loNov., 1910.] Tlic Spread of VCeeds and of Plant Diseases. 693 



of plants has been definitely proved, and that the previously unexplained 

 losses to which various crops are subject ha\e been shown to be due to 

 various injurious plant organisms. It has, for instance, been recently 

 shown that the mvcorrhiza fungi which develop on the roots of many 

 forest trees and are generallv supposed to aid them in obtaining food 

 from the humus in the soil may. under special circumstances, become para- 

 sitic and cause the death of the tree, particularly when in the young 

 seedling state. Large losses of one and two year old oak seedlings are 

 reported to have been produced in Russia in this w-ay and this may be one 

 of the causes of the hea-\-y losses which sometimes occur in forest plan- 

 tations without apparent cause. 



Even in the case of such a well-known disease as the common potato 

 disease, PhytopJitlwra infestans, wrong ideas as to its spread were long 

 prevalent. 1 he spores were .supposed to be carried any distance by the 

 wind, that is to say, by natural conditions which it was impossible to 

 control. It is now known that, in ninety-nine cases out of a hundred, the 

 disease is conveyed from one district to another by means of infected 

 tubers which formerly were supposed not to be able to carry the disease. 

 The spores are apparently responsible for the local spread of the disease 

 over small areas from each centre of infection so that a careful farmer using 

 clean seed mav keep his fields free from disease for a considerable time 

 in the middle of an infected area, particularly if his land is surrounded 

 by a belt of timber and his fields by good hedges, and may escape infec- 

 tion if timely measures are taken to suppress the disease in the surround- 

 ing districts. 



Half a century ago belts of timber were found to delay for a time the 

 spread of the terrible coffee disease {Hemihea vastatrix) which rapidly 

 wiped out the coffee industry in Ceylon, and it is possible that, if a suffi- 

 ciently broad quarantine belt had been maintained and proper regulations 

 enforced, the plantations might, in part at least, have been saved. Without 

 regulations to restore the balance of nature, and to counteract the artificial 

 conditions which favour the spread of disease in any widely cultivated 

 plant, a similar history is capable of repetition at any time and in any 

 country. 



Many other instances of similar character could be given, but sufficient 

 has perhaps been said to emphasize two points, which are of great im- 

 portance. Firstly, that the spread of weeds and of plant diseases takes 

 place to a far greater extent as the direct or indirect result of the activities 

 of man than it does by uncontrollable natural agencies. Secondly, this 



being so, very much more can be done to prevent the spread of weeds 

 and disease by the increase and spread of knowledge, and by suitably 

 directed regulations for bringing that knowledge into play, than would 

 otherwi.se be the case. It is easy, for instance, to prevent the sale and 

 planting of diseased potato tubers, or the sale of wheat or other seeds 

 infested with the spores of parasitic fungi or mixed with the seeds of 

 injurious weeds, but it is not possible to any extent to prevent the spread 

 of spores or seeds bv the wind, although it is of course possible to give 

 them no opportunity of developing. It has been possible in man by 

 suitable precautions and regulations to practically suppress certain diseases, 

 and there can be nO' doubt that the same will ultimately be found possible 

 in the case of certain at least of the more injurious diseases of plants. 



