1920.] 



Potato Blight. 



-215 



It has been brought to the notice of the Ministry that there 



are being put upon the market certain proprietary dressings 



for seed potatoes which are stated to be 



Potato Blight: effective in preventing the development 

 Usele3sness of Seed ^ „ p^^^^^ g^^^ „ 



Dressings. , . ^ . ^ , , 



As this disease is reproduced each season 



from the mycelium or vegetive threads of the fungus actually 

 within the tissues of the tuber, it is useless to expect any 

 external dressing to kill this mycelium without also injuring 

 the tuber. Moreover, the subsequent epidemic character of the 

 disease is due to the development of spores on the foliage of 

 affected plants. These spores are carried from plant to plant 

 and from field to field. The Ministry, therefore, advises growers 

 to exercise caution. 



The only really effective preventive of blight is spraying at 

 the proper season, i.e., in late June or early July, with a good 

 fungicide such as Bordeaux mixture (copper sulphate combined 

 with lime) or Burgundy mixture (copper sulphate combined with 

 washing soda), applications used for many years in the vineyards 

 of the C ontinent . By the timely use of these dressings the spores 

 of the fungus are prevented from germinating and producing 

 the threads which grow into the tissues of the leaf. Well- 

 sprayed haulms, instead of withering under an attack of blight, 

 remain healthy and green ; the crop is increased and the 

 tubers themselves remain free from disease. It should be 

 remembered that even the most careful spraying can never 

 be so complete as to prevent all risk of infection, but it is 

 nevertheless a very efficient safeguard. It should be regarded 

 rather as a means of insurance, which wiU enable the plant to 

 tide over the time during which it is most liable to infection. 

 This period once well past, the work of tuber formation sufters 

 no check, and the yield is larger than would have been obtained 

 from a crop where the plants had been infected. The accumu- 

 lated evidence of many years justifies the conclusion that, in 

 an average season, the cost of insurance by spraying is amply 

 repaid by the greater yield of healthy tubers. 



Spraying too early is wasteful rather than helpful. With the 

 exception of Devonshire, Cornwall and the neiglibouring 

 counties it is not usually necessary to begin spraying until the 

 end of June or the beginning of July. 



Further information regarding " Blight " and its prevention 

 will be found in the Ministry of Agriculture's Leaflet Xo. 23, 

 which may be obtained gratis and post free from the otfice 

 of the ^Ministry, 3, St. James's Square, London, S.W. i. 



***** :ic 



Q 2 



