DISEASES OF LIME AND OTHER CITRUS TREES 213 



the world, and has been reported from time to time as the cause 



of heavy losses. 



It is not a prominent disease in the Lesser Antilles, as the 

 principal citrus tree, the lime, shows little susceptibility to it 

 under local conditions. Apparent instances have been met 

 with on this host in St. Lucia, Dominica, and Grenada, but only 

 under special circumstances. The case is very different with 

 seedling orange trees, which, in Dominica at least, invariably 

 develop this trouble as they begin to pass their prime. Similar 

 experience is recorded in the United States and Cuba. In a 

 district of Barbados which has lost most of its orange trees 

 the old surviving trees examined by the writer were affected 

 in this way. 



In the heavy clay lands of British Guiana the lime is subject 

 to the disease. 



Symptoms. 



Foot-rot proper, as distinguished from other forms of gum- 

 mosis on citrus trees, is confined to the collar and crown roots. 

 It begins in a patch or patches, often in the hollows formed in the 

 neighbourhood of the crown ; there is often an exudation of 

 gum through cracks in the bark, the bark dries up or is rotted 

 by fungi and bacteria, and the wood is eventually left bare and 

 begins to decay. A sour smell usually accompanies the disease. 

 The patch spreads around the base of the stem and for some 

 distance along the main roots and the tree is slowly killed. 

 Usually, as in other bark diseases, a thinning and yellowing of 

 the foliage occurs, and, as the encircling of the stem approaches 

 completion, heavier crops of fruit are set. 



Causation. 



Various fungi have been found associated with this type of 

 disease in different parts of the world, but it is generally regarded 

 as of non-parasitic origin. Recently H. S. Fawcett has suggested 

 the complicity of the fungus Pythiacystis citrophthora in at 

 least part of the cases passing as examples of foot-rot in America. 



The more recent investigations of H. E. Stevens in Florida 

 have shown that the disease there is caused by Phytophthora 

 parasitica Dast [Ph. terrestria Sherb.) and this fungus has also 

 been found in cases of foot-rot in Cuba, Isle of Pines and the 

 Argentine. 



Apart from this question, there is general agreement as to 

 the conditions under which the disease is liable to appear. 

 Heavy or compacted soils, imperfect drainage, too deep 

 or too close planting are recognised as predisposing causes. 

 As noted above, it is also very liable to appear, even under 

 seemingly favourable conditions, on seedling trees of the suscept- 

 ible species after they reach a certain age. It seems probable 



