' RUST ' AND ' MILDEW ' OF WHEAT 727 



Symptoms. — In early summer a wheat crop suffering from 

 this disease rapidly loses its green colour, becoming much 

 yellower in a few days. Soon 

 after this is observed, a close 

 examination reveals reddish- 

 orange elongated spots on 

 the lower leaves and stems 

 of the plants. With a pocket 

 lens the spots are seen to 

 be cracks or slits in the 

 epidermis of the plant from 

 which an orange - coloured 

 powder is shed {A, Fig. 



245)- 



Frequently towards the 

 end of the summer the 

 orange or ' rusted ' spots 

 change into or are replaced 

 by darker ones {C, Fig. 245) 

 which are often prominent 

 on the stems and leaf- 

 sheaths; the crop is then said to be 'mildewed.' Formerly 

 ' rust ' and ' mildew ' were believed to be distinct from each 

 other; they are, however, now known to be caused by one 

 and the same fungus. 



' Mildewed ' straw has not the shining golden colour so 

 characteristic of healthy, well-ripened wheat stems, but is 

 greyish-brown and dirty in appearance as well as brittle and 

 rotten. 



When ' rust ' and ' mildew ' are extensively, developed on a 

 wheat crop, the yield of grain is much reduced, and the in- 

 dividual grains are often shrivelled and small in size. 



Cause. — 'Rust' and 'mildew' are caused by the fungus 

 Puccinia graminis Pers, The yellow dust is composed of 



Fig. B45. — A , ' Rust ' spots on wheat leaf (twice 

 natural size) ; B, a single * rust ' spot more highly- 

 magnified ; C, 'mildew' spots on leaf-sheath of 

 wheat (twice natural size) ; Z>, two ' mildew ' 

 spots more highly magnified. 



