178 



the amount of diseased leaf-tissue' on each stalk. Of the two, the amount 

 of diseased leaf-tissue is the more important, since, as is indicated in 

 Table 3, it varies more than the number of infected stalks and is more 

 closely in accord with the estimates of loss. 



In 1918 and again in 1983, when the estimated loss from rust was 

 low, the infected plants numbered more than 80 per cent. The increase 

 in the number of infected plants in 1922, when loss was estimated at 

 not less than 10 per cent, was not more than 15 per cent, while the in- 

 crease in diseased leaf-tissue on each plant at the same time was 24.1 per 

 cent greater than in 1923, and 47.5 per cent in excess of that in 1918. 



Variations in the amount of infection in dift'erent places and in dif- 

 ferent years are definitely attributable to climatic conditions throughout 

 the growing period of the crop. Leaf-rust infection of wheat in Illinois 

 begins in the fall previous to the harvest. How far fall infection may 

 influence the seriousness of the spring attack is not entirely evident, but 

 it is noteworthy that in the fall preceding the very severe epidemic of 

 1922, winter-wheat fields were in many places so thoroughly infested 

 with rust as to assume an orange tinge, visible from a long distance. 



Moisture and warmth in the fall are conducive to abundant infec- 

 tion of the young crop, and the survival of the fall infection is limited 

 in varying degrees by the severity of the winter. Whatever infective 

 material remains alive until spring serves as a source of inoculum for 

 the spread of the rust, and the amount of new infection at this time will 

 be influenced favorably or unfavorably by the presence or absence of 

 suitable climatic conditions. 



The importance of leaf rust as a limiting factor in the production of 

 wheat in Illinois is not a matter of light concern. It is true that in certain 

 sections even in years of serious epidemics, there is only a slight loss, 

 but these sections are not the large wheat districts of the state. The part- 

 of the state in which leaf rust is most abundant and generally most 

 severe concides with the region of greatest wheat production. Estimates 

 made following the 1922 epidemic and reproduced in Table 4 show the 

 seriousness of the disease in 12 of the most important wheat counties 

 of the state. The total-loss estimate for these counties was 1,3T3,588 

 bushels, which may be valued at $1,469,T39.1G. 



Complete prevention of leaf-rust attack and its consequent injury 

 to the wheat crop in Illinois is at present impossible. The fact that the 

 rust overwinters upon the wheat and does not commonly have an alter- 

 nate host precludes the possibility of control through eradicating other 

 hosts. Hard winter-wheats arc more resistant than the soft varieties, 

 and losses may be considerably reduced in places where these wheats 

 make a good crop. Where hard wheats are not satisfactory the ulti- 

 mate use of special resistant strains of the soft wheats appears to be 

 the ultimate solution. The experiments of the U. S. Department of 



' Tlie amount of (ii-sTasid leaf-tissue is measuii d by a scale prepared by tho Offict" 

 of Cereal Inve.«itisations of the tl. S. Departmont of Agrriculture. in terms of the 

 amount of leaf-surface occupied by the rust spots. 



