258 DISEASES OF CROP-PLANTS 



control lies in the cutting out and absolute destruction of diseased 

 plants, or their affected parts, before the spores are ripe. 



Head Smut. 



Another smut disease which sometimes occurs on maize is 

 due to the fungus Sorosporium reilianum (Kiihn) McAlp., which 

 causes the head smut of sorghum. It attacks the cobs and 

 tassels, and is usually confined to them, but may exceptionally 

 occur on the leaves or bracts. It reduces the cob or tassel to a 

 formless sooty mass, and can be distinguished by the absence 

 of the monstrous swellings characteristic of the common corn 

 smut. The spores of the species under discussion are minutely 

 but abundantly warty, those of the latter are prominently though 

 rather bluntly spiny. 



Head smut is uncommon in North America, and has not, to 

 the writer's knowledge, been recorded for the West Indies. In 

 Victoria, Australia, it has been reported as occurring to the 

 extent of one per cent in some districts. It occurs in South 

 Africa 



It does not yield to seed treatment, and can only be con- 

 trolled by the collection and destruction of diseased material. 



Leaf Scorch. 



A disease variously known as leaf-scorch, leaf-blight, and 

 helminthosporiose occurs on maize in N. America, S. Africa, and 

 various localities in Europe and Asia. Two instances of its 

 occurrence are reported from Porto Rico. It does not appear to 

 have been noticed in the Lesser Antilles ; possibly the local types 

 are resistant. The affection is capable of causing serious loss 

 of crop. 



Nature of the Disease. 



The disease begins on the leaves with the production of pale 

 blotches, which spread and may extend to nearly the length of 

 the blade. Wilting takes place, and the leaf becomes dry and 

 brittle. The conidiophores are then developed and are visible 

 as a dark hairy mould. Infection is said to take place b\' way of 

 the stomata. 



There is definite varietal resistance to the disease, amounting 

 in some cases to almost complete immunity. 



The Causative Fungus. 



Numerous species of Helminthosporium cause leaf diseases, 

 and biological races are believed to occur on specific plants. 

 The fungus attacking maize in America has usually been referred 

 to H. inconspicuum C. & Ell., but this is probably a synonym of 

 H. turcicum Pass., described at an earlier date from S. Europe. 



