urticola has been reported as one 

 of the causes of root rot of the beans in Porto Rico. 



PEA 



Powdery Mildew. The powdery mildews are among 

 the most destructive pests of both garden and field. 

 They are widely distributed throughout the world, and 

 under favourable conditions are likely to occur wherever 

 these crops are grown. The stems, leaves, and pods 

 are covered with a more or less dense white mycelium, 

 which produces an abundance of conidia, by which the 

 disease is spread. It can be controlled by the use of 

 Bordeaux mixture. 



Blight. This disease, caused by Ascochytapisi, Lib., 

 is well known from some parts of the world, but so far 

 as the writer knows has not been reported from the 

 tropics. It often attacks the stems of the young plants 

 near the ground, causing more or less circular spots ; 

 and on the pods it causes spots which resemble the 

 spots on the bean which are caused by the anthracnose. 

 The disease is carried from season to season in the seed. 

 It can be controlled by spraying with Bordeaux, careful 

 seed selection, burning of debris from the old fields, and 

 crop rotation. 



Other Diseases. Other diseases which are known to 

 attack the peas are the rust, which is caused by 

 Uromyces pisi, (Pers.) De Bary, the root rot which is 

 caused by Thielavia basicola, (B. & C.) Zopf., and 

 the root and stem rot which is caused by Corticium 



