72 DISEASES OF TROPICAL PLANTS CH . 



with a dense coat of fine plant hairs (see page 219). 

 The rust or russetting of the orange and lemon is also 

 the work of a species of mites (Eriophyes oleivonu). 

 Other species of mites are the cause of peculiar enlarge- 

 ments of stems and the formation of witches' brooms 

 (pp. 182, 220), while still others attack the young foliage 

 of plants, causing the formation of excessive growths of 

 plant-hairs or the production of well-defined cecidia. 

 When very abundant the plants are frequently con- 

 spicuous for long distances. 



Insects are frequently of great assistance to fungi 

 in their work of destruction and may therefore be con- 

 sidered as indirect causes of disease. Many fungi 

 depend almost or entirely on insects to carry them from 

 place to place, while many others depend upon the 

 insects to make wounds by which they gain admission 

 into the plants. In fact, many fungi cannot penetrate 

 plants in any other manner than through wounds. 



Nematodes. An enormous number of plants suffer 

 from the attacks of worms which are so small that it is 

 impossible to detect them without the use of the micro- 

 scope. In most cases the attack is made upon the 

 roots of the plants but in some few cases other parts of 

 the plants suffer. The nematode plant disease was first 

 discovered in Europe in 1859 by Schacht. In 1871 

 Schmidt described the organism under the name of 

 Heterodera Schachtii. In 1872 Greef described 

 Anguillula radicicola which Miiller transferred to 

 Heterodera radicicola in 1884. Either this or a very 

 similar species was afterwards discovered on the roots 

 of the sugar-cane in Java and described by Treub under 

 the name of Heterodera javanica. The disease has 

 been referred to in an Agricultural paper in the United 

 States as early as 1857, and in 1889 Dr. Neal referred 

 the organism provisionally to Anguillula arenaria, but 

 in the same year Atkinson referred the organism to 

 Heterodera radicicola. It was reported from Brazil 

 in 1878 as attacking the coffee trees and described by 

 Golbi under the name of Meloidogyne. In 1897 Cobb, 



