K.— BOTANY 185 
there are some features of these investigations to which I would like to 
refer. Simmonds ® in Canada first directed attention to the rapid deterio- 
ration of inocula of fungi which cause foot-rot of cereals when added to 
unsterilised soil, an effect which was attributed by Broadfoot * to the 
antagonism of other micro-organisms in the soil. Henry ® shewed that 
the growth of Helminthosporium sativum, one of the fungi causing foot-rot 
of wheat, in sterilised soil might be completely suppressed by adding 
small quantities of unsterilised soil or by simultaneous inoculation of the 
sterilised soil with certain other fungi and bacteria. In a later paper 
Henry ® has elucidated some puzzling results concerning the effect of 
temperature on the pathogenicity of some of these foot-rotting fungi. 
Since the optimum temperature for the growth of wheat seedlings is about 
15° C. and that for the growth of the foot-rotting fungi ranges from 24° C. 
to 30° C., it might be thought that there would be a greater incidence of 
foot-rot at 24° C. than at 15°C. The reverse effect, however, is sometimes 
seen. Henry explains the decrease in infection by Ophiobolus graminis 
with rise of temperature in unsterilised soil as being due to the antagonism 
of other soil micro-organisms to the pathogen, which is not operative at 
the lower temperature. In sterilised soil there is the expected increase in 
infection by this fungus with rise of temperature to about 24° C., and 
Garrett ® has found that in a naturally occurring sand in South Australia, 
which is practically devoid of micro-organisms, O. graminis is most patho- 
genic at 24° C. 
Further work will doubtless elucidate the nature of this antagonism to 
pathogenic fungi in the soil, which is exhibited by other constituents of 
the micro-flora. It may be that saprophytic organisms sometimes starve 
out the pathogenic fungus, but another explanation is that the saprophytes 
secrete toxins which kill the pathogen. One example of antagonism has 
been elucidated by Weindling ® in the latter manner. He has shewn that 
the saprophytic fungus Trichoderma lignorum secretes a lethal principle 
which destroys the hyphae of Rhizoctonia Solani, one of the common 
causes of the ‘ damping off’ of seedling plants. By adding Trichoderma 
spores to Rhizoctonia-infested soil under conditions which favour the 
secretion of the toxic principle he has been able to control this disease in 
citrus seedlings. Weindling’s results concerning the toxic influence of 
T. lignorum on certain pathogenic fungi in the soil have been confirmed 
by Allen and Haenseler.®* Pathogenic fungi which live in the soil are 
notoriously difficult to control. When more is known about the antago- 
nism of other micro-organisms to them it may be possible to devise 
methods of biological control, such, for example, as altering soil conditions 
in such a way as to favour the antagonistic action of other members of the 
micro-flora. Some interesting data on this subject have been presented 
60 Report of Dominion Botanist, Canada, 1927, p. 98. 
81 Report of Dominion Botanist, Canada, 1930, p. 92. 
62 Canadian Jour. Res., 4, p. 69 (1931). 
63 Canadian Jour. Res.,'7, p. 198 (1932). 
64 Jour. Agric. South Australia, 87, p. 664 (1934). 
65 Phytopathology, 22, p. 837 (1932). 
66 Phytopathology, 25, p. 244 (1935). 
