POTATO DISEASES 



1723 



has not rotted off, yet the plant was too 

 young and succulent to withstand the at- 

 tack for any length of time. 



Rhizoctonia, as it occurs in this coun- 

 try, probably does not directly damage 

 the tissue of the tubers, but it frequently 

 forms small, irregular growths on the 

 surface which give them a scabby or 

 dirty appearance. These fungous growths 

 are so firmly attached that a vigorous 

 scrubbing is necessary to break them 

 away. When wet they are black in color. 

 (Pig. 2.) 



Cause 



The fungus causing this disease is 

 closely related to the mushrooms and 

 toadstools. It was, for many years, con- 

 sidered a sterile fungus, and supposed to 

 have no true spore stage. This has been 

 disproved and the classification changed, 

 since it has been found that spores are 

 produced. 



The fungus can live indefinitely with- 

 out the intervention of the spore stage, 

 and it is probable that it depends largely 

 for its distribution and perpetuation on 

 the small patches of fungous tissue 

 (sclerotia) that are formed on the sur- 

 face of the potatoes and stems. (Fig. 2.) 



Control 



Since this fungus is able to live in the 

 soil indefinitely, it is necessary to ob- 

 serve special precautions against intro- 

 ducing it. It is introduced, in most cases, 

 on seed. Careful inspection for the pres- 

 ence of the small black bodies on the sur- 

 face of the potatoes should always be 

 made before planting and, in case they 

 are found, the general seed treatment 

 as recommended for potato scab should 

 be used. 



Liming the soil and rotation with crops 

 which are not affected is also recom- 

 mended where the disease is prevalent. 



Formaldehyde and Corrosive Sublimate 

 for Scab and Rhizoctonia in Potatoes 



The following discussion is from the 

 New York (Geneva) Station: 



It seems best to advise potato growers 

 who wish to disinfect potatoes they use 

 for seed to use the gas treatment only 

 in cases where it is impracticable to use 



either the liquid formaldehyde solution 

 or corrosive sublimate. The safety and 

 efficiency of both the liquid treatments 

 for scab have been thoroughly estab- 

 lished, while the evidence proves the gas 

 treatment unreliable for controlling 

 Rhizoctonia, and casts considerable doubt 

 on its effectiveness against scab. When 

 it is desired to treat potatoes for both 

 Rhizoctonia and scab the corrosive sub- 

 limate solution should be used; but in 

 treating for scab alone the formaldehyde 

 solution is effective, while the corrosive 

 sublimate solution, though effective, is 

 in many ways less desirable to use. 



Directions for Using Treatments 



The method to be used in applying any 

 one of these treatments is summarized 

 in the following paragraphs, which 

 should be carefully read before begin- 

 ning the work. 



?', AV ** ' 



Fig. 2. The small black patches on the sur- 

 face of the tuber are a resting: condition of 

 Rhizoctonia and furnish a means for its 

 distribution. 



Formaldehyde Gas 



Use a thoroughly tight, unheated room. 

 Place the seed tubers in shallow, slatted 

 crates, not over eight inches deep, and 

 so arranged that the gas may circulate 

 freely on all sides of the potatoes. For 

 each 1,000 cubic feet of space in the dis- 

 infection room use three pints of formal- 

 dehyde (40 per cent solution) and 23 

 ounces of potassium permanganate (slen- 

 der, needle-shaped crystals). Spread the 

 potassium permanganate over the bottom 

 of a large pan or pail having a capacity 

 equal to about one quart for each ounce 

 of permanganate. Pour on the formalde- 

 hyde, close the door at once and keep 

 it closed for 24 hours. It is important 

 that the disinfection room contain ap- 



