240 DISEASES OF TROPICAL PLANTS 



CH. 



young the root becomes spherical or oval instead of 

 conical, matures early and rots soon after harvesting. 

 It is very frequently localized in a field and is spread 

 by setting diseased plants, transfer of dirt from place 

 to place, poor irrigation and poor drainage. It can be 

 controlled by proper attention to the above points and 

 by rotation of crops. 



Downy Mildew. This disease has been reported from 

 Jamaica. It attacks the young tubers or heads, and 

 originates and spreads first through the fibro- vascular 

 bundles, and then to the surrounding parenchyma. 

 The fibro- vascular bundles are at first bright yellow, 

 eventually becoming brown or blackish. However, the 

 diseased parts remain dry instead of becoming putrid, 

 as in the case of some other plants when suffering from 

 similar diseases. 



The disease is due to Peronospora trichotoma, 

 Massee, which reproduces by means of both conidia 

 and resting spores. It apparently gains entrance to 

 the host through wounds only, and if the healthy new 

 plants are allowed to dry some time before planting, 

 it will no doubt prove a great protection against this 

 disease. Fields in which the disease is prevalent should 

 not be reset with this crop for two or three years. 



Pliytoplitliora \colocassiae, Rac., is the cause of a 

 leaf and corn disease of the colocassia in India and Java. 



Leaf Spots. The yautia of Porto Rico is affected 

 with two leaf fungi, Pericoria pysnospora and Gloeo- 

 sporium sp. 



It also suffers from a supposed bacterial disease 

 commonly known as " el mal," which attacks the base 

 of the rhizomes, clogs the fibro-vascular bundles, and 

 causes a gradual discoloration and decay, extending 

 over a period of six months before the plants are com- 

 pletely destroyed. All the upper parts of the plants 

 may become attacked, but the tubers rarely suffer, and 

 will germinate, sending up perfectly healthy plants. 



The yautia also suffers from some other diseases, 

 which are of little consequence. 



