288 



Potato Leaf-curl. 



[JUNE, 



The curling of the lower leaves, a feature which has been 

 largely overlooked in the past, is a very important one by 

 which to identify the disease, as it serves to distinguish it 

 from other maladies such as Blackleg and Verticillium Wilt, 

 where the upper leaves only exhibit this tendency. 



Leaf-curl is perpetuated by means of the " seed/' Its 

 normal course in the southern and drier counties, where it is 

 abundant, is as follows : During the hrst season that the attack 

 manifests itself (i.e., usually the second year's crop in the south) 

 the lower leaves in a certain number of plants show curl, and 

 there is a reduction in yield in these plants of about 25 per cent. 

 If tubers from such plants are saved and planted the following 

 season, more serious curling will be e\ddent and a further and 

 very serious reduction in yield will occur, most of the tubers 

 being merely of seed size. If these tubers are planted, growth 

 is often a complete failure and the crop almost nil. The 

 impojiance, therefore, especially to seed-potato growers, of recog- 

 nising the disease in its early stages (i.e., during the first year 

 when the lower leaves only are curled) and of rejecting all such 

 plants for seed purposes, is obvious. Other features of leaf-curl 

 are the faihire of the parent " set " to decay, and the 

 tendency for the new tubers formed by an affected plant to 

 cluster round the bases of the stems. 



Cause of Leaf-curl. — ^Though leaf-curl was formerly regarded 

 as being due to a parasitic fungus, it is now known that this 

 is not the case ; and by plant pathologists it is generally assigned 

 to the group of so-called " deterioration diseases," its actual 

 cause still being obscure. The symptoms manifested, such 

 as stunting of the growth, curling of the leaves and poorness 

 of crop, clearly indicate that the functions of the plant are not 

 proceeding normally. One important fact has been determined, 

 namely, that much of the food manufactured by the leaves of 

 diseased plants does not pass down to the new tubers as it 

 normally should, but remains in the leaf in the form of starch. 

 Other s^/mptoms of abnormal physiology have been discovered. 

 But whatever the primary cause of these disturbances, the eftect 

 is sufficiently great to influence the seed-tubers profoundly, 

 since tubers produced by affected plants give rise to diseased 

 plants the following year. The disease, therefore, may, in a 

 somewhat loose sense, be said to be inherited. 



In this country the most commonly accepted explanation of 

 the origin or cause of leaf-curl is that it is due to the use of 

 over-mature seed. Such a condition in the tubers might arise 

 as the result of cultivation in dry soils, particularly in those 



