NON-PARASITIC, OR PHYSIOLOGIC PLANT DISEASES 577 



cases, when it becomes a lighter green sometimes with brown or reddish 

 flecks, where the tissues are dying. This malady is distinguished from 

 leaf-roll by the buUate, downward curling of the leaves, the persistence 

 of the normal leaf green and the general firmness of the leaves. It 

 results in the reduction in the yield of tubers, and in several cases no 

 tubers have been found. 



The nature and cause of this disease remain inexplicable. That it is 

 an hereditary trouble has been attested by German plant pathologists. 

 The tubers from diseased hills all develop into curly-dwarfs, while those 

 from healthy hills remain normal. The disease which is found in 

 Europe and in this country plays a large role in the deterioration of 

 potatoes. It seems from our knowledge of the disease that it is a 

 physiologic disorder resulting in a permanent deterioration of the 

 potato stock. It may develop at any time under the influence of 

 conditions not yet fully understood, and the vigor of the strain is reduced 

 apparently without any chance of its restoration. Perhaps it is concerned 

 with the senescence of the particular race of potatoes attacked or in 

 other words a varietal decline. 



The disease can be controlled to some extent by selecting tubers 

 from healthy hills, and if it is prevalent in a field of potatoes, it would 

 be better not to use any of the tubers from such a field for seeding 

 purposes. ' 



Bean Mosaic^ — Hundreds of acres of pea beans Phaseolus vulgaris in 

 New York showed the mosaic disease in 191 6 and in some fields prac- 

 tically every plant was affected and these plants rarely form pods. The 

 malady is not confined exclusively to the pea beans, but affects varieties 

 of dry and snap beans and perhaps is the same disease described by Mc- 

 Clintock as attacking pole and bush lima beans. The leaves of the 

 plants attacked by mosaic show irregular crinkled areas, somewhat 

 deeper green than the surrounding yellowish-green tissue. The dis- 

 ease is transmitted through the seed for diseased seedlings develop 

 from bean seeds taken from mosaic parents. The disorder has been 

 induced experimentally by rubbing healthy seedlings with crushed 

 leaves from diseased plants, the reaction taking place four weeks later. 

 The first signs of the "disease are seen about the time of blossoming. 



"Orton, W. a.: Potato Wilt, Leaf Roll and Related Diseases, Bull. U. S. 

 Dept. Agric. 64, 1914. 



' Stewart, U. B. and Reddick, Donald: Bean Mosaic, Phytopathology 7: 61. 



37 



