187 



166. This fungus belonging to ( l ) ph y to P h - 



the great group of the PJiycomycetes causes the most virulent \nfestans 

 disease from which the potato plant (Solanum tuberosum) is De B ar y. 

 known to suffer, and in countries where potatoes are exten- Species 

 sively grown the damage done by it not infrequently amounts attacked< 

 to a national calamity. In Ireland, for instance, one of 

 the worst famines of modern times has been caused by 

 this disease, and again in 1879 the loss occasioned by it 

 in that country was estimated at nearly 6,000,000. Up to 

 the present the disease in India has been chiefly confined to the 

 moister localities such as Assam, the Eastern Himalayas and the 

 Nilgiris, and experience in Europe has shown that an exposure 

 of 4 or 5 hours to a dry heat of 104 F. is fatal to the furigus. 

 From the fact, however, that the disease has of recent years 

 established itself also in the plains of Bengal it appears probable 

 that it will gradually spread throughout the country wherever 

 potatoes are cultivated. This fungus is also known to attack the 

 tomato (Lycopersicum esculentum) and a few other plants belong- 

 ing to the Solanaceae and Scrophidariaceae. 



The presence of the disease is usually first indicated by the Signs of 

 occurrence of brown patches on the leaves of the potato plants. the Dlsease - 

 These increase in size and depth of colour, coalesce with neigh- 

 bouring patches, and gradually extend over the whole leaf 

 surface. Similar patches often occur also on the petioles and 

 stem. The attacked leaves eventually shrivel up and drop off 

 leaving the bare stalks standing, or the leaves and stalks are com- 

 pletely converted into a rotten mass which emits a characteristic 

 and disagreeable odour. This latter is chiefly due to the decom- 

 posing action of bacteria on the tissues which have been killed 

 by the fungus. The extension of the disease is particularly 

 rapid in damp warm weather. 



If one of the attacked leaflets is now turned over and one of Life History 

 the characteristic brown patches on the under surface examin- and Dama s e 

 ed with a lens, a number of pale silky threads will be seen 

 standing up from the leaf surface at the edges of the dark spot, 

 thus producing a characteristic pale mouldy margin to the patch 

 which is particularly noticeable in damp weather. See 

 Plate XVI (1). These more or less erect threads are the 

 serial hyphae, or conidiophores, of the fungus which have been 

 sent out singly or in tufts through the stomata on the under 

 surface of the leaf for the purpose of bearing the conidia, or 

 asexual reproductive organs. These fine tubular conidiophores 

 are branched, and at the tip of the main filament and of each o{ 

 its branches the minute egg-shaped conidia are produced 



