28 
demonstrated its pathogenicity by several series of experiments. 
He recommends various remedial and preventive measures, and 
urges the need of study of the relations of this parasite to species 
allied to the Butternut, such as the Black Walnut and English 
Walnut. 
Birch Canker.—Work begun in 1g18 on the Nectria canker of 
the birch has been continued by Dr. Graves during the past sum- 
mer. He has found this disease, which is very destructive in 
this region on the Cherry Birch (Betula lenta), on the Yellow 
Birch (Betula lutea), and the Paper Birch (Betula papyrifera) in 
Maine. Inoculation experiments on the latter species in Maine 
are now in progress. 
Disease Resistance 
Disease Resistance —The utilization of disease-resistant varie- 
ties of plants has long been a recognized method of plant disease 
control. Plant growers are familiar with the fact that certain 
varieties are able to withstand the attacks of parasites which 
prove to be very serious to other varieties. The discovery and 
cultivation of varieties which show a clearly defined resistance 
to a particular disease is an obvious way to avoid loss. In the 
long run the use of disease-resistant types is the most effective 
method of disease control. 
The problem of developing suitable resistant varieties is a 
very complicated one. For its solution, there is involved an in- 
timate knowledge of the relations between the host and the para- 
site. In view of the fact that many parasitic fungi are highly 
specialized to particular hosts, a further problem in the develop- 
ment of resistant varieties arises. Progress in breeding disease- 
resistant varieties requires that the resistance or susceptibility 
towards the various races of the parasite be fully determined. 
To do this the importance of environmental conditions in the ap- 
pearance and spread of a disease must be fully recognized, in 
order to avoid mistaking resistance for mere chance escape from 
a disease. | 
It also is highly important that a complete study of the in- 
heritance of the disease-resistance character be made. In recent 
years important progress has been made in this field of genetics, 
