PlvANT DISEASES 



DISEASES NOT DUE TO PARASITES 

 OR OF UNKNOWN ORIGIN. 



803 - Gummosis in the Citrus Plantations of Florida. — Stevens h. e. in University of 



Florida, A'^ncultuial Expfrmu-nt Station, Kipcrl lor the Fiscal Year cndiifj June 30th, 1914, 

 pp. 57-71. Tallahassee, Fla., 1915. 



Gummosis is common in the citrus plantations of Florida, causing great- 

 er damage year by year, especially in the north. Among citrus trees, C. 

 Aurantium amara alone appears exempt. 



Very little is known as to the origin and cause of this disease. It is 

 certain that its development is influenced by several factors, and although 

 many species of fungi are associated with gummosis, none of them can be 

 regarded as the true pathogenic agent. 



With the object of making a contribution to the study of this interest- 

 ing question, the writer undertook a series of researches and experiments 

 and a very careful investigation into the course of development of the 

 infested areas. vSeveral inoculation trials were made, both with crude 

 diseased material and with pure cultures of Diplodia natalensis and Pho- 

 tnopsis Ciiri. 



The 18 infested areas studied for a period of 13 months were selected 

 so as to represent as closely as possible the different stages of development 

 of the disease. 



Three stages are distinguished : i) the tissues seem impregnated with 

 water over small portions of the bark ; small lesions and a few cracks appear 

 from which a slight amount of gum exudes ; 



2) The secretion of gum increases, the bark tissue hardens and 

 cracks all over ; 



3) Below the diseased part, the cambium forms a new parenchj'nia, 

 and the diseased portions are ultimately completely eliminated, leaving 

 a dry rough scar. 



The first observations were made in May 191 3 and afterwards repeated at 

 intervals of three, ten and thirteen months. 



