POTATO SCAB 255 



h t stoma of Potato leaf; /, conidiophore ; j, conidium. F, a 

 free conidium germinating : k y conidium (spore) ; /, germ- 

 tube. G, a free conidium with contents breaking up into 

 zoospores. H, a conidium germinating and liberating zoo- 

 spores : m, conidium ; n, free zoospores with cilia (hairs). I, 

 zoospore become passive and germinating : 0, passive ; p, ger- 

 minating. J, zoospore germinated and germ-gum entered 

 Potato leaf through stoma ; f, stoma ; r, germ-tube. A-C, 

 two-thirds natural size. D-J, enlarged two hundred and fifty 

 diameters. 



POTATO SCAB. 



This affection on the skin of Potatoes may be due 

 to different agents. There are at least three forms 

 found on Potatoes. 



i. Scab not caused by parasitic organisms. The 

 brownish scale on the skin and cracking in the bark 

 of Potatoes, often forming large rough patches and 

 seriously affecting their market value, begins at a 

 very early stage of growth in the tuber. Both are seen 

 at first as small corroded spots, or minute open pus- 

 tules. In bad cases the spots and cracks become 

 confluent, and the whole skin or bark of the Potato is 

 unsightly. If the weather be dry, the tubers are not 

 injured more than the waste of substance for use, 

 indeed, the so-called dry-scab is a sure indication that 

 the Potato attacked is good and floury, though the 

 market value is depreciated in ratio to the degree of 

 scab. When the weather proves wet and the inner 

 portion of the tuber is exposed, as in scab, to the soil, 

 affected Potatoes acquire an earthy and disagreeable 



