376 The Study of Plant Diseases. 
In the case of the heteroecious Rust Fungi, the eradication of 
one of the hosts involved in the life-cycle has been advocated in 
some cases. In this connection the Law of the State of 
Massachusetts passed in 1755, forbidding the planting of Barberry 
bushes in the neighbourhood of wheat fields, may be recalled. But 
where such a fungus possesses a uredo stage, the work of Marshall 
Ward 1 has shown the possibility of this phase of the life-history 
surviving the winter and causing re-infection the following season 
without the intervention of the alternate host. 
Another means of controlling certain fungous pests is by the 
action of fungicides. The practice of spraying dates from the 
discovery by Millardet in 1883 of the toxic effect of Bordeaux 
Mixture (Copper Sulphate and Lime in certain proportions) upon 
diseases of the Vine. Bordeaux Mixture has ever since been 
deservedly popular with growers. Many other spraying fluids such 
as Liver of Sulphur, Lime-Sulphur, Formalin, etc., have come into 
use and sometimes are to be preferred to Bordeaux Mixture. It is 
clear that spraying will be most effective where the fungus is 
superficial, as in the Mildews. Spraying, however, is only a 
palliative in many cases and sometimes fungicides have almost no 
effect. 
A more promising line of advance lies in the introduction of 
varieties of cultivated plants which are immune to certain fungous 
pests. When one begins to consider the question of disease in any 
group of organisms one is struck at the outset by differences in the 
degree of susceptibility present in various individuals. In agricultural 
and horticultural practice, variations in this respect are clearly 
recognised. Thus some varieties of potatoes are known to be 
immune from the Blacl$ Scab or Wart Disease, and there are 
strains of Wheat which are not attacked by certain Rusts. It must 
be remembered, however, that cases are known in which a variety 
may be immune in one locality and susceptible in another, and 
there is some evidence that the same variety may be resistant at 
one period of the year and readily affected at another. 2 
Biffen 3 has recently shewn that immunity from Yellow Rust 
(Puccinia glumarum) of a certain variety of wheat (Einkorn) is a 
character which can be bred on Mendelian lines and in this way 
wheats have been produced by hybridisation which combine heavy 
cropping properties with the power of rust-resistance. 
1 Ann. Bot., 1905. 
1 Cf. Howard. “Journal of Agricultural Science,’’ 1907, p. 279. 
3 Biffen. “ Journal of Agricultural Science,’’ 1907. 
