THE CONTROL OF PLANT DISEASES DUE TO FUNGI. I5 
Palinkas, and F. Savoly in Hungary in connexion with vine mildew ; 
we must study the behaviour of the fungi causing epidemics, the 
growth of mycelium, the germination of spores, perithecia, &c, in 
relation to temperature, humidity, and other environmental influ- 
ences, following the example of Mengel, Ravaz, Verge, and other 
investigators in France. 
Clearly we have not yet sufficiently applied our scientific knowledge, 
whether of fungi, of physical conditions, or of chemical reactions. 
Let us take care, however, that the application is aptly made, bearing 
always in mind Marshall Ward's review of the great chemist Liebig's 
influence on biological science. " That Liebig was indispensable in 
1811-1850 is one thing ; but that his influence should extend to the 
present day is quite another, and his inevitable mistakes were almost 
as powerful for future evil as his clear exposition of the chemistry of 
his day was productive of immediate good." 
Some examples of the kind of pitfall awaiting the botanist or 
mycologist who attempts to deal with practical problems without 
sufficient field experience may be taken from the writings of the 
brilliant author of " Disease in Plants " himself. Writing of potato 
disease and the symptoms presented in the foliage, in "Diseases of 
Plants" Marshall Ward explains that Phytophthova passes down 
the haulm of the plant and reaches the growing tubers. Forthwith 
attention became fixed on the haulm as the carrier of infective 
mycelium, and the idea once rooted became firmly established, and 
on this basis spraying efforts were devised to prevent the infection of 
tubers, but often foredoomed to failure, as I shall presently show. 
Now, as a matter of fact, the course of the fungus down the haulm 
had not been demonstrated to occur, or at all events to any extent, 
under field conditions, and recently Dastur has shown that the castor 
oil Phytophthora, P. parasitica, shows no propensity to travel either 
upwards or downwards from a stem inoculation. Neither is there 
any weight of field evidence to support this view. In the 1915 epi- 
demic, which threatened at one time a potato failure of serious 
dimensions, the attack took place by way of the "eye" and skin, a 
fact clearly demonstrable at the Daily Mail show held at Vincent 
Square, in a number of exhibits derived from all parts of the 
country. Again, with regard to direct infection of growing tubers 
by the zoospores of Phytophthora, we are told that infection can 
only succeed as a rule when the tubers are still young, since the 
coat of the older potatos, being thick and corky, resists the inroads 
of the fungus. Here the lenticels, skin affections, or abrasions due 
to soil and weather conditions, as providing means of ingress, are 
presumably ignored. Again, the power of dormancy was ascribed 
to Phytophthora \ it could be assumed that Phytophthora is present 
in healthy tubers, and some authorities held even that tubers are 
rarely free from it. But this is certainly not the case ; the mycelium 
can winter only in the diseased tissue of diseased tubers. The view 
arises apparently from a false comparison with certain rust fungi 
