482 '' BlacMerf of the Potato 



browning is limited to the bundles at the "heel" end only; this is 

 usually the only sign of disease exhibited in the young tuber, the storage 

 tissue having quite a normal appearance. Microscopic examination of 

 a section across one of these bundles shows the vessels filled with a 

 mass of bacteria : from the vascular bundles the organism under suitable 

 conditions rapidly invades the starchy tissue reducing it to a soft 

 pulpy mass with very offensive smell. Potatoes in such a condition 

 are frequently found in the field, especially if rain be followed by a hot 

 baking sun. In the important potato-growing districts of Lancashire 

 these tubers, thus rotted in the soil, are locally termed " par -boiled.''' 

 Microscopic examination of the pulpy mass reveals the cells separated 

 from one another, the substance of the middle lamella being apparently 

 dissolved by an enzyme ; the starch grains appear to be quite unaltered. 

 The propagation of the disease is usually assumed to be due to the 

 planting of diseased "sets"; von Hegyi(8) however who investigated 

 "Blackleg" in Galicia and Prussian Silesia has stated that in every 

 case the "blacklegged" shoots examined by him bore evidence of the 

 attack of wire-worms, and that biting insects are a necessary factor 

 for the entry of the parasite into the host. The fact that the parent 

 "set" from which the diseased shoots have arisen will almost invariably 

 be found to have decayed points to the planting of diseased "sets" 

 as the source of the disease in most cases. This is further emphasised 

 by the fact that the disease makes its appearance in isolated individuals 

 and is only rarely to be found affecting even small patches. The general 

 lack of anything in the nature of an epidemic of "Blackleg" spreading 

 from a certain focus of infection would seem to indicate that propagation 

 by wire-worms is not usual, though undoubtedly biting insects may be 

 instrumental under certain conditions in introducing the parasite from 

 the soil. Morse(i4) gives one instance of such an epidemic spreading 

 over a patch of potatoes where the soil was exceptionally moist, and it 

 is probable that insects were the responsible agents in this case. 



ETIOLOGY. 



The disease known as "Blackleg" appears to be caused by a number 

 of different parasites. B. phytophthorus was isolated and named in 

 1903 by 0. Appel(i) in Germany and the same organism has been found 

 by E. F. Smith (18) and L. R. Jones (lo) as causing the disease in 

 America. In 1911 Pethybridge and Murphy (15) described the disease 

 under the name of "Black Stalk Rot." The organism which they 

 isolated from diseased plants in Ireland differs only slightly from that 



