K.—BOTANY 173 
fungicides, and the protection of wounds in woody plants against parasitic 
invasion have all received much attention in recent years and have met 
with a considerable degree of success. 
More care is now paid than formerly to growing plants under the best 
environmental conditions with a view to diminution of parasitic attack, 
including modifications of cultural practice which tend to favour the host 
at the expense of the parasite. ‘The ecological study of disease in plants, 
as I may term it, is only in its infancy, but it promises to be one of the 
most fruitful aspects of pathological investigation in the future. Environ- 
mental conditions often determine whether a disease will become serious 
or not. In the Malayan rubber plantations for instance Pink Disease, 
caused by Corticium salmonicolor, is only severe in the regions of highest 
rainfall. Reinking has summarised our knowledge of the relation- 
ships which often exist between certain types of soil and Fusarium wilt 
diseases. He points out that the wilt disease of bananas caused by F. 
cubense is much more severe in Central America on sandy than on clay 
soils. The study of the temperature-relationships of parasitic fungi 
which infect their hosts below soil level has already yielded results of the 
highest value, notably at the University of Wisconsin under the leadership 
of Prof. L. R. Jones. For instance, Walker and Jones 2 have shewn that 
at soil temperatures of 29° C. and above Urocystis Cepule, the cause of 
the smut disease of onions, cannot cause infection because the spores do 
not germinate normally ; at such temperatures, however, the host grows 
well. It is in connection with the action of weak parasites, especially root 
parasites, that attention to the well-being of the host will prove to be of 
the greatest consequence. Claims are occasionally made that by growing 
plants in the best environment pathogenic organisms will be reduced to 
impotence. Such claims, however, cannot be justified in general. Al- 
though there may be some truth in this belief with regard to certain weak 
parasites, it is not true in connection with the incidence of obligate para- 
sites such as Downy Mildews and Rusts. Fungi of this kind generally 
thrive best when their hosts are in a vigorous state. The same consid- 
erations apply to many virus diseases. The ‘ Spotted Wilt’ virus, for 
instance, is at least as severe in its many hosts grown under good conditions 
as when they are enfeebled. It is not implied that the incidence of virus 
diseases is entirely unrelated to the condition of their hosts; Spencer 3 
has shewn that tobacco plants which made the most rapid growth were 
somewhat less susceptible to certain mosaic viruses than plants in which 
growth was retarded by excess nitrogen. It is maintained, however, that 
plants grown under normal nutritive conditions shew no particular 
resistance to appropriate virus diseases. 
Of greater academic interest to botanists are the mutual relations between 
parasitic micro-organisms and their hosts. ‘The province of the plant 
pathologist is particularly intriguing in this respect, for he has to study a 
complex of two organisms in relation to environmental conditions. The 
pioneers of plant parasitology such as the brothers Tulasne, de Bary, 
la Zentralbl. f. Bakt., 11, 91, p. 243 (1935). 
2 Jour. Agric. Res., 22, p. 235 (1921). 
3 Phytopathology, 25, p. 178 (1935). 
