178 PARASITIC FUNGI 



In a dry summer the effects of the disease are usually 

 negligible. 



Potato blight cannot be cured once it has got a 

 hold on the plant, but its spread can be very largely 

 checked by spraying the leaves with " Bordeaux 

 mixture." This is a " solution " of copper sulphate 

 and lime ; it dries on the leaves, and when wetted 

 again by rain or dew it forms a poisonous copper 

 solution which kills the zoospores, germ tubes and 

 young hyphae. In this way the crop can be very 

 efficiently protected against the onset of the disease. 

 Even if the disease is only moderately severe spraying 

 will increase, or more strictly will prevent a diminution 

 of, the potato crop. By destroying the zoospores and 

 young hyphae it enables the leaves to go on making sugar 

 and proteins, essential for the growth and stocking with 

 starch of the potato tubers, during August for instance, 

 at a time when they might otherwise be injured and 

 their chlorophyll largely destroyed by the fungus, even 

 if the shoot of the plant were not entirely killed, 

 j Rust of Wheat (Puccinia graminis and P. glumarum). 

 Puccinia graminis is an example of a highly speci- 

 alised parasite with a much more complicated life 

 history than that of Pythium or of Phytophthora. On 

 the leaf or stem of the wheat plant straw-coloured or 

 reddish streaks may appear in June or July. These 

 are masses of uredospores, a special kind of conidia 

 (Fig. 19, A), which are formed on the ends of hyphse that 

 have burst through the surface of the leaf or stem 

 from a mycelium in the tissues below. Detached and 

 blown about by the wind, the uredospores germinate 



FIG. 19. Life history of Rust Fungus (Puccinia). A, isolated 

 uredospores of P. graminis (black rust). X 475. B, germination 

 or uredospore showing penetration of the germ tube through a 

 stoma into the wheat leaf. Note the futile attempts at branching 



