58 



INJURIOUS INSECTS AND FUNGI. 



[Sept. 181)4. 



A New Disease of the Potato. 



In the Zeitschrift filr Pjlanzenkranhheiten a new disease of 

 the potato is described by Dr. Sorauer, which he says may be 

 called the " black dry rot of the potato." 



Dr. Sorauer says the affected tubers are large and exhibit 

 here and there a tendency to the formation of tubercles. The 

 cylindrical-shaped specimens are unusually light, and on tapping 

 the tubers it is easily ascertained that they are either hollow 

 inside or of an abnormally spongy consistency. Every specimen 

 exhibits externally traces of the dry rot, which almost always 

 enters the tubers from the root end. There are also here and 

 there in many tubers isolated patches of rot, whereas the ordinary 

 dry rot proceeding from the root or stalk end, and also per- 

 ceptible on the fibrous remains of the stalk, spreads in connected 

 patches. 



But the difference between this new disease and the true 

 " dry rot " is the circumstance that when the tubers are cut in 

 half, the healthy portion of the flesh, which is perfectly white 

 at the moment of cutting through the tuber, turns in 10 or 15 

 minutes, either in parts or over the whole of the cut surface, to 

 a rusty red colour. The rust-red surface changes later to black. 

 Individual tubers are filled inside with a black sappy mass so 

 that only the outer portions have maintained their normal white 

 condition and hardness. 



The cut surface of the black mass gets lighter in colour in 

 drying, it also contracts and becomes spojigy and tinder-like, 

 while the outer portions which have remained hard are at first 

 of a glistening white and change later to a normal chalky 

 white. 



Between the latter and the inner spongy mass there are thick 

 layers of fungus of a whitish-grey colour, and between these 

 the dark scabby patches formed by the mycelium of Bhizoctonia 

 Solani. 



The black, spongy, inner mass is generally entirely inter- 

 penetrated by a vigorous mycelium, which develops into dense 

 layers of white filaments in the holes gradually formed by the 

 contraction of the diseased flesh of the potato, and absorbs a 

 little of the brown colouring matter of this diseased flesh. Here 

 and there crystals are deposited which are not oxalate of lime, 

 but are apparently phosphate of lime. These crystals, which 

 are also secreted in the process of decomposition of other 

 reserve matter cells, point to putrid changes, as do likewise 

 the unusually large anguillidse to be found in the holes in the 

 flesh, and the myriads of bacteria infesting the entire tissue. 



In connection with this disease the result of an examination 

 with the Trommer probe is especially worthy of attention. By 

 this it is shown that the above-mentioned zone of transition, 

 lying between the black internal flesh infested with mycelium 

 and the healthy exterior zone, is very rich in glucose or similar 



