NON-PARASITIC DISEASES 71 



The diseased condition may supervene upon healthy growth, or 

 vice versa, or the plant may be uniformly affected throughout 

 its vegetative phase. The disease is not so clearly dependent on 

 weather conditions as curly-leaf, but the only severe recorded 

 epidemics of the two affections occurred together in St. Kitts, 

 in 1914. Previous to that time loggerhead was only known to 

 occur in plants raised out of the ordinary season, and since 1914 

 its presence has not been reported. Both affections are indepen- 

 dent of the source of seed. 



To some extent in St. Kitts, Nevis, and much more distinctly, 

 according to specimens and accounts received, in St. Croix, the 

 leaf-curl disease exhibits also the character of the tomosis 

 described by O. F. Cook as occurring on cotton seedlings and 

 young plants in the United States, namely, the fraying of the 

 edges of the leaves and their perforation with small irregular 

 holes. 



The blossom-end rot of tomatoes is an example of a well- 

 marked specific disease which develops in close relation with 

 soil and water factors, is not infectious, and has in its inception no 

 recognisable connection with any parasite. It is marked by the 

 appearance of a water-soaked spot, which soon turns black, near 

 the blossom end of the fruit when the latter is about one-half or 

 two-thirds grown ; the spot increases rapidly in size and finally 

 spreads deep into the tissue of the fruit ; the affected tissue 

 becoming black, hard, and leathery, and the fruit much flattened. 

 The affection is not uncommon in the West Indies. 



The disease may be induced or increased in vigorously growing 

 plants by irregular watering or excessive transpiration, by raising 

 the soil temperature, and by the use of organic and certain 

 chemical manures. In the writer's experience it has been com- 

 pletely arrested by lightly shading the plants, protecting the soil 

 with trash, and regular moderate watering. C. Brooks, from a 

 very thorough study of the disease, arrived at the conclusion that 

 it is probably due to the development of harmful humic and 

 ammonium compounds and an accompanying decrease in the 

 supply of nitrates. 



A good deal of importance is attached in the study of some 

 other non-parasitic diseases, particularly certain obscure affec- 

 tions of Citrus species, to the relation of the plants to the nitrates 

 and ammonium compounds in the soil. The subject is a complex 

 one, involving the activities and interactions under given con- 

 ditions of the son bacteria, and the results obtained, though 

 highly suggestive, are hardly definite or general enough for concise 

 statement. It is a matter of frequent experience, however, of 

 which local examples could be given, that in particular circum- 

 stances the application of rich nitrogenous manures has been 

 followed by serious damage to citrus trees, and this might be 

 accounted for, in relation to the soil flora, on the lines of the 

 theory regarding blossom-end rot quoted above. 



