Pethybridge — On the. Rotting of Potato Tubers. 535 



tuber, causes them to be more or less firmly cemented together, so that when 

 the tuber is lifted from the soil these clusters of soil particles adhere to the 

 surface of the affected portions of the tuber, aud in some cases so tenaciously 

 that it is difficult to dislodge them even with a brush under a stream of water, 

 without tearing the skin. If the tuber is affected with one or other of the 

 vaiious forms of scab, as is frequently the case, or if the skin is otherwise 

 wounded, the same blackening is generally apparent at such spots, and soil 

 also adheres to them. 



Tlie skin over the attacked portions of the tuber, besides being darker 

 than usual, is also rather loosely attached to the underlying dead tissues, and 

 in many cases can be peeled off somewhat easily. Sometimes gas accumulates 

 between the skin and the underlying diseased tissues, forming a kind of 

 blister, and occasionally it escapes along with a certain amount of liquid, 

 forming accumulations of a frothy nature on the surface. The formation of 

 gas, like the development of pustules, is a secondary concomitant of the 

 disease. 



An attacked tuber is comparatively firm, but has a softish feel, and is to a 

 certain extent resilient like india-rubber. But if the tuber be squeezed really 

 hard, a quantity of the watery juice previously alluded to is expressed, which 

 is very distinctly acid to litmus, more so than the slightly acid normal juice 

 of the potato, and the tuber does not regain its original shape as rubber 

 would do. Affected tubers which are left lying for a day or two on a flat 

 surface become flattened beneath, and adhere to the object on which they are 

 placed. 



Partially affected tubers have been allowed to stand until in a few days 

 they become completely rotten, and further for a very prolonged period, in 

 order that the final stages of the rot might be observed. These vary according 

 as to whether the tubers are allowed to remain uncovered or covered. In tiie 

 former case the pustules developing on the surface are fewer aud the exuding 

 liquid evaporates almost as quickly as it is formed, so that the tuber gradually 

 dries up and becomes hard and woody, being covered with its wrinkled aud 

 contracted skin, and somewhat resembles one destroyed by the " dry rot '' 

 fungus, Fusarium. When the tubers are left in a covered dish, the exuded 

 liquid does not evaporate, and the decay would be designated a form of " wet 

 rot." The skin remains intact, not mucli wrinkled or folded, and within ii, but 

 by no means filling it, lies a pulpy, watery mass of dead tissue of foetid 

 odour, and containing a variety of animals such as mites, worms, maggots, 

 &e., as well as various fungi and bacteria. In the field or store, therefore, the 

 rot would probably be regarded as of the " wet " type. 



As regards the odour emanating from recently diseased tubers, this is not 



