REFLECTIONS ON DISEASE TREATMENT 191 
when exposed to conditions of infection under which 
normal plants of the same variety are unable to function, 
Many reasons have been brought forward to account for 
disease resistance, but with these the practical man, in 
the present state of our knowledge, is unconcerned. It is 
sufficient for him to know that such phenomena do exist, 
and to realize the great possibilities of disease control 
which they open before him. Resistance is variable in 
extent—some individuals being slightly resistant, others 
completely resistant. Plants belonging to this latter 
class are commonly said to be immune. Plants which 
are resistant to one disease are not necessarily resistant 
to another. This is well known to farmers, who find 
that certain varieties of potatoes immune to wart disease 
are yet susceptible to blight caused by Phytophthora 
infestans. While this is disappointing it should not 
prevent us from taking full advantage of the benefits to 
be derived from the existence of resistant varieties, even 
though these are limited in the scope of their resistance. 
All growers of plants, and especially those of glass- 
house plants, have long since experienced the difficulties 
connected with disease control, and many have voiced 
an opinion that the control may be worse than the 
disease, for it requires infinite care and patience before 
it is effected. Moreover, many diseases are only imper- 
fectly controlled by any known means, and the production 
of resistant varieties in such cases would solve the 
problem satisfactorily. The use of resistant varieties is 
becoming more and more important, and in the future 
the production of such varieties must become one of the 
most important problems of the pathologist and grower. 
The chief means by which disease resistant varieties 
can be obtained is by selection or by hybridization. 
Selection 
By this method plants are grown under conditions 
which expose them to a considerable amount of infection 
