VEGETATIVE VIGOR OF THE HOST AS A FACTOR INFLUENCING 



SUSCEPTIBILITY AND RESISTANCE TO CERTAIN 



RUST DISEASES OF THE HIGHER PLANTS 



II 



M. A. RAINES 

 (Received for publication July 13, 1921) 



FURTHER FIELD STUDIES AND EXPERIMENTS 



The Rust History of Plots of Cereals Sown at Successive Intervals 

 through the Summer 



To test the susceptibility to rust of plants of different ages at each period 

 of the summer and of young plants at different times in the summer, and 

 1o follow the subsequent history of the disease on plantings of cereals 

 started at successive intervals through the season, plots of wheat, rye, 

 oats, and barley were started in the breeding plot of the New York Botanical 

 Garden on June 10, June 23, July 6, July 20, August 5, and August 25, 1916. 

 Observations were made at intervals on the height of the plants, the time of 

 first appearance of the rust, the amount of infection, and the proportion 

 of the rust in the teleuto stage. 



The results for the different cereals are shown in table 3 and are discussed 

 below. In the table, the days on which the observations were made are 

 :iven at the top. The height of the plants is given in inches ("). When 

 the plants have headed, it is indicated by an "H." The degree of rust 

 infection is indicated by a numeral, and was estimated on a scale of 10, 

 ihe values of the numbers in the scale being: i = an occasional pustule 

 here and there; most of the leaves not affected. 2 = most of the leaves 

 with from one to five sori. 3 = about ten pustules on each leaf. 4 = 

 leaves heavily infected. 5 = leaves bearing the maximum possible amount 

 of rust. 6 = leaf sheaths infected as well as the leaf blades. 7 = a 

 sprinkling of rust sori on the stem and leaf sheaths; blades heavily infected. 

 <S = infection on sheath and stem well developed. 9 = heavy infection 

 on the stem. 10 = heaviest possible infection on the whole plant. The 

 leleuto stage is indicated by its Roman numeral, "III." A fraction pre- 

 ceding the "III " gives the proportion of the rust in the teleuto form. 



As is indicated in table 3, the plots of wheat planted on June 10 and 

 June 23 headed out in 77 and 64 days respectively. Those planted July 6, 

 July 20, August 5, and August 25 never headed and never exceeded a 

 height of 20 inches. The rust invariably appeared on the young plot as a 

 thin, evenly distributed infection when the host plant was putting out its 



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