1146 



Potato Disease (Blight) in 1920. 



[Mar., 



POTATO DISEASE (BLIGHT) IN 1920. 



The continual wet weather of the summer of 1920 gave rise 

 to fears (not unfounded) that there would be a very severe out- 

 break of Blight, with considerably lessened production in 

 consequence. That these fears were not confirmed to the full 

 appears to be due to the fact that the wet was accompanied by 

 continued cold. Once again it was found that excessive wet alone 

 is not sufficient to cause a ' ' Blight year " ; a high temperature 

 is also necessary if the fungus is to become rampant. Yolkart,* 

 in a Swiss paper states that the fungus (Phytophthora infestans) 

 does not develop and spread until the temperature attains 20° C. 

 (68° F.), and this is borne out by seasons such as the present. 



The first signs of the disease in the South West were observed 

 m the Penzance district as early as 30th April, and it was fairly 

 general there by the second week in May. By June it had 

 attacked the main crops and by July it was prevalent throughout 

 Cornwall, many parts of Devon and Dorset, and was appearing 

 in Somerset. In the meantime it appeared in other districts, 

 especially in Lincolnshire (one exceptional case was reported 

 the third week in xApril) and by the middle of Jane it was of 

 common occurrence, and in July it was present throughout most 

 of the country. 



In low-lying and marshy districts, especially where sea mists 

 are common, so that there is almost constantly a damp atmo- 

 sphere, the disease was bad. Such areas were noticed in 1920 

 in both Lincolnshire and Sussex. 



On the higher grounds in Lincolnshire and elsewhere the 

 disease was later in appearing and not so bad when it did appear. 

 The same applied further north, although disease became very 

 bad towards the end of the season in Lancashire. This was 

 especially the case on heavy soils, and disease in such cases 

 considerably reduced the crop, which was already poor owing to 

 difficulty of cultivating and late planting due to continued wet. 

 On the other hand, disease was not as evident on the lighter 

 soils, and as the wet season suited these, heavy crops largely 

 free from blight were obtained. It was a general occurrence 

 this season to find that, notwithstanding -the almost universal 

 appearance of blight on the haulms, the tubers from light soils 

 were remarkably free, while on the heavy soils the trouble was so 

 prevalent on the tubers as severely to reduce the crop and make 

 damning almost inrooesible owing to the further risk of rotting. 



On the whole very little spraying was done. Owing to varying 



*A. Volkart. LanHw. Jahrb. d. Schweiz., Jahrg. 33, 1919, p. 38 [Abs. in 

 <Jentr: f. Bakt. 51, 1920]. 



