246 SECTIONAL ADDRESSES. 
cultivation. In some classes of disease, such as the Rusts, the intensity 
of attack tends to rise with the increased vigour of the plant, and Melhus *4 
found that with unhealthy plants it was almost impossible to obtain 
satisfactory infection by Cystopus candidus. 
On the other hand, there is a large class of infectious diseases in which 
the degree of natural resistance can be markedly enhanced by good 
cultivation. Under good conditions such diseases, which with Nowell 2 
may be termed ‘ Debility diseases,’ are of little importance; they only 
become serious when the crops are growing under unfavourable conditions. 
Diseases of this class are usually caused by saprophytes which are only 
weakly parasitic. The question of the nature of the changes occurring in 
the plant in conditions of so-called debility are quite unknown. The 
problem is sure to be a complex one, but it is possible that one of the 
factors may be an increased permeability of the superficial tissues of the 
less vigorous plants, so that the spores on the surface of the host find 
conditions especially favourable for vigorous growth. In addition to the 
relation of general health to the incidence of certain plant diseases, we 
have the undoubted effect of certain fertilisers, such as potash, in reducing 
the intensity of fungal attack. Exploration of such fields of physiological 
research, though no doubt the difficulties of investigation are considerable, 
should certainly provide results of great scientific interest. A clue to the 
nature of the changes occurring in plants which can reduce their liability 
to disease may also open the way to the enhancement of natural 
resistance by other and possibly more economical ways. Clearly it is on 
plant physiology that plant pathology is largely dependent, not only for 
the elucidation of the relationship of host and parasite, but also for 
fundamental scientific knowledge which may profoundly affect economic 
practice. 
24 J. EK. Melhus : ‘ Experiments on Spore Germination and Infection in certain 
Species of Oomycetes.’ Wisconsin Agr. Exp. Stat. Bull., 15, 1911. 
26 Nowell : ‘ Diseases of Crop Plants in the Lesser Antilles.’ 1923. 
