PLANT INDUSTRIES 331 
In some cases where solutions have not been forthcoming, 
attention is being centered upon securing resistant varieties 
of plants rather than upon preventing the disease. In open 
nature many plants thrive and are not affected by diseases 
which affect other plants of the same kind; that is, some 
plants are resistant and some are susceptible. Other plants 
may have certain diseases but are not killed by them, as in 
the case of the lilac and the disease known as lilac mildew. A 
good illustration of the value of the study of disease resistance 
is presented in connection with grape plants and an insect 
(phylloxera) which is parasitic upon the roots of the grape. 
The grapes cultivated in Europe are descended from a 
European wild species; the principal varieties cultivated in 
the middle and eastern United States are descended from 
American wild species. Since the French grapes produced a 
quality of wine that differed from that made from the grapes 
of the United States, European grapes were brought to this 
country. Their roots were soon attacked and the plants well- 
nigh destroyed by the phylloxera. It was found, however, 
that the roots of the American grapes were able to withstand 
attacks from phylloxera and were not seriously affected by it. 
It was also found that when European grapes were brought 
to this country and grafted upon American stock, the quality 
of the European fruit might be secured without the accom- 
panying dangers from the insect. But when grape growers 
transplanted American grapes into Europe, the phylloxera 
was also transferred, and soon the native grapes of Europe 
were attacked and serious damage was done in the vineyards 
of France. It was found that by treating the soil with carbon 
disulphide the phylloxera were killed, but this method usually 
proved too expensive for growers of grapes. Many French 
grape growers adopted the practice of planting American 
plants and then grafting their own grapes upon this intro- 
duced stock. The grape industry of France has been greatly 
increased by thus growing French varieties upon the stronger 
and more resistant American stock. 
