202 TEXT-BOOK OF FUNGI 



cetes, as Polystigma rubrum, forming reddish thickened 

 blotches on the leaves of various rosaceous plants, and 

 Rhytisma acerinum, forming large black blotches on the 

 leaves of sycamore and maple. The whole of the Erysi- 

 pheae, which cover living leaves with a white mildew, also 

 belong to this category. 



Parasitic fungi affect the host-plant in different ways. 

 Some species, as Phytophthora inftstans, the cause of the 

 too well-known potato disease, in severe cases, or in other 

 words, when climatic conditions favour the parasite, kills 

 the host-plant within a very short space of time. Pythium 

 debaryanum, the cause of damping off in seedling plants, 

 acts in a similar manner. Trees are also soon killed when 

 the roots are attacked by Agaricus melleus, or by species 

 of the Ascomycete called Roscllinia. 



Other fungi, as Rhytisma, Polystigma, and numerous 

 other species belonging to the Ascomycetes, Sphaerop- 

 sideae, etc., only cause local death, that is, the mycelium 

 only extends for a short distance from the point of infec- 

 tion, the result being the destruction of a small patch of 

 living tissue. Many of the dead patches on otherwise 

 living and vigorous leaves are caused by parasitic fungi, 

 the minute fruit of which is situated on the dead patch or 

 immersed in the tissue. 



Proceeding one step further, we come to a group of fungi 

 which, after inoculation, live in the tissues and at the 

 expense of the host for a considerable length of time 

 without, so far as we know, inflicting the slightest injury or 

 inconvenience, and whose presence is not revealed by any 

 outward sign, the host continuing to grow in a perfectly 

 normal manner. The members of the Ustilagineae are 

 past masters in this method of parasitism. Sedges and 





