WALLROTHIELLA ARCEUTHOBII 



By James R. Weir, 



Forest Pathologist, Investigations in Forest Pathology, 



Bureau of Plant Industry 



INTRODUCTION 



The fungus Wallrothiella arceuthobii * has the distinction of causing the 

 only disease of the leafless mistletoes so far described. Comparatively few- 

 botanists have seen this interesting fungus either in nature or in museums, 

 and all reference to it has been omitted from works dealing strictly with 

 plant diseases. The fungus was apparently never collected again from 

 the region where it was originally discovered, and no record exists of 

 preserved material from the place of its second discovery. The dis- 

 covery of the fungus by the writer in the Northwest adds a new interest 

 to its study, especially since it is found to be so abundant as to have 

 some economic significance. 



Owing to the fact that the fungus has only been reported twice and 

 from widely separated stations, its literature is very meager. It was 

 originally discovered by Peck,^ who published a short account of it under 

 the following name and description: 



Sphaeria arceuthobii, n. sp. 



Perithecia small, densely caespitose, oblong or cylindrical, very obtuse, shining 

 black; asci subclavate, fugacious; spores crowded, globose, colorless, .00016'' in 

 diameter. 



Capsules oi Arceuthobium pusillum. Forestburgh. Sept. (Plate I, figs, ia-14). 



It forms little black tufts, crowning the fruit at the tips of the stems and branches. 

 I have not seen it on the staminate plant. I am not fully satisfied that the generic 

 reference is correct, as the perithecia seem to be mouthless. It is interesting to 

 observe the extent to which parasitism prevails. The Arceuthobium is a parasite 

 on the spruce, this fungus is parasitic on the Arceuthobium, and in a few instances a 

 third parasite, a minute white mold, was seen on the perithecia of the fungus. 



The second discovery of the fungus was by Wheeler ' in the Upper 

 Peninsula of Michigan, who reports it as follows: 



I fotmd that the mistletoe was also attacked by a fungous parasite, which must 

 have a tendency to check the spread of this pest. Each fruit is attacked at its apex 

 by the fiuigus Wallrothiella arceuthobii, Peck, and, of cotu-se, destroyed. 



Both Peck and Wheeler published some excellent drawings of the 

 fungus, from which a very good idea of the character of the disease may 

 be obtained. 



' If the system of classification of Engler and Prantl is employed, the fungus would be referred to Rosel- 

 linia. (Engler, Adolf, and Prantl, K. A. E. Die naturlichen Pflanzenfamilien. T. i, Abt. i, p. 394, 

 400, 404, fig. 258, A, B. Leipzig, 1897.) 



2 Peck, C. H. Report of the botanist. In 27th Ann. Rpt. N. Y. State Mus. Nat. Hist. 1873, p. iii, 

 pi. I, fig. 10-14. 1875. 



'Wheeler, C. F. The geology and botany of the Upper Peninsula experiment farm. In Mich. Agr. 

 Exp. Sta. Bui. 186, p. 27-28, 4 pi. 1900. 



Journal of Agricultural Research, Vol. IV, No. 4 



Dept. of Agriculture, Washington, D. C. July is, 1915 



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