ORANGE LEAF RUST OF WHEAT. 19 



be iiiii)ossible to make any general estimate from the per cents of dam- 

 age given in Table 1, as they show the losses for only one or two seasons 

 and in comparatively few localities. In the writer's opinion, however, 

 the average annual loss from rust throughout the Ignited States far 

 exceeds that due to any other enemy, insect or fungous, and often 

 equals those from all others combined. 



ORANGE LEAF RUST OF WHEAT. 



{Puccinia rubujo vera tritici). 



Physiological relations. — It was not until a late date in the writer's 

 inoculation experiments that attention was given to this rust, most of 

 the previous work having been with P. (/ra minis and P. coronata of 

 oats. Many observations made in the field have shown that the leaf 

 rust of wheat and of rye probably do not pass from one host to the 

 other, but are distinct specialized forms. Where wheat and rye are 

 grown in adjoining fields, rust not only attacks them at ditterent dates 

 (which may be partially accounted for in that rye is in a condition most 

 susceptible to rust earlier than wheat, maturing, as it does, earlier), 

 but is occasionally very prevalent on one and not abundant or even 

 present on the other during the entire season. Again, the writer has 

 never been able to find any relation between the abundance of this 

 rust on wheat and its occurrence on native grasses in the vicinity of 

 the wheat fields. 



The writer's first experiments, reported by Hitchcock and himself 

 (31), pp. 3, 4), showed that uredospores of this rust would not infect 

 oats, rye, or orchard grass, and that a uredo (supposed to belong to 

 P. ruhigo-rera) of Kentucky blue grass would not infect wheat or oats. 

 The results of all subsequent inoculations ^ with this rust are summa- 

 rized in the table following, 



' Unless otherwise explained, all inoenlation experiments reported in this bulletin 

 consisted in treating one small pot containing from two to twelve seedling plants 

 of the same kind 10 to 30 days old from the seed in case of cereals and 20 to 40 

 days in case of other grasses. An average of nine inoculations were made in each 

 experiment, there being never less than six, and rarely more than twelve. The 

 manner of performing the experiment is as follows: The plants to be inoculated are 

 first wet with a very tine spray of water from an atomizer. Then by the use of a 

 thin, narrow-bladed scalpel material is scraped from the diseased plants and is 

 applied in spots here and there on the healthy ones, that is, on the upper surface, 

 the lower surface, or on both, or on the stem also, depending on the particular 

 oxi^eriment. No incisions are ever made. After another spraying the plants are 

 covered with a bell jar, and the latter wet with cold water to aid in preserving a 

 coating of moisture ou the plants. On an average, the bell jar is not removed lor two 

 days, except just for an instant to allow a further spraying with water. During 

 periods of hot sunshine a shade is used to shut off the sun's rays. These experi- 

 ments are made in the greenhouse even during the summer, and the dift'erent rusts 

 are kept growing the year through. All experiments of the same date are, as a rule, 

 accompanied by one check experiment, in which inoculations are made upon plants 

 of the same host as those from which the inoculating material was taken. All 

 inoculations are made with uredospores unless otherwise stated, 



