Pei'hybridge and Murphy — Bacterial Disease of the Potato. 33 



become euriched with au orgamsin pathogenic to the potato. On large farms, 

 where a proper system of rotation can be carried out, this is, perhaps, not of 

 so much practical consequence ; but on small farms, and in gardens where 

 potatoes are, unfortunately, too often grown for several successive seasons on 

 the same land, there is the not inconsiderable risk of the new season's crop 

 contracting the disease fi-om the organisms left in the soil by the previous one. 

 For although it has not as yet been definitely proved, in the case of this parti- 

 cular organism, that plants may contract the disease from the soil, there can 

 be httle doubt, arguing from analogy with similar species, that such is likely 

 to be the case ; and experiments are arranged for next season which, it is 

 hoped, will settle the point. It is just on such small areas that remo^ong the 

 individual diseased plants would be most feasible. 



It is, however, doubtful whether infection of the plant from the soil is ui 

 the main responsible for the occurrence of this disease, or, at any rate, for its 

 spread in this coimtry. It seems certain that this is due rather to the planting 

 of aifected tubers. It has abeady been shown that, imder certain eircmn- 

 stanees, a tuber may be infected, and the rot may come at least to a temporary 

 halt. Such a tuber might show little or no external signs of being diseased, 

 and might easily be passed as suitable for " seed." When, however, such a 

 tuber is planted in the soil, the conditions of warmth and moisture are such 

 as to facHitate the resumption of activity by the parasitic bacteria, with the 

 result that sooner or later the stalks developing from such a tuber will 

 succumb to the disease. 



The following experience bears out this view. In 1909 tubers of tlie 

 variety British Premier were planted on land which did not grow potatoes in 

 the previous season, and which was manured with farmyard manure and 

 artificials. Early in July the plants were formd to be suffering to the extent 

 of 25 per cent, from Black Stalk-rot. The disease increased, and at the end 

 of the third week in the mouth the whole crop was raised. It consisted of a 

 considerable number of small potatoes, some of which it was easy to see were 

 attacked ; and they were therefore destroyed. The remainder were\boxed for 

 planting in 1910. The tubers kept weU over the winter ; and at planting 

 time there was nothing to indicate from an external examination of them 

 that they were otherwise than soimd. Half a square perch of laud, which 

 had grown healthy potato plants in 1909, and which received artificial manure 

 only, was planted with these tubers in 1910. No less than ninety-four per 

 cent, of the plants suecimibed to the disease during the simimer. Oir a 

 control plot of the same kind of land, similarly treated and planted with the 

 same variety of potato, the " seed, " however, being the produce of a perfectly 

 sound crop the pi'evious year, not a single plant was attacked by the disease. 



R.I. A. PROC, VOL. XXIX., SECT. B. [K] 



