10 DISEASES OF TROPICAL PLANTS CH . i 



which consist of masses of spores of certain fungi : they 

 are usually black and powdery and are most destructive 

 in the flowers and ovaries of the cereals. Rusts, which 

 are fungi and usually noted in their fruiting stages : 

 they are widely distributed with reference to both host 

 and localities ; they are of greatest importance on the 

 cereals. Some diseases are referred to as rusts which 

 are not caused by the true rust fungi (Uredinales). 

 Yellowing, refers to a discoloration of leaves or wilting 

 of twigs and leaves (namely, reduction of size and 

 increase in number) : the causes are usually obscure. 

 Mosaic or calico, which in most cases is due to enzyme 

 action within the cells, but may be due to other causes. 

 Chlorosis refers to reduction of chlorophyll, and may be 

 due to any one of many causes. Cankers, which are 

 roughenings or splittings of the bark, usually due to 

 fungi but may be due to any one of many other causes. 

 Galls or cecidia are enlargements on roots or stems, or 

 on leaves and fruits, and may be caused by insects, 

 mites, fungi, bacteria, slime moulds, or mechanical 

 irritations of many kinds ; they are sometimes very 

 injurious. Witches 1 brooms are malformations which 

 consists of great masses of distorted twigs and some- 

 times of the flowers : they are caused by fungi and 

 mites. Fasciations are abnormal growths of stems, 

 leaves, flowers, or fruits ; they may take various forms 

 and are sometimes of considerable economic importance : 

 the causes are usually obscure. 



