386 



How Living Things Affect One A?2other unit vii 



Fig. 335 Sin/it is one of the parasites of corn. 

 These balls each cojitain billions of spores, (u. s. 



DEPARTMENT OF AGRICULTURE) 



Parasites on plants are harmful to us. 



Plants as well as animals are subject to 

 disease. Their parasites are commonly 

 viruses and molds; but they may be bac- 

 teria, roundworms, and insect larvae. 

 Corn is frequently attacked by a fungus 

 known as corn smut. You may have seen 

 corn ears that consisted partly of a sooty 

 black mass. Such a mass may also appear 

 on the corn tassel or the stem. The black 

 mass is a mass of spores which are very 

 small and long-lived. They are easily 

 blown about by wind. When one of the 

 spores alights on a tender young part of 

 a growing corn plant it begins to form 

 threads (hyphae) which grow into the 

 tissues of the corn plant, using the food 

 made by the host. The threads in time 

 produce a large mass which becomes 

 black as spores are formed. The smut 

 does little harm to the plant unless it ap- 

 pears in the ear, but this is where it fre- 



FiG. 336 /i white pine infected with blister 

 rtist. How cafi this disease be controlled? (u. s. 



FOREST service) 



quentlv forms. Control of corn smut is 

 very difficult. 



Rusts are parasitic fungi that have 

 caused great damage to plants. The 

 white pine blister rust, which was 

 brought to the United States from Eu- 

 rope in the early part of this century, 

 killed many A\hite pine trees throughout 

 New England. The rust must spend part 

 of its life in a second host, a wild goose- 

 berry or currant bush. Spores are formed 

 in the berry bush. These, when released, 

 infect the pine. The spores formed there 

 must find their way back to the ooose- 

 berry or currant. To save the pines, it is 

 necessary to destroy wild gooseberry 

 and currant plants. 



Another serious fungus disease, called 

 the Dutch elm disease, threatens to de- 

 stroy the American elm trees so com- 

 monly planted along streets in many 

 cities for shade and decoration. The fun- 



