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 phyllpxera. 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. 



