Apr., 1922] RAINES VEGETATIVE VIGOR OF THE HOST 185 



REVIEW OF LITERATURE 



Physiological studies on the rusts have been reported almost entirely 

 from the point of view of a direct relation between the environmental factor 

 ' concerned and the rust fungus, the essential intermediary between the two, 

 the host, being kept more or less indistinctly in the background.- A marshal- 

 ing of the available data on the physiology of the rusts from the point of 

 view of the present study, the possible correlation between host vigor and 

 virulence of the parasite, reveals a very general agreement in favor of the 

 concept that in the rust diseases of the higher plants there is a tendency for 

 the parasite to exhibit a higher incidence of infection and greater virulence 

 on the host of greater vegetative vigor. Similar instances in other classes 

 of plant diseases are cited in the general discussion. 



Host Nutrition : Nitrogen Nutrition and Mineral Nutrients 



Field Studies 



Butler (1918, p. 73) describes the coffee leaf disease (Hemileia vastatrix 

 B. & Br.) as having first appeared on some of the best coffee in Ceylon, and 

 states that it is, in the case of this disease, considered as established that 

 fungous infection and growth occur better in strong leaves, rich in nutriment, 

 than in those with less food supply. 



The greater susceptibility to rust of wheat grown on highly fertile land 

 has been noted repeatedly. Little (1883, p. 634) states that high manuring, 

 especially with nitrogenous manures, predisposes wheat plants to rust. 

 And Bolley (1889, p. 17) writes that 



It is a matter of common note that soils rich in organic plant foods, such as low-lying 

 loams, are quite liable to produce rusted crops; and in England, where great quantities of 

 nitrogenous fertilizers are used, much has been said as to the liability of the crop to rust 

 upon fields to which such manures have been applied. Such observations go to confirm 

 the belief that soils excessively rich in nitrogen, either natural or applied, produce wheat 

 easily attacked by rust. 



Freeman and Johnson (1911, p. 69), in their general review of the cereal 

 rust problem in the United States, state: 



... It is now well established that where there is an excess of nitrogen in the soil, 

 other things being equal, grains are more severely attacked by rust than crops on soil con- 

 taining less nitrogen. . . . Where barnyard manures have been applied heavily the result 

 is similar, and where grains are grown after a crop of clover, beans, or vetch, rusts may be 

 expected. In fact it may be generally stated that where soils are rich in nitrogen, pro- 

 ducing rank and succulent plant growth, rust attacks will, as a rule, be most severe on 

 account of increased succulence of the plants, increased rankness of growth, delay in drying 

 out after showers and dews, and slight delay in the ripening period. On the other hand 

 phosphate of lime tends to shorten the ripening period and thus acts as a rust preventive 

 to some extent. ... In general, a rust attack is most virulent on a healthy plant. 



Peacock (1911) states that rust in wheat and oats is favored by pre- 



