BUREAU or PLANT INDUSTRY. 277 



FOREST DISEASES. 



WHITE-PINE BLISTER RUST. 



Investigation of the white-pine blister rust has been continued. 

 The method of distinguishing between the white-pine rust and the 

 similar pinon-pine rust, formerly confused with it in the stage 

 on currants, has been improved. Examination of American species 

 of white pine planted in western Europe showed the rust killing 

 large trees in 15 different places. In British Columbia large trees 

 of western white pine were found dying. In many cases trees were 

 killed even before the fungus reached the trunk, as the result of 

 universal killing of the branches by individual infections. These 

 foreign investigations give further evidence of the damage which 

 the disease may be expected to do on longer establishment in this 

 country and especially to the western white pine, which has been 

 found to be more susceptible than the eastern white pine. 



Further information has been obtained by field and greenhouse 

 experiments as well as by field inspections as to the species of cur- 

 rants and gooseberries which cause greatest damage to pine and are 

 therefore most necessary to eliminate in selective eradication work. 

 It has been found that all the northwestern species of Ribes which 

 have so far been subjected to infection either in nature or in green- 

 house experiments (a total of 16 species) are susceptible to the 

 disease. Rihes hracteosum and R. petiolare^ wild currants, native to 

 the Northwest, are especially susceptible. 



Field evidence has confirmed earlier indications that the elimi- 

 nation of cultivated black currants, a cheap and rapid procedure, 

 will greatly delay the establishment of the disease in new territory. 

 The black currant {Rihes nigrum) is much more susceptible to 

 the disease than any other species. 



Field surveys at the end of the 1921 season showed that un- 

 protected pine areas in the Northeastern States were becoming 

 generally infected and that the volume of seciospore production fol- 

 lowed by currant and gooseberry infection and later by pine infec- 

 tion was rapidly increasing. An examination of 53,838 pines on 

 138 miles of rod-wide strip line and 256 plats in Maine, New Hamp- 

 shire, Vermont, Massachusetts, and New York showed 16 per cent 

 of the trees infected. In local areas where the disease had been 

 established many years, 60 to 100 per cent of the white pines were 

 dead or dying, and between these areas new infections were gen- 

 erally distributed. 



During the past season 20 per cent of the pines on 12 miles of 

 rod-wide strip line in northeastern New York were found diseased, 

 and an extensive study of a white-pine plantation established in 1916 

 in the same region showed 24 cent infection. In northeastern 

 Pennsylvania the rust was found on currant and gooseberry bushes 

 in Wayne and Lackawanna Counties, but no infected pines were 

 located. In Michigan the blister rust was found on currants and 

 gooseberries in Oakland County, for the first time in the State. 

 Also, three infected planted pines were found in Oakland County 

 and one in Kent County. All the diseased host plants were destroyed. 

 In Wisconsin and Minnesota the disease is not advancing so rapidly 

 as in New England and New York, partly because of the scattered 

 distribution of the host plants, particularly the white pine, and 



