98 POTATO DISEASE. 



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lished itself on the foliage of previously healthy plants. They 

 assumed precisely the appearance of field plants attacked by the 

 disease in August. Leaf after leaf was affected, and in a few weeks 

 the plants above ground were entirely destroyed, while nearly 100 

 shoots of the same kind of potato planted at the same time and 

 placed under similar circumstances, save that they were shielded 

 from contact with the fungus, kept perfectly healthy, and remained 

 so for mouths afterward. 



It is at present rare that perfectly sound potatoes are employed 

 as seed. Actually rotten or badly infected tubers are of course 

 not used, but according to De Bary it may easily happen that 

 apparently sound potatoes actually contain the fungus. The fact 

 is well known that tubers which have been slightl}^ diseased, never 

 so to speak, recover from the injury without decay, the diseased 

 parts being separated from, though adhering to the sound, by a 

 layer of cuticular matter. The small scabs or brownish spots seen 

 on the surface of the healthy tubers, are not unfrequently the lurk- 

 ing places of the dormant fungus, which only needs the moisture 

 of the soil to develop abundantly. 



De Bary gives the following summing up of the cause and course 

 of the potato disease, viz : A parasitic fungus, Peronospora in- 

 festans, exists only by feeding on the potato plant. Its mycelium 

 penetrates the tubers in order to hibernate in them. Kept cool 

 and dry, it vegetates but slowly, or makes no growth ; but in the 

 warm season, or under favorable circumstances, it increases luxu- 

 riantly. Then the mycelium extends itself into the stems of the 

 potato plant, in order, earlier or later, to develop its spore-sacks, 

 which, transferred to neighboring parts of the plants, yield spores 

 that speedily penetrate the healthy tissue, and produce the leaf blight. 

 The parasite spreads from one or many such sources over the 

 field, and from one field to another — the foliage of the potato 

 becomes discolored, and the tops die down. Of the number- 

 less spore-sacks formed anew on the foliage, a large part lodge 

 in the pores of the soil, and there yield myriads of spores which 

 penetrate the earth. Some of them reach the tubers, and within 

 them develop again the mycelium, Avhich serves to ensure the con- 

 tinuation of the life of the fungus as the tuber ensures that of the 

 potato plant. When developed in large quantity it destroys the 

 tuber, producing the rot. In smaller amount it causes slight, often 

 imperceptible patches of disease, through which it comes another 

 year into the field, and renews its life, and perhaps its ravages. 



