Nov. 29, 191S Varietal Resistance of Plums to Brown-Rot 367 



PHYSIOLOGICAL REVIEW 



A considerable amount of literature has appeared, especially in recent 

 years, on the subject of resistance and immunity to disease. The cereal 

 crops have perhaps received the most attention. BoUey (1889) and 

 Anderson (1890) attempted to correlate resistance with certain morpho- 

 logical characters. Cobb (1892, p. 1 81-2 12) advanced the theory of 

 mechanical resistance due to morphological characters, such as thick 

 cuticle, waxy coating, and small stomata. Freeman (1911) showed that 

 barley might escape rust owing to variation in amount of bloom produced 

 on the leaves, which could be varied by growing in soils of different 

 degrees of alkalinity. This escape from rust is not true resistance, but 

 is due to the inability of the water to wet the surface of the leaves so that 

 the drops containing the spores roll off. When these plants were infected, 

 however, they "exhibited large and vigorous growths of the rust." 



Marryat (1907) showed in the case of Piiccinia glumarum grown on a 

 semi-immune host that it killed small areas of the host tissue and formed 

 only small or abortive pustules, while in the case of the susceptible forms 

 the host cells, though containing haustoria, were apparently normal. 



Comes (191 2) reported that Rieti wheat, which is very resistant to 

 rust, contained a higher percentage of acid than other more susceptible 

 forms and also that the acid content increases with the altitude at which 

 wheats are grown, as does also the ability to resist rust. 



Jones (1905) showed that some varieties of potatoes are much more 

 resistant to certain potato diseases than others. He based resistance 

 more on chemical composition than on morphological differences in the 

 host. 



Kinney (1897) noted that "fruit of different varieties of plums varies 

 in susceptibility to injury by rot fungus" and attributed the difference in 

 resistance to variations in texture of the skin. He also stated that early 

 varieties are usually injured more than those which ripen their fruit later. 



Miiller-Thurgau (1900) noticed that varieties of apples in Switzerland 

 showed different degrees of susceptibility to a wilt or blight caused by 

 M. fructigena. 



Quaintance (1900) observed a marked variation among varieties of 

 drupaceous fruits in their resistance to attacks of the brown-rot fungus. 

 Among the peaches the varieties densely covered with down were the most 

 susceptible. Of the plums some varieties of the Miner group were prac- 

 tically free, those of the Wild Goose rotted about 10 per cent, while the 

 varieties of Prunus americana, P. tri flora, and P. pumila were very sus- 

 ceptible. He suggested that the firmness and thickness of the skin of 

 the Miner plums might have something to do with their resistance. The 

 relative resistance of some varieties of P. domestica to brown-rot is given 

 by Alwood and Price (1903). 



