INVESTIGATIONS ON THE CONTROL OF DISEASE IN PLANTS. 317 



possibility of controlling disease by the addition of almost negligible 

 amounts of salts to the soil. 



A still more direct way of minimizing the losses caused by fungi 

 is to cultivate disease-resistant plants. It appears to be almost 

 invariably the case that when many varieties of any plant are grown 

 under the same external conditions they show considerable differences 

 in their capacity to resist disease. Some few may even be found 

 which are practically immune. 



Unfortunately, immunity to any particular disease is not necessarily 

 associated with other characters almost equally desirable from the 

 cultivator's point of view. For instance, the potato ' Evergood ' 

 possesses a considerable capacity to withstand the attacks of Phyto- 

 fihthora infestans, but it can never come into such general cultivation 

 as the susceptible * Up-to-Date,' simply because in many districts 

 the quality of its tubers is far from desirable. Hence the question 

 arises whether immunity to disease and its converse susceptibility 

 are heritable characters. If so, it would seem possible by cross- 

 breeding to associate the valuable feature of immunity with the other 

 characteristics the cultivator requires. 



A brief survey of more than one of our garden plants indicates 

 that such is the case, but really critical data are difficult to obtain, 

 owing to the lack of precise knowledge with regard to the origin of 

 many, if not of most, garden varieties. The numerous varieties of 

 Roses in cultivation nowadays provide the best evidence in this 

 direction, and fortunately the pedigrees of many are known with some 

 degree of certainty. Further, the Rose is ideal from another stand- 

 point, though possibly one not appreciated by the grower, for it happens 

 to be attacked by a considerable number of fungoid pests, chief of 

 which are the rose mildew, orange rust, and black spot. These fungi 

 appear with great regularity each season, and consequently offer 

 good material for investigating the phenomena of disease susceptibility. 



Three groups of " florists' " Roses are generally recognized. These 

 are the Teas, the Hybrid Perpetuals, and the Hybrid Teas. The 

 Teas and Hybrid Perpetuals form two sharply marked groups easily 

 distinguished by their foliage and floral characters. The third group, 

 the Hybrid Teas, have resulted from crosses between members of 

 the first two groups. Some are first crosses, others seedlings from 

 first crosses. Naturally the limits of the group are somewhat ill- 

 defined, and the raisers of new Roses often arbitrarily class their 

 novelties as Hybrid Teas when the ordinary observer is inclined to 

 put them in other groups. This is in part due to the fact that it is 

 not generally recognized that when such first crosses are again crossed 

 the resulting offspring shows segregation into each of the three classes. 

 The Tea Roses as a group are characterized by the fact that they are, 

 to quote Foster Melliar, " particularly exempt from orange rust." 

 The Hybrid Perpetuals, on the other hand, can fairly be described as 

 particularly susceptible to it. The Hybrids from these two groups 

 are worthy of extended investigation with regard to their capacity 



