344 



GENERAL PLANT PATHOLOGY 



is induced in all probability by the presence of oxidizing enzymes in 

 patches of cells where the chlorophyll pigment is destroyed and not in 

 other adjoining areas. 



The formation of spots on leaves (Fig. 134), stems, flowers, or fruits is 

 due to a variety of causes. The grayish or whitish spots on the under 

 surface of grape leaves are due to mildews, on the stems of cruciferous 

 plants to white rusts and on the leaves of the parsnip are found white 

 spots due to a fungus, Cercosporella. Grayish spots on the prickly pear 



Fig. 134. — Apple leaves showing leaf spots produced by natural infection with 

 Sphaeropsis malorum. (After Scott, W. M., and Rorer, J. B., Bull. 121, U. S. Bureau 

 of Plant Industry, 1908.) 



and on the leaves of the box trees are occasioned by a disease known as 

 anthracnose. Many leaf spots are yellow as in violets, oaks, cucumbers 

 and melons. The red or orange spots on plants usually suggest the 

 presence of rusts as on wheat, rye, alfalfa and a host of other cultivated 

 and wild plants. The so-caUed tar spots of the maple leaves are black 

 in color and such discolorations of the leaf surface are traceable to the 

 attack of a fungus, Rhytisma acerinum. Apples are frequently marked 

 by fly specks which are usually clustered as small circular black spots. 

 A fungus is the causal agent. 



