377 
The Study of Plant Diseases. 
Practically nothing is yet known as to the factors which confer 
immunity from specific fungous diseases. In the variety of wheat 
known as “ Einkorn,” which is immune from Yellow Rust, Miss 
Marryat 1 has shown that immunity is brought about by the too 
virulent attack of the parasite in its early stages of development. 
Thus instead of the fungus establishing a kind of symbiotic relation¬ 
ship with the host, it kills the cells in the immediate neighbourhood, 
and in consequence its progress into other living tissues is effectively 
barred. But the factors in the struggle between host and parasite, 
i.e., the parts played by toxins and anti-toxins, and the influences 
of chemotropism are still obscure. Potter 2 , working on a bacterial 
disease of turnips, has established the interesting fact that the 
disease can be checked by treatment with a sterilised extract of a 
culture on which the bacteria have been living for some time. The 
explanation of this phenomenon is that the organism secretes some 
substance which, upon being accumulated beyond a certain concen¬ 
tration, is detrimental to its own activity. 
One of the most interesting discoveries of recent times in 
regard to the fungi is that of the existence of the phenomenon 
known as “ specialised parasitism ” in certain groups. Eriksson 3 
was the first to show that Yellow Rust on Wheat was not physio¬ 
logically the same as Yellow Rust on Barley, although these fungi 
cannot be distinguished morphologically. Thus uredospores of this 
Rust derived from wheat will not infect Barley, and vice versa. 
Eriksson has determined the existence of many such specialised 
forms amongst the Rust Fungi which affect cereals. 
Marshall Ward 1 followed up this work by determining that 
there may be “ bridging host species ” within certain cycles of 
alliance. Thus he found that the Brown Rust of the Brome grasses 
present in host species “A” will not infect host species “ C,” but 
can infect another species “ B ” and subsequently pass from “ B ” 
to “ C.” 
Salmon, 5 working on the Mildews, has ascertained that the 
phenomenon of specialised parasitism is present in this group also. 
The same phenomenon has been discovered in other groups, but 
Botrytis and others appear to be pleophagous. 
1 Miss Marryat. “ Journal of Agricultural Science,” 1907. 
2 Potter. “ Journal of Agricultural Science,” 1908, p. 102. 
3 Eriksson. Ann. des. Sci. Nat., Vol. XIV. 
* Marshall Ward. Ann. Myc., Vol. I. 
4 Salmon. Ann. Myc., Vol. II. 
