New York Agricultural Experiment Station. 779 



in the laboratory, and it was only by laboratory study that the effi- 

 ciency of the treatments could be determined in such an investiga- 

 tion, where the number of tubers used in each test was small and 

 where the tests were so numerous. To have depended on field trials 

 in this case would have been out of the question. 



Accordingly another fungus was selected, Rhizoctonia, which pro- 

 duces several different potato troubles and is believed to be a common 

 cause of " skips " in the stand of plants. The means by which this 

 fungus maintains itself over winter are somewhat different from 

 those of most fungi. It does not penetrate the tuber with its mycelium 

 and remain alive therein as does late blight, nor mar the potato 

 with scars and deformities as does scab, nor does it form spores 

 that cling to the potato as do those of some of the smut fungi to 

 grain. But as the potato matures the Rhizoctonia forms on the 

 surface of the tuber, on which it grows in the soil, small rounded 

 collections of tightly-packed fungus tissue. These little bodies, 

 sclerotia, are very resistant and serve almost as well as spores or 

 seeds to maintain the life of the fungus. They appear on the potato 

 as small, brownish bodies, much like tiny lumps of dirt, but they 

 cannot be dislodged by washing as can dirt. They are frequently 

 so numerous as to make the potato rough and unsightly, and may 

 lessen its market value. 



Like other fungus tissues, these sclerotia are destroyed by fungi- 

 cides, but in them the microscopic cells are so tightly packed together 

 that some of them in the interior may not be reached by the chemical 

 and may remain alive even though the outer portion be destroyed. 

 When placed under proper laboratory conditions they easily start 

 into growth unless completely destroyed; so they make a very good 

 means for studying the efficiency of any treatment. Accordingly 

 tubers showing several of these sclerotia, which are very easily 

 recognized, were used in many of these tests and, for purposes of 

 comparison, others were soaked in formalin solutions and in cor- 

 rosive sublimate (mercury bichloride) solutions, using different 

 strengths of each. 



In many cases when exposed to the formaldehyde gas at standard 

 strength, for 24 hours, the Rhizoctonia sclerotia remained alive on 

 the tubers so that the fungus developed well in the laboratory cul- 

 tures. In many cases, also, other fungi appeared in these cultures, 

 showing that the gas treatment was not thoroughly reliable. In a 

 general way it may be said that the efficiency diminished as the 

 quantity of tubers treated increased, but there were some notable 

 exceptions to this. In a few cases in which the number of potatoes 

 was so small that the tubers themselves were injured by the gas, 

 quite a percentage of the sclerotia came through alive. Bacteria 

 in large numbers also developed after this gas treatment. As moist- 

 ened tubers showed more injury from the gas than dry ones, so 



