70 DISEASES OF CROP-PLANTS 



with the virus diseases, that a similar set of symptoms, as for 

 example in the conditions known as gummosis and chlorosis, 

 may arise from different causes, some of which may be parasites 

 and others physical factors. 



The systematic discussion of atmospheric and soil conditions, 

 and the effects their variations produce on plants, is more appro- 

 priate to a treatise on plant ecology than to a handbook of plant 

 diseases. A few of the more definite examples occurring locally 

 will be mentioned as illustrations, and for fuller information as to 

 these the reader is referred to Part II. The sections on cotton 

 and citrus diseases, particularly, may be consulted in this con- 

 nection. 



The red leaf blight of cotton and chlorosis of sugar-cane 

 illustrate the kind of effect that may follow from soil defects. 

 Growing plants of Sea Island cotton may at any stage begin to 

 show a rusty discoloration of the foliage, and the leaves so 

 affected fail prematurely. The symptoms may be slight or 

 severe and in the latter case the development of the plant is 

 permanently checked. The condition is at the present time very 

 prevalent in St. Vincent, where it has been shown by S. C. 

 Harland to be a result of potash deficiency. A yellow leaf 

 blight somewhat similar in its nature sometimes occurs, which 

 renders the plants very susceptible to the attacks of certain 

 fungus leaf-spots. 



Chlorosis of sugar-cane occurs in Antigua, Barbados, and other 

 islands with limestone soils, and persists on a particular spot 

 year after year. According to H. A. Tempany, this is due in the 

 first instance to the interaction of salt subsoil water with lime- 

 stone, producing sodium carbonate, and from recent research 

 in Porto Rico, the effect would appear to be produced by a 

 further reaction which prevents the plant from obtaining iron, 

 an element necessary for the development of chlorophyll. 



The leaf-curl of Sea Island cotton which occasionally appears 

 on a large scale in these islands is in some way intimately con- 

 nected with dull rainy weather and is especially liable to occur 

 in shaded and sheltered situations. The internodes grow out 

 to abnormal length, the leaves are strongly crimped at their 

 margins, and the flower buds are shed while very small. The 

 condition may come on at any stage, and passes off entirely with 

 a change of weather. 



The loggerhead disease of the same plant occurs sporadically 

 in St. Kitts and Nevis, and in July, 1914, appeared on a large 

 scale. The plant assumes the form of a low dense bush with 

 crowded foliage. The leaves are mottled with dark and light 

 green areas, and the veins often develop imperfectly, causing 

 internal puckering of the leaf blade. The flower buds mostly 

 fail as in curly-leaf, while those surviving develop with a crumpled 

 corolla. There is no evidence of infectiousness, and distribution 

 may either be scattered or nearly general throughout a field. 



