' RUST ' AND ' MILDEW ' OF WHEAT 729 



rapidly the fungus may spread in a crowded crop of plants 

 when conditions for the distribution and germination of the 

 spores are suitable. 



This form of reproduction is carried on throughout a good 

 part of the summer, and accounts for much of the extensive 

 distribution of ' rust ' in wheat, although it is important to note 

 that, according to Eriksson's investigation, the uredospores from 

 any individual infected plant only spread the disease a com- 

 paratively few yards around their point of production in a 

 field. 



Although certain varieties and species of 'rust' to be men- 

 tioned hereafter attack other cereals and grasses, the form under 

 present consideration lives as a 'rust' upon wheat alone, so that 

 there is no danger of the disease being carried to neighbouring 

 crops of oats, rye, barley or grasses from an infected field of 

 wheat. 



Towards the end of the wheat's growing-season, the produc- 

 tion of uredospores ceases, and as they do not retain their power 

 of germination more than a few months it would appear that 

 the fungus does not usually live through the winter in this form, 

 though it is possible that in some districts at any rate the uredo- 

 spores infect autumn-sown wheat, and the fungus remains upon 

 the latter in an inconspicuous condition until the following 

 spring, when it develops and spreads as 'rust' through the 

 crop. 



The mycelium in the tissues of the host, after producing 

 uredospores for a considerable time, begins to give rise in July 

 and onward to another form of chlamydospores of dark colour. 

 The ' rust ' spots in consequence change from orange to black, 

 especially on the leaf-sheaths and stems of the straw, after which 

 the crop is said to be ' mildewed.' 



This new form of chlamydospore is designated a tekutospore, 

 a name meaning final spore, and given to it in consequence of 

 the fact that it is developed at the end of the season. 



