New York Agricultural Experiment Station. 543 



There are few fungous diseases of cultivated plants which are 

 equally destructive to all of the varieties of the species which they 

 attack. Usually some varieties are much more severely attacked 

 tha:? are others. Some varieties may be but slightly affected while 

 others are ruined. Observant fruit growers know that Flemish 

 Beauty "scabs " worse than most other varieties of pears, while the 

 fungus which produces the leaf-blight and cracking of the pear 

 {Eydomosporiuin maculaiam), has a preference for the variety. 

 White Doyenne. Wheat growers know that some varieties of wheat 

 are more liable to rust than are others. These are but a few examples. 

 Many more might be mentioned. In the case of lioestelia pirata, 

 this preference for certain varieties is carried to extremes. We 

 know of no other fungus which attacks some varieties of a species 

 so severely and yet can not even be inoculated upon a large number 

 of other varieties of the same species. Carnation rust, Uromyces 

 Caryo2)hyllinus, (Schrank) Schroeter, perhaps most nearly 

 approaiches it. This rust is exceedingly destructive to some varieties 

 of carnations, while several other varieties are nearly exempt from its 

 attacks. One variety (Wm. Scott) is notably immune. We know 

 of but one well authenticated case in which the true rust ( Uromyces) 

 has been found upon this variety, although we have repeatedly 

 observed it growing in green-houses where other varieties were 

 badly rusted. 



In the present state of knowledge concerning the conditions of 

 parasitism it is impossible to completely explain the immunity of 

 v^arieties. The structure and chemical composition of a variety are 

 intimately associated with its susceptibility or non-susceptibility to 

 the attacks of a particular fungus ; but what is the relative import- 

 ance of these or what part is played by the mysterious factor called 

 ^* inherent vigor," we do not know. 



In conclusion we will record our observations on the effect of 

 moisture on the prevalence of Gymnosporangium and Roestelia. 



In the spring of 1894 G. macropus was fairly abundant at Ames, 

 but tlie spring and summer were very dry, and as a consequence of 

 the drought, Roestelia pirata on Pirns coronaria was rare. As 

 previously stated, even attempts at inoculation of P. cononaria 

 failed that season. In the spring of 1895, showers were frequent 

 during the month of May. This season Roestelia was so abundant 

 on P. coronaria that it was difficult to find leaves which were not 



