PHYSALOSPORA MALORUM ON CURRANT 1 
By Neil E. Stevens 
Pathologist , Fruit-Disease Investigatio7is , Bureau of Plant Industry , United States 
Department of Agriculture 
A fungus having pycnidia and spores similar to those of Sphaeropsis malorum 
Berk, was reported on dead canes of cultivated currant (Rites sp.) more than 
twenty years ago by Grossenbacher and Duggar. 2 These writers, however, 
made no suggestions as to its life history or specific identity. From the infor¬ 
mation now available it is apparent that the perfect stage of this organism is a 
Physalospora, and that the fungus must be regarded as identical with P. malorum 
(Berk.) Shear (P. cydoniae as used by Hesler) the cause of black rot of the apple 
(Pyrus malus) . This fact is of pathological interest, for obviously the problem 
of disease control is complicated when a fungus of economic importance on one 
host occurs on various unrelated hosts. Morphological and cultural comparisons 
have proven also that the difference in the size and appearance of the fruiting 
structures of the fungus on the two hosts is due to differences in the substratum. 
The taxanomic significance of this is apparent, for once it is proven, as in the 
present case, that a readily observable morphological character of a fungus is 
not constant but varies according to the nature of the host upon which it is found, 
such a character can no longer be accepted as a basis of generic or specific 
segregation. 2 
REVIEW OF EARLIER WORK 
The suggestion that the apple blackrot fungus may occur on hosts other than 
those closely related to apple is of course not new. In 1899 Paddock 4 proved by 
inoculation that Sphaeropsis sp. from such unrelated hosts as Diospyros virginiana 
L., Rhus typhina L., Celastrus scandens L., and Sambucus canadensis L., will 
cause an apparently typical “blackrot” of apples. Paddock further maintained 
that the average size of the pycnospores varied in many cases according to the 
host on which they grew, yet the spores produced on apple fruits inoculated 
with cultures from these hosts were usually of the same size and character. 
Hesler 5 gives a long list of hosts, including Ribes, on which Sphaeropsis malorum 
has been collected and cites cross inoculation experiments with material from 
fourteen hosts. 
As noted above, Grossenbacher and Duggar 6 report finding on currant canes 
a “Sphaeropsis-like fungus” which they refer to the genus Haplosporella in 
their early studies of currant cane blight. 
Received for publication April 26, 1924. 
» Grossenbacher, J. G., and Duggar, B. M. a contribution to the ufe-history, parasitism, and 
biology OF botryosphaeria rib is. N. Y. State Agr. Exp. Sta. Tech. Bui. 18, p. 184. 1911. 
* Many of the slides on which the morphological part of this paper is based were prepared by Rhoda 
Benham during the years 1919 and 1920, the rest by Ruth Colvin in 1923 and 1924. Much of the culture 
work was done by Marguerite Wilcox. The photomicrographs were made by Miss Colvin. 
♦ Paddock, W. the new york apple-tree canker. N. Y. State Agr. Exp. Sta. Bui. 163, p. 194. 1899. 
^ Hesler, L. R. blackrot, leaf spot, and canker of pomaceous fruits. N. Y. Cornell Agr. Exp. 
Sta. Bui. 379, p. 95-98. 1916. 
• Grossenbacher, J. G., and Duggar, B. M. op.cit. 
Journal of Agricultural Research, 
Washington, D. C. 
( 583 ) 
Vol. XXVIII, No. 6 
May 10, 1924 
Key No. G-422 
