152 DISEASES OF CROP-PLANTS 



word blight is inappropriate, as the fungus does not enter living 

 tissue, but derives its nutriment from the materials on which it 

 occurs only after they are dead. 



Horsehair blight occurs on cacao in the West Indies in 

 Dominica, St. Lucia, Grenada and Trinidad. Similar forms 

 occur in India and Ceylon on various jungle plants and on tea and 

 nutmeg, and there are also records from Java, Australia, Africa, 

 and South America. 



Fructifications are occasionally produced from the threads 

 while they are still on the tree, but more commonly from the leaves 

 to which they are attached after these have fallen to the ground. 

 They reveal that the horsehair-like material is a special type of 

 mycelium pertaining to a group of species of the genus Marasmius. 



The West Indian specimens at Kew, according to Petch, 

 represent the species M. sarmeniosus, Berk. The ideas as to 

 its parasitism have apparently arisen from confusion with the 

 effects of thread blight. 



Algal Disease, Red Rust. 



An algal organism, best known as the cause of the troublesome 

 affection of tea twigs in India and Java named Red Rust, is 

 widely distributed and common in the tropics, including appar- 

 ently all the West Indian islands, occurring especially on the 

 leaves of numerous trees. The twigs of cacao are occasionally 

 attacked, and dieback due to this cause is not uncommon in 

 Trinidad, is recorded from Jamaica and St. Lucia, and probably 

 occurs to some extent in the other cacao-growing islands. This 

 form of the disease occurs also on mango both in India and the 

 West Indies. 



Causation. 



Cephaleuros mycoidea, Karst. (C. virescens, Kunze, Mycoidea 

 parasitica, Cunn), the causative organism, is one of a group of 

 mostly epiphytic algse, and affords an instance of partial para- 

 sitism, which outside this genus is exceedingly rare in the algse. 

 The organism is very common on leaves, especially those with 

 a smooth surface and somewhat leathery texture. 



In this type of situation it forms, generally on the upper 

 surface, orange or rusty red, roundish or less often irregular, 

 slightly raised patches up to a centimetre in diameter. As a 

 rule the presence of the alga on leaves causes little or no damage. 

 In the case of tea, cacao, and mango alike it is in its occurrence 

 on the twigs that destructive powers may be developed. 



The first outward sign of the disease on cacao shoots is the 

 occurrence during the dry season, on the twigs of the previous 

 season's growth, of dark purplish or black spots ranging up to 

 1-2 centimetres in diameter. With the coming of the rains 

 these take on a rusty-red colour, due to the development of a 



