22 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



certain of the citrus fruits to impress the industry with its commer- 

 cial importance. It is believed that the same principles will be 

 applicable to many other varieties of fruits. 



Continued studies of the effect of the length of day upon crop 

 growth' are yielding good results and promise to be helpful in con- 

 sidering varieties of plants to be used in breeding work for different 

 regions. 



Great progress has been made in recent years in solving the 

 problem of the cause and control of many formerly obscure plant 

 ailments, commonly spoken of as physiological diseases — such, for 

 example, as the mosaic disease of sugar cane, corn, cucumbers, po- 

 tatoes, and many other of the cultivated crops. Some of the diseases 

 of potatoes and beets apparently belong in this same category. It 

 is being found that these are infectious diseases which may be trans- 

 mitted by different insects. Each new discovery in this most inter- 

 esting field brings nearer the possibility of controlling or eliminat- 

 ing these troubles, or of developing varieties and strains which may 

 be resistant to them. Much of the failure in controlling some dis- 

 eases is now known to be due to failure to recognize the fact that 

 plants might be infected and capable of transmitting the disease 

 without showing external symptoms. These researches have re- 

 sulted in throwing much light on a field in which scientific workers 

 previously have been almost helpless. 



BARBERRY ERADICATION. 



Efforts to wipe out some plant diseases by exterminating the 

 intermediate host are encouraging. The warfare against the black- 

 stem rust of wheat in the Northwest and against the white-pine 

 blister rust in the forests are cases in point. The part played by 

 the barberry in the transmission of the wheat-stem rust is now 

 generally recognized, and scientists, extension workers, farmers, 

 and people and communities interested in the wheat trade are co- 

 operating in an extensive campaign to eradicate the barberry. The 

 first annual appropriation ($150,060) for barberry eradication be- 

 came available on July 1, 1918. This was increased to $350,000 on 

 July 1, 1922. During the first two years of the campaign most of 

 the effort was spent in getting bushes out of the cities, towns, and vil- 

 lages, on the supposition that the greater number of b.arberry bushes 



