CHAPTER IV 



OTHER CAUSES OF PLANT DISEASES 



Slime Molds (Mycetozoa). The organisms of this 

 group are considered by some as the simplest of animals, 

 and by others as the 'simplest of plants. The great 

 majority are saprophytes, although a few are parasitic 

 in habit and cause diseases of both animals and plants. 

 When active they are protoplasmic and jelly-like. The 

 fruiting or spore-bearing bodies are usually small, but 

 in some few species are quite large. 



The mature spores have a very close resemblance to 

 the spores of smut. Under favourable conditions they 

 germinate, giving rise, not to a mycelium, but to a 

 small protoplasmic body which moves, engulfs food, and 

 lives very much as the simplest form of animal (the 

 amoeba). Different individuals fuse and eventually 

 form a jelly-like mass known as a plasmodium, which 

 may be of considerable size. These bodies are often 

 abundant in forests where there are quantities of decay- 

 ing woody materials. In time they dry up, forming 

 the fruiting bodies. In the case of the parasitic forms, 

 they penetrate the hosts during the amoeboid stage, 

 stimulating the cells and causing abnormal growths, as 

 in the case of Plasmodiophora brassicae, Wor. , of the 

 cabbage and related plants (page 232). 



Algae. Some of the blue green algae live in cavities 

 in the higher plants and have assumed either a partially 

 or completely parasitic existence. However, they are 

 not users of manufactured foods, and do not cause 

 diseases of great economic importance. 



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