134 DISEASES OF CROP-PLANTS 



origin in trees grown for shade or as windbreaks, or shrubs grown 

 as marginal hedges. The most susceptible of the trees commonly 

 grown in or about cacao are avocado, breadfruit, and pois-doux 

 {Inga spp.) Mango and the immortelles are sometimes but less 

 frequently concerned. A year or two after these trees have been 

 cut down, after a large root has been cut in digging a drain, 

 or when they have died from natural causes, the fungus is 

 commonly found to be established on their roots in the 

 same way that it occurs on the stumps in new clearings, and 

 the adjacent cacao trees begin to pick it up. In valley 

 cultivations the disease most frequently appears in situations along 

 the lower courses of ravines or on flats where flood water over- 

 flows, a distribution which suggests that the infective material 

 is often water-carried, probably from the upland forest. On 

 hill estates the distribution is usually more irregular. 



The cases sometimes met wiili of cacao trees becoming 

 diseased in the absence of stumps can be accounted for by 

 infection of the surface type. The instances noted have been in 

 the wettest districts, where production of organic matter is at a 

 maximum, and where cloudy days, the depth of shade produced 

 by luxuriant growth, and the frequent saturation of the soil 

 with water, all reduce the rate of its destruction. Where rainfall 

 and shade are not excessive the addition of organic matter to the 

 soil does not keep pace with its decay, and the soil met with on 

 the ordinary cacao estate is usually not particularly rich in humus, 

 nor is there much in the way of leaf-mould on its surface. 



The spread of the disease along closely planted hedges and 

 windbreaks, of which some striking instances have occiurred in 

 St. Lucia, is characterized and probably mainly effected by 

 infestation of the surface soil. The leaves and twigs which 

 accumulate along the base of the windbreak, and decaying, 

 enrich the soil with humus, and the shelter aftorded by the trees 

 and by the vegetation which grows up under their protection, 

 provide conditions especially suitable for the fungus. It creeps 

 along the line like a smouldering fire, killing off the trees and 

 their seedlings, and most of the shrubby and herbaceous vegeta- 

 tion, as it comes to them. Windbreaks of pois-doux and galba 

 {Calophyllum Calaba) and hedges of Hibiscus and Aralia are 

 very susceptible to the disease. 



The Course of the Disease. 



The course of the disease is much the same in old or new 

 plantations. Typically it starts from one or from several scattered 

 centres and extends slowly but persistently in a widening circle. 

 Its present or past existence in a field may always be suspected 

 where irregular groups of trees are missing or are notably of 

 lesser age. 



The fungus spreads at two levels. It works its way along the 

 roots and passes from one root to another in the network that 



