16 DISEASES OF ECONOMIC PLANTS 



Mildew upon any plant surface consists of a whitish pow- 

 dery fungous growth. 



Burn is a general term applied to all cases where the leaves 

 turn red or brown, especially if the edge first shows 

 the symptom. 



Smut consists of a mass of spores, usually black and pow- 



FiQ. 6. — Clustercups of rust enlarged. After Clinton. 



dery. It is most common in the ovaries and other 



floral parts of the cereals. 

 Rust should properly be applied only to cases where the 



special rust fungi (Uredinales) are present. Rusts 



usually show the spores in sori. 

 Yellowing of leaves, due to diminution in green coloring 



matter, is a common sjTnptom of disease. 

 Chlorosis is a lack of proper green, a whitening of tissues 



which should be normally green. 

 Canker consists of a roughening or splitting of the bark. 



