' WET-ROT ' 707 



spot should be wetted and ' gently ' scraped with the point of a pen-knife : 

 transfer the small portions scraped off into water and examine with a low 

 power. If present, the dark-coloured multicellular conidia are readily 

 observed. 



(iii) Diseases of the potato tuber. 



The fungi previously mentioned mainly destroy the leaves 

 and stems of the potato plant, and although it is possible 

 that under certain circumstances Phytophthora infestans does 

 direct damage to the tubers, the most extensive destruction 

 of the tubers is chiefly due to other causes, many of which 

 are at present imperfectly understood. 



(r) 'Wet-rot.' — In damp, warm seasons the tubers under 

 ground often suffer from what is termed ' wet-rot,' and the 

 same or a similar malady is very frequently observed among 

 potatoes stored in 'pies,' clamps and sheds. Moreover, 

 the sound tubers of early Ashleaf varieties, on the leaves of 

 which no Phytophthora has been observed, become affected 

 with this sickness if left in the ground for a time in such 

 seasons. 



The disease begins with the formation of dead patches 

 immediately beneath the skin of the potato, and the whole 

 interior is soon altered into a brown, watery and slimy pap, 

 often distended with gases. It is observed that the cell- 

 walls of the tissues are disunited, and the cell-contents, 

 except the starch-grains, changed and fermented by the ac- 

 tivity of a number of different species of bacteria. Recent 

 work appears to indicate that some of these bacteria present 

 are capable of attacking sound tubers and are primarily 

 responsible for the disease, although much of the putre- 

 faction is due to saprophytic bacteria and fungi which obtain 

 access to the tissues, after the latter are killed by parasitic 

 species. 



The fungus Rhizoctonia Solani Kiihn. with a purple violet 



