28o 
W. A. ORTON 
Meanwhile a development of great biological significance was 
taking place. The old beds of susceptible American asparagus were 
killed out or plowed under as the weakened plants were no longer 
profitable, and replanting with Palmetto, Argenteuil, and Reading 
Giant was generally practiced throughout the eastern districts. At 
the same time the rust became less prevalent, except where infection 
centers of uncut, susceptible asparagus were permitted to remain. 
The elimination of the old non-resistant kinds is proving to be nearly 
as important as the introduction of the new resistant stocks. 
We shall shortly be on the same basis as Europe, where asparagus 
and its rust parasite are both native, and where no serious losses occur 
since asparagus highly susceptible to rust has been eliminated. This 
fortunate result, which has taken place during our time and under 
our eyes, illustrates the fundamental principles which should guide 
our work with other crops, to the end that American agriculture may 
be protected against excessive losses from plant diseases. 
Disease resistance in plants, as in animals, is nature's method of 
restricting parasites. Nature has been breeding disease-resistant 
plants since the world began. Evolutionary factors tend to modify 
the toxin formation of parasites and to build up the resistance of the 
host plants. Organisms react in this way upon each other throughout 
the range of their natural geographic distribution. The principal 
barriers are the seas, and we must therefore lay much emphasis upon 
the intercontinental relations of this problem of disease resistance. 
The native parasites of our native plants, growing in their natural 
surroundings, offer no parallel to the ravages of introduced pests. 
The most serious plant diseases are, in all countries, due to the 
bringing together of a host and a parasite native to different continents. 
When our forefathers cleared away the American forests and planted 
the European pears, the pears were ravaged by blight, Bacillus 
amylovorus, an endemic parasite of American pome fruits, which are, 
however, very resistant. Countless attempts to introduce the Euro- 
pean grape, Vitis vinifera, into the eastern United States have failed 
because of the Phylloxera, black-rot and mildew, all native here; yet 
these diseases present a relatively slight obstacle to the culture of 
American species of grapes. The ravages of these diseases when 
carried from America to Europe nearly wiped out the viticulture of 
Europe, and they were checked mainly by the use of resistant American 
stocks. 
