178 SECTIONAL ADDRESSES 
strains are known which differ in pathogenicity. In Fusarium cubense, 
the cause of the devastating Panama or Wilt disease of bananas in the 
West Indies and Central America, Hansford 2” has shewn that some 
strains, morphologically indistinguishable from F. cubense, are non- 
pathogenic to the banana. Botrytis cinerea compr:ses innumerable 
small genetic units which differ appreciably in morphological characters 
and to some extent in pathogenicity, as demonstrated by Brierley. As 
research extends and as these forms hitherto included in Botrytis cinerea 
become more clearly differentiated by morphological and cultural criteria 
some of them are elevated to the rank of distinct species. 
How have these innumerable minor forms of parasitic fungi arisen ? 
Doubtless hybridization between genetically different strains of fungi 
which reproduce themselves sexually has played an important part in their 
evolution, as has been demonstrated for Puccinia graminis in North America 
and Australia. But how did these inherently different strains of sexually 
reproducing fungi arise? They may, of course, be the outcome of sexual 
interactions in the distant past, but this does not entirely account for their 
abundance at the present time. Although evolution may proceed mainly 
by the interactions of dissimilar gametes, as claimed by some authorities, 
there is abundant evidence in the fungi that evolution has occurred to a 
considerable extent by gene mutation. Sexual processes rarely occur in 
the life-cycle of Puccinia triticina and its sexual stage probably does not 
exist in this country. Puccinia glumarum is now probably entirely asexual. 
Yet in both these fungi there are many physiologic forms. It seems 
likely, therefore, that evolution in such fungi as regards pathogenicity has 
proceeded by changes of a mutational character. Mutants in fungi com- 
monly arise as regards colour and other cultural characteristics without the 
intervention of sexual processes, so it is not surprising if changes in patho- 
genicity occur in the same manner. Some such changes have been ob- 
served under experimental conditions. For instance, Miss F. M. Roberts” 
in her investigation of the British physiologic forms of Puccinia triticina 
has found that one form, which had been stable during a long succession 
of uredospore generations, suddenly gave rise to a form which differed 
markedly in pathogenicity from the parental stock. Such a change can 
only be accounted for by mutation, for the original culture was started 
from a single spore so that any question of an admixture of forms can be 
ruled out. 
It may be asked whether these physiologic forms of fungi are stable 
entities. There is no doubt that in general they are, apart from occasional 
alterations through hybridisation and gene mutation. Although these 
forms are usually constant in their parasitism it would be rash to be too 
dogmatic about the matter. Miss Roberts has discovered one form of 
Puccinia triticina which seems to be genetically unstable in its behaviour 
on the differential host varieties : under constant environmental conditions 
it apparently varies in pathogenicity at different times. 
The prevailing opinion of the general stability of physiologic forms of 
27 Kew Bulletin, p. 257 (1926). 
28 Ann. App. Biol., 18, p. 420 (1931). 
2° Cambridge Ph.D. thesis, 1935 (not yet published). 
