GENERAL ADVICE 687 



diseases. — Although various specific remedies are given in the 

 chapters dealing with individual plant-diseases, it is advisable 

 to bear in mind the following points which are applicable to all 

 diseases induced by parasitic fungi. 



(a) Remember that the diseases are spread silently and 

 invisibly by means of spores of very minute size which are 

 readily carried about by the wind, by insects, and on the hands, 

 boots, and clothes of labourers in the field and garden, as well as 

 by implements used for cultivating the ground. 



{b) Never allow refuse from a diseased crop to remain in the 

 field or garden longer than is absolutely necessary ; wherever 

 possible collect it and burn it at once. 



(c) N€ver throw diseased leaves, stems, roots, or tubers upon 

 manure heaps, for many injurious fungi remain dormant through 

 winter in such places without damage to their vitality, and 

 are liable to spread disease among crops again when the manure 

 containing them is subsequently applied to the soil. Moreover, 

 many parasitic fungi are capable of living an active saprophytic 

 existence on manure heaps, and become parasitic again on plants 

 when opportunity offers. 



{d) Take active steps to check the development of disease 

 as soon as it is first noticed; early action always saves much 

 future trouble. The timely removal of a diseased specimen 

 from its neighbours often saves the latter from infection. 



(«) Keep down all weeds as far as possible, for many fungi 

 live upon and spread from these to useful crops. 



(/) After severe attacks of disease it is advisable to change 

 the cropping of the ground, selecting totally distinct genera 

 of plants to follow each other whenever possible : many fungi 

 live only on one species of cultivated crop. 



ig) Avoid overcrowding of plants, and endeavour to promote 

 their healthy growth by careful tillage, drainage and thorough 

 supply of fresh air to all parts of the plants. Plants weakened 

 by excessive or inadequate supplies of manure, heat and water, 

 often fall an easy prey to fungi. 



