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W. A. ORTON 
Equally remarkable are the results of James Johnson (5), who has 
selected strains of tobacco resistant to Thielavia basicola, a root 
parasite of many species and varieties of plants. 
It is especially fortunate that resistant varieties may be employed 
against these root parasites which cannot be reached by fungicides 
or eliminated by rotation. 
The problem now before us is to produce adapted races, resistant 
to disease, and bring them into general cultivation. There is reason 
to expect that when such a wheat as the Kanred, just described by 
Melchers, is in state-wide cultivation, stem rust will become in- 
significant in prevalence. 
All plant breeding should take disease resistance into account. 
Strains under test should be exposed to infection by all the parasites 
that they are likely to meet in order to bring them into equilibrium, 
for it is possible by breeding plants in the presence of their diseases 
to produce resistant varieties. Such resistance will be reasonably 
permanent, at least as long as admixture and intercrossing with other 
non-resistant varieties are prevented. 
There will, however, be serious danger that the advent of a new 
parasite, or even of a new biological strain, would result in losses. 
Such introductions, as we have seen, will be from other continents. 
Consequently, there is a fundamental biological argument for a policy 
of exclusion from North America of all living plant material from other 
continents, or at least for strict regulation and admission under 
safeguards as to disinfection. 
International commerce has developed enormously in the last 
generation. Plant products may come from the ends of the earth; 
corn and potatoes have come from Australia, fruit from South Africa, 
and beans from Manchuria. These food products are to some extent 
a source of danger, since they may bring in new parasites; but the 
importation of nursery stock is a greater risk, for living plants are 
constantly accompanied by parasitic fungi and insects. 
Complete success in excluding plant diseases is perhaps not to 
be hoped for, when one takes account of the myriad articles of com- 
merce which may carry infection, such as wool, hides, and lumber; 
nevertheless, asparagus rust was not brought over until about 1896, 
chestnut blight not until after 1900, and citrus canker not until about 
191 1. That these diseases were kept out for so long when we had no 
exclusion law argues strongly for a hopeful view of the future. 
