i8o DISEASES OF CROP-PLANTS 



scattered red dots in otherwise sound living tissue, which mark 

 the upper extension of the red zone in the stem and the beginning 

 of infestation in the leaf-stalk, are initial nests containing usually 

 a few adult worms together with larva and eggs. The red zone 

 in the stem is in its every part simply a vast infestation with 

 myriads of active worms. 



In sections of the tender white meristem of the upper ex- 

 tremities of the stem and of the embryonic leaves the worms may 

 be seen in large numbers threaded along between the cells like 

 fungus hyphas or lying coiled up in the larger intercellular spaces. 

 In this kind of tissue they appear to be able to travel in any 

 direction and are by no means confined to the discoloured spots 

 which mark their breeding places. 



In the roots the worms are present in numbers which have 

 been estimated by N. A. Cobb to exceed 20,000 to the linear foot. 

 These are adult and larval stages, eggs having been rarely found 

 in the roots. On the other hand abundant reproduction takes 

 place in the red zone of the stem, being apparently most active 

 in its upper sections. 



The infestation is in all cases confined to the ground tissue, 

 the vascular bundles remaining to all appearance unaffected 

 in any way. Neither the species concerned in this disease, nor 

 any remotely comparable infestation, has been found in careful 

 examination made of trees failing or dead from other causes. 



Infection experiments, carried out by the writer in Grenada 

 (with the help of J. C. Moore) and later in Trinidad have given 

 uniformly consistent results, while the equal number of controls 

 have been unaffected. Introduction of a fragment of infested 

 tissue (i) into the stem, by way of an auger hole immediately 

 closed by a tarred plug, (2) into a wound made by cutting out a 

 small cube from the petiole of an upper leaf, (3) into one or 

 several leaf axils, without artificial wounds, produces typical 

 infestation which becomes externally evident usually in about 

 60 to 70 days. The characteristic red zone in the stem is de- 

 veloped in infections by each method even in trees 10 or 12 

 years old, and is dense at the base and lighter above as in natural 

 cases. In leaf axil infections purposely examined at an early 

 stage infestation of a small number of leaf bases, without evidence 

 of extension into the stem, has been found, and very occasionally 

 such cases have been met with by chance in the field. The next 

 phase of infestation is the appearance of the red ring at the base 

 of the stem. 



It is rare for trees to show signs of infection at less than four 

 to five years of age. The writer has met with one plant judged 

 to be not more than three years old which was changing colour 

 and was found to be infested in the leaves only. In the great 

 majority of cases the trees develop the disease during the bearing 

 of the first few crops of nuts. The most critical period may be 

 said to extend from the fourth to the seventh year, but cases 



