336 DISEASES OF CROP-PLANTS 



scale, as in Antigua and Montserrat, the affection is occasionally 

 troublesome. 



Symptoms. 



The disease may become prevalent in the fields about reaping 

 time, or during storage or transit of the harvested crop. Usually 

 the rot starts at the neck of the onion, and it may be confined 

 to the heart, to one or more scales enclosed between others 

 which remain sound, or the outside may be sound with the whole 

 interior decayed, or vice versa. The organism is unable to pene- 

 trate the epidermis and only passes from one scale to another 

 by way of the base. The presence of the rot may often be de- 

 tected by a reduction in resistance to pressure. 



Conditions of Occurrence. 



Various kinds of injury can induce the appearance of the rot» 

 but the scale of its occurrence in the field is generally determined 

 by the prevalence of wet weather during ripening and by condi- 

 tions as to moisture generally. Good drainage and open clean culti- 

 vation tend to reduce it. Careful harvesting, curing, and packing, 

 and good ventilation of the place of storage lessen the losses sub- 

 sequent to reaping. Experience in Montserrat leads to the con- 

 clusion that not only are onions grown under wet conditions 

 more liable to decay in the field, but that they suffer much more 

 during storage and shipment than the lots which are grown in 

 drier situations. 



Sweet Potato. 



To a reader familiar with the long and formidable list of 

 sweet potato diseases occurring in the United States the absence 

 of recorded diseases of this important vegetable in the West 

 Indies will appear remarkable. 



So far as the storage rots are concerned the explanation is 

 to be found in the fact that these are so much more severe under 

 tropical conditions that storage for any considerable period is not 

 practised. A somewhat similar explanation is probably true in 

 part of field diseases ; under conditions where they become severe 

 the sweet potato is not grown. The chief area of production 

 is the dry and open cane land of Barbados. A third reason may, 

 perhaps, be found in the existence of sweet potato cultivations 

 in America on the margin of the climatic range of the plant. 



White Rust. 



Effects on the leaves ranging from small spots to considerable 

 pocket-like distortions are caused by infestation with the fungus 

 Albugo ipomoecB-panduranos (Schw.) Sw. White, somewhat wax- 

 like swellings appear under the epidermis, and this eventually 



