K.—BOTANY 183 
In the domain of plant pathology I will first mention some examples in 
which a host, attacked by one fungus, is thereby rendered more susceptible 
to a second fungus. If a variety of wheat normally resistant to Puccinia 
glumarum is attacked by bunt (Tilletia Caries) it becomes susceptible to 
the rust, the effect of the bunt mycelium being apparently to break down 
the resistance to the rust. Again, Johnston 47 and Roberts * have shewn 
that if a variety of wheat normally resistant to Puccinia triticina is affected 
by Erysiphe graminis it becomes susceptible to the rust in the immediate 
vicinity of the patches of mildew. Fawcett *° has indicated that combined 
inoculations of Diplodia natalensis and Colletotrichum gleosporioides into 
slight wounds in the bark of citrus trees produced a much greater effect 
than did either organism applied alone. In another series of experiments 
on citrus trees he ®° found that inoculations of Phytophthora citrophthora 
combined with a Fusarium led to the formation of more rapidly enlarging 
lesions than did inoculations with the Phytophthora alone ; the Fusarium 
introduced by itself did not spread at all, so that in this instance an 
innocuous organism facilitated the progress of the parasite. 
In a bacterial disease of cocksfoot grass (Dactylis glomerata) my col- 
league Dr. Dowson *! has found, as Smith °? had previously indicated, 
that the predominant yellow bacterium (Bacterium Rathayi) is constantly 
associated with a white bacterium : inoculations with the slime containing 
both organisms reproduce the disease but all inoculations with the yellow 
bacterium alone have failed. 
In recent investigations on virus diseases of plants, Kenneth Smith * 
and others have demonstrated that the symptom expression of a complex 
or association of two viruses in certain hosts is quite different from that of 
either virus acting alone. Viruses profoundly modify the metabolism of 
the plants they infect and a promising line of enquiry is the influence 
which they have on the incidence of specific fungus and bacterial diseases. 
In this connection Prof. Murphy of Dublin informs me that the potato 
variety Champion, which is the early days of its cultivation in Ireland was 
very resistant to Phytophthora Blight and is now very susceptible, has, in 
his opinion, lost its resistance through more or less universal mosaic infec- 
tion. This opinion is borne out by the investigations of Davidson,** who 
has shewn that virus-free stocks of this variety are still markedly resistant 
to Blight. 
On the other hand, the effect of micro-organisms on one another is 
frequently one of antagonism. ‘Two organisms may mutually inhibit the 
development of each other, or one may be greatly impeded in its growth 
by the other. In the latter case one organism may exercise some toxic 
influence on the other or it may utilise the available food material so 
rapidly as to starve the second organism. Factors of this kind may perhaps 
47 Phytopathology, 24, p. 1045 (1934). 
‘8 Cambridge Ph.D. thesis, 1935 (not yet published). 
49 Florida Agric. Exp. Sta., Annual Report, 1912. 
50 Jour. Agr. Res., 24, 191 (1923). 
51 Ann. App. Biol., 22, p. 23 (1935)- 
52 Bacteria in Relation to Plant Diseases, vol. iii, p. 155 (1914). 
58 Proc. Roy. Soc., B, 109, p. 251 (1931). 
54 Econ, Proc. Roy. Dublin Soc., 2, p. 319 (1928). 
