Chocolate Spot Disease 



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CHOCOLATE SPOT DISEASE OR 



STREAK DISEASE OF BROAD BEANS. 



Sydney G. Paine and Margaret S. Lacey, 

 Department of Plant Physiology and Pathology, Imperial 

 College, London. 



This disease occurred as a serious epidemic in the summer 

 of 1920. Tt was recognised first by an extensive marking of the 

 leaves with chocolate-coloured spots, and was in many places 

 confounded with " Rust " (Uromyces fabae). Simultaneously 

 with the occurrence of the spots on the leaves there appeared 

 upon the stems long and short streak-like markings of a rich 

 bronze-brown colour, which recalled very forcibly the markings 

 on the stems of tomato plants suffering from the " Stripe " 

 disease. Investigation has shown that the bean disease is 

 caused by the same organism as that producing " Stripe " in 

 tomatoes. Now this organism was first described by Manns and 

 Taubenhaus as the cause of " Streak " disease in sweet-peas, 

 and was subsequently shown by them to produce streak disease 

 of many leguminous plants. It therefore seems advisable to 

 use the term " Streak " for this disease of beans, although 

 perhaps " Chocolate Spot " would more adequately describe the 

 most obvious symptom. 



Occurrence of the Disease. — It is probable that field beans 

 are never, or seldom, quite free from this disease, but it is only 

 under exceptional weather conditions that it assumes the form 

 of an epidemic, or does any considerable amount of damage. 

 Such exceptional conditions prevailed in the spring and early 

 summer of 1920. Hot, wet and thundery weather seems to h ive 

 been general just previous to the appearance of the first 

 symptoms of disease. The trouble was first reported from 

 Hampshire in the latter part of April and South Wales in May, 

 and rapidly spread from various centres. It was observed by 

 the authors in Devon during June, in Sussex in July, and was 

 reported to them successively from Buckinghamshire, Cam- 

 bridgeshire and Lincolnshire. It was undoubtedly very general 

 throughout a large part of England and Wales. 



Description of the Disease. — In a typical case, beans planted 

 in October, 1919, first showed signs of disease on 25th Max . 

 1920, small purplish-brown spots on the loaves and streak 

 lesions on the stems being observed on plants about five feet 

 high. When next observed, 8th June, the plants were largely 



