THE RHIZOCTONIA DISEASE OF POTATOES. 201 



much below the average in size some of these plants were 

 normal in general characteristics, but many of them had begun 

 to show some evidence of disease in that they were slightly yel- 

 lowish in color or at least lighter green than normally, often 

 having the beginning of a rossette appearance of the leaves. 

 Frequently these might readily pass from outward appearances 

 for plants suffering from an early attack of blackleg. 



An examination of such plants showed that their stems were 

 invariably diseased below ground. These stems which at first 

 had been kept covered as fast or faster than they grew, by 

 the method of cultivation or hilling, had blanched so that they 

 were nearly perfectly white. As a result the lesions produced 

 by the fungus stood out in marked contrast on this background. 

 Frequently, even though the crown of leaves on the shoot 

 appeared green, the stem would be so nearly cut off that it 

 would be impossible to remove the plant from the row without 

 having it break oft'. Figs. 65 and 66 illustrate the appearance of 

 some of the plants at this stage. Other instances were found 

 where one or more shoots from a seed-piece would reach the 

 surface of the soil before becoming badly diseased while other 

 sprouts from the same seed-piece had been killed back entirety 

 soon after they were put forth. In this connection it may be 

 mentioned that several bags of tubers produced on the field in 

 1913, after having stood in a warm cellar for some time, were 

 examined by the writers in the spring of 1914. At this time 

 the tubers bore sprouts from two to six inches long. A con- 

 siderable per cent of these sprouts showed lesions similar to 

 those observed on shoots in the soil the year before and in 

 many instances the sprouts had been entirely killed. Both in 

 the field and in the sacks the lesions on the shoots and sprouts 

 were plentifully covered with the threads of the Rhizoctonia 

 fungus which could be made out readily by the aid of a small 

 hand lens. As a rule the healthy shoots and sprouts did not 

 show the presence of these threads upon their surfaces. How- 

 ever, cases were observed in the field, much more frequently on 

 the Green Mountain than on the Irish Cobbler and more es- 

 pecially on plants which showed the Corticium stage above 

 where the filaments of the fungus grew up over the parts below 

 ground without any apparent injury to them. 



