172 ANNUAL, REPORTS OF DEPARTMENT OF AGRICULTURE. 



BLACK-CHAFF OF WHEAT. 



Studies on the biology of the bacterial organism causing the black- 

 chaff of Avheat have continued, and numerous field observations were 

 made on its distribution. It occurs in many places in Iowa, Kansas, 

 Nebraska, and the neighboring States. This trouble is transmitted 

 on the seed wheat, and wherever it is possible farmers are urged to 

 i:)rocure seed wheat from fields which have not sho^vn the disease. 



A method of control by means of seed treatment has been discov- 

 oi-ed, which works remarkably well in the laboratory, greenhouse, 

 and experimental plat, but is yet to be tried on a large scale 

 under field conditions in the Middle West. This treatment consists 

 in soaking the infected seed (after proper screening) for 10 minutes 

 in formalin diluted with water in the proportion of 1 part to 400 

 parts of water. The former objection to the use of formalin, that 

 either it did not kill all the surface organisms or else that it de- 

 stroyed also a considerable portion of the seed, has been overcome 

 by causing the seeds to absorb considerable water in advance of treat- 

 ment. By givingtheseedwdieat a preliminary water bath for 10 min- 

 utes, after which it is drained and kept moist (covered) for six hours, 

 the seeds absorb about 30 per cent of water and become much more 

 resistant to the germicide. They are then i^lunged for 10 minutes 

 into the formalin water, drained, and are kept moist (covered) for 

 another six hours in order to secure the full surface effect of tlie 

 small amount of formaldehyde vapor remaining. They are then 

 spread out thinly to dr}^ and should be sowed the next day. 



CROP DISEASES. 



POTATO DISEASES. 



The investigations of potato mosaic, extending over several years, 

 have yielded particularly valuable results during the past season in 

 the way of proof of the means of dissemination of this disease and 

 the possibilities of control through the roguing of seed plats. It has 

 been shown that the mosaic is transmitted through stem grafts and 

 tuber grafts, through transference of the plant juices mechanically, 

 and by aphids. The establishment of these facts is an important 

 step toward the working out of control measures for this disease. 



Following the discovery in the fall of 1918 of the potato wart in 

 tlie mining districts in the vicinity of Hazelton, Pa., the investiga- 

 tions already in progress were extended and enlarged. In coopera- 

 tion Avitli the Federal Horticultural Board and the Pennsylvania 

 autliorities a careful survey was made of the territory contiguous to 

 the point where the wart was first discovered, and many gardens were 

 found to be infested. A laboratory for the investigation of the dis- 

 ease has been established at Freeland, Pa., in cooperation with the 

 Pennsylvania Department of Agriculture and the State Agricultural 

 Experiment Station. 



The studies of potato leaf-roll during the past year have included 

 observations of the symptoms of tliis disease under varying con- 

 ditions throughout the country. Our knowledge of this disease has 

 been further increased through the results of comparative studies of 

 the anatomy of healthy plants and those infected with leaf-roll. 



