234 DISEASES OF TROPICAL PLANTS 



CH. 



concerning the occurrence of these diseases in tropical 

 countries and their character is such that we have every 

 reason to expect them at any time. 



ONION 



Bermuda Onion Disease. This disease is due to 

 Peronospora Schleideni, De Bary. The first appear- 

 ance of the disease is the formation of a silvery-white 

 powder on the upper surface of the leaves about three or 

 four inches from the base. The disease spreads rapidly 

 and the powder turns a greyish black. Finally the 

 tips turn downward, become dry and withered, and the 

 growth is arrested. It does not attack the bulb, and 

 there is no rot. The air passages of the plants become 

 choked. The fungus reproduces and spreads in the 

 usual manner (see page 39). It is especially abundant 

 in warm damp weather. The remedies are the use of 

 good fertilizers, burning of rubbish, and spraying with 

 Bordeaux. 



Macrosporium parasiticum is a fungus which 

 sometimes follows the attacks of P. Schleideni, but 

 does not attack healthy plants. 



Bacterial Rot of onions in Barbadoes attacks one or 

 more of the inner scales and causes considerable loss. 

 The onions should be kept as dry as possible both while 

 growing and afterwards. Avoid the use of barnyard 

 manure. 



There are other diseases of the onion, especially rots, 

 which have received little or no attention. 



BEET 



Leaf Blight. This disease (Fig. 82) is widely dis- 

 tributed and no doubt occurs wherever beets are 

 grown. The disease is due to Cercospora beticola, 

 Sacc., which causes numerous spots with ashen-grey 

 centres. The leaf becomes dry and black, the disease 



