186 SECTIONAL ADDRESSES 
by King, Hope and Eaton, *™ who have found in Arizona that the applica- 
tion of organic manures to soil infested by the cotton root-rot fungus 
(Phymatotrichum omnivorum) greatly reduces the disease. They consider 
that the organic manures so stimulate other soil organisms as to bring 
about conditions unfavourable for the root-rot fungus. The fungi causing 
foot-rot of cereals are very diverse, and further study of biological an- 
tagonism may throw light upon the reasons why this disease is chiefly 
caused by Fusarium culmorum in England, by Ophiobolus graminis in 
South Australia, by Helminthosporium sativum in New South Wales, 
and by O. graminis, H. sativum, Gibberella Saubinetii and Fusarium spp. 
respectively in different parts of Canada and the United States. 
Much attention is being paid at present to the fungus root diseases of 
perennial tropical crops such as rubber, tea, cocoa and oil-palms. The 
plantations are often established in land previously under high forest in 
which these pathogenic fungi are indigenous, but in which root diseases 
never assume alarming proportions. When the jungle is felled, however, 
and a rubber plantation for instance is made, the balance of nature is upset, 
with the result that fungi such as Fomes lignosus and Ganoderma pseudo- 
ferreum spread underground rapidly by means of rhizomorphs and become 
a potential menace to the plantation. Considerable progress has already 
been made towards a proper ecological interpretation of this class of root 
diseases, and I should like to pay a tribute to the work accomplished by 
tropical mycologists in this respect, especially in Malaya, Ceylon, the Gold 
Coast and the West Indies. : 
Certain important bacterial diseases of plants have been intensively 
studied in recent years, especially as regards their incidence in relation to 
environmental conditions. When I was a student there was considerable 
scepticism as to whether bacteria were ever pathogenic to plants, but it is 
now universally acknowledged that many serious plant diseases are caused 
by these organisms. I can refer to only a few investigations in this field. 
Stoughton ®? has made a special experimental study of the environmental 
conditions requisite for infection by Bacterium malvacearum, which causes 
the ‘ black-arm’ or ‘ angular leaf spot’ disease of cotton in the Sudan, 
Uganda and other countries, and Massey ® and Hansford and others ® 
have made notable contributions to our knowledge of the epidemiology 
of this disease in the field. Riker and his colleagues 7° in the United 
States have thrown further light on the Crown Gall disease of numerous 
plants caused by Bacterium tumefaciens, and have clearly distinguished 
the galls produced by it from the overgrowths which are sometimes the 
response to wounding. Wormald 7 has shewn that one of the most 
serious diseases of plum trees in this country is caused by Bacterium 
mors-prunorum. Day “ has brought forward evidence that the ‘ water- 
mark ’ disease of the cricket-bat willow in the eastern counties of England 
86a Tour. Agr. Res., 49, p. 1093 (1934). 
6 Ann. App. Biol., 20, p. 590 (1933). 
68 Empire Cotton Growing Review, 11, p. 188 (1924). 
8° Ann. App. Biol., 20, p. 404 (1933). 
70 Jour. Agr. Res., 48, pp. 887 and 913 (1934). 
71 Jour. Pomology and Hort. Sci., 9, p. 239 (1931). 
7 Oxford Forestry Memoirs, No. 3, 1924. 
