INVESTIGATIONS ON THE CONTROL OF DISEASE IN PLANTS. 



315 



towards providing external conditions unsuitable for the growth of 

 these pests. Nevertheless commercial crops of such plants as cucum- 

 bers are deliberately grown under the identical conditions the myco- 

 logist uses when he requires to carry out infection experiments. The 

 plants are forced as rapidly as possible, with the result that the tissues 

 are far more succulent than they should be, and the high temperatures 

 and saturated atmosphere, often so requisite for the germination of 

 fungus spores and the growth of their mycelium, are provided. In 

 such circumstances one cannot wonder that the plants readily fall 

 a prey to fungoid attacks which, under such conditions, are excessively 

 difficult, if not impossible at present, to cope with. There can be 

 little doubt that critical investigations on the best methods of culti- 

 vating such plants under glass would modify existing methods in a 

 direction which would provide less ideal conditions for the spread of 

 fungoid pests. Even in the open much can be effected by a judicious 

 choice of external conditions. Larch canker, for example, has to a 

 certain extent been kept under control by avoiding planting in situa- 

 tions known to offer especially favourable conditions to the fungus 

 causing it. 



Whilst temperature and water-content in many cases play an 

 important part in the incidence of plant diseases, the supply of food 

 and other materials in the soil is often of the greatest importance. 

 The rapidly increasing employment of artificial manures is gradually 

 calling attention to this fact by demonstrating that the more intensive 

 cultivation becomes the more plant diseases abound. As evidence for 

 this the following table showing the intensity of an outbreak of yellow 

 rust on the wheat plots at Rothamsted is worthy of some attention.* 

 Throughout the plots the variety of wheat grown was the same, the 

 plots only differing in the manurial dressing applied. This is given 

 in column I., whilst in column II. are marks assigned to each plot on 

 the basis that 0 indicates no disease, 12 an excessively bad attack. 



Table. 



I. — Manure. II. — Extent of Disease. 



None .......... 1-2 



Minerals. ......... 2 



Minerals + 200 lb. ammonium salts ..... 3 



„ + 600 lb. „ 3-4 



„ -J- nitrate of soda ...... 3 



Ammonium salts only ....... 6-7 



„ „ and superphosphate .... 10 



„ „ „ „ and sulphate of magnesia 12 



Ammonium salts ^ . , ( 3-4 



Minerals j alternating each season . ... £ q 



Rape cake ......... 8 



Minerals + 550 lb. nitrate of soda ..... 9-10 



A similar investigation of the manurial wheat plots on the Woburn 

 Experimental Farm of the Royal Agricultural Society revealed a very 

 similar state of affairs, whilst water-culture experiments designed 



* Biffen, R, H., Journ. Agric. Set. vol. iv. p. 425. 



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