316 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



to test the matter further gave similar results. The results may be 

 summarized by saying that susceptibility to this particular disease is 

 increased by supplying the growing plants with large amounts of 

 available nitrogen. There is little to choose, as a rule, between the 

 evil effects of nitrate of soda and of sulphate of ammonia. On the 

 other hand, mineral manures, more especially the salts of potash, 

 decrease susceptibility, whilst starvation, e.g. on the unmanured plot 

 which has grown wheat continuously for many years, leads to a marked 

 degree of disease-resistance. In fact, by starving highly susceptible 

 varieties practically immune plants can be grown. Unfortunately 

 such treatment, though it checks the incidence of disease, does not 

 lead to the production of profitable crops, and the most the cultivator 

 can do is to strike a balance for himself and decide, as far as may be, 

 what particular system of manuring will give him, on the whole, the 

 most profitable crop. It is too early to say yet how far these results 

 apply generally, but tentative experiments carried out with Roses, 

 Hollyhocks, and Chrysanthemums seem to indicate that dressings of 

 potassium phosphate may help considerably in preventing severe 

 attacks of the diseases these plants are liable to. 



These observations suggested an interesting line of inquiry which 

 is being followed up by Spinks,* namely, the effects of various salts, 

 not usually available for plant growth, on the disease-resisting capacity 

 of plants. His preliminary observations were made at the Pot Culture 

 Station at Woburn. Here cultures of wheat were being grown under 

 uniform conditions, except that traces of salts had been added to the 

 soil. The plants had become attacked with the common mildew, 

 Erysiphe graminis, and there was no reason for supposing that all of 

 the plants had not had precisely the same opportunities for becoming 

 infected. The cultures growing in normal soil were moderately 

 attacked, the intensity of the disease being expressed by 4 marks. 

 The addition of magnesia resulted in a more severe attack, 8 marks 

 being assigned to these cultures. Zinc salts gave variable results. 

 The carbonate slightly decreased the severity of the attack as com- 

 pared with the control plants in normal soil (3 and 4 marks respectively) , 

 the phosphate produced no effect on the amount of the disease, whilst 

 the nitrate increased the severity of the attack to an extraordinary 

 extent (10 marks). The corresponding salts of lead gave much the 

 same result, the nitrate again markedly increasing the amount of 

 mildew present. The addition of traces of lithium salts, on the 

 contrary, served to diminish the severity of the attack. The phosphate 

 and carbonate produced practical immunity in the plants, whilst, with 

 the exception of one case, the. nitrate also diminished the amount 

 of disease compared with that on the control plants. Various experi- 

 ments have since confirmed and extended these results, and we now 

 have to recognize that the merest traces of different salts may pro- 

 foundly modify the plant's susceptibility to disease. The lines these 

 observations open up look to be of great promise, for they indicate the 



* Spinks, Journ. Agric, Sci. vol. v. p. 231. 



