June 18,19x7 
Rhizoctonia solani on Potato 
4 T 5 
Inoculations on the tubers were also made. The method followed 
was to select clean and healthy tubers, immerse these for io minutes in a 
i to 1,000 mercuric-chlorid solution, wash with sterilized water, make a 
slight incision with a sterile scalpel, and insert a bit of a pure culture in 
the wound. Control tubers were treated in a similar manner, the agar 
medium without the fungus being inserted. The lesions produced with 
any of the strains were never very large, but a very distinct lesion was 
produced with R5 as compared with the other strain. Plate 26, J3, illus¬ 
trates a tuber inoculated with R5. For comparison a control tuber is 
shown in Plate 26, C. In several instances the lesion as a result of the 
inoculation extended to % inch in diameter. 
The strain R5, therefore, is not only more pathogenic on the stems than 
the remaining strains isolated from the potato, but is in fact able to 
produce a distinct necrosis of the tissues of the potato tuber. 
PHYSIOLOGICAL CHARACTERS 
The various strains were grown on a variety of media. Only those 
on which R5 has shown any marked distinguishing characteristics will 
be pointed out here. 
Potato agar. —On this medium, when grown in test tubes, R5 at the 
end of a week or 10 days produces a very marked discoloration of the 
medium. This coloration is dark brown, approaching black. The dis¬ 
coloration, if produced at all by the other strains on this medium, is very 
much less pronounced and never approaches the intensity of color pro¬ 
duced by R5. 
Corn-meal agar. —On this medium, when grown in test tubes, R5 
produces light-gray, loosely formed sclerotia as compared with the 
darker, brownish, and more compact sclerotia formed by the other 
strains (PI. 26, A), The character is very striking and can be relied upon. 5 
Uschinsky’s solution. —One hundred cubic centimeters of this solu¬ 
tion was poured into 200-c. c. Erlenmeyer flasks and inoculated with 
small bits of pure cultures of the various strains grown on potato agar. 
The rate of growth of R5 is far in excess of the remaining strains. At 
the end of 10 days R5 entirely covered the surface and was growing on 
the side of the flask, while the growth of the remaining cultures were still 
below the surface of the liquid. 
morphological characters 
In holding up against the light some petri-dish cultures on string-bean 
agar of the various strains, the writers were struck with an apparent 
difference in the fineness of the mycelial strands of R5 as compared with the 
remaining strains. A mount made and examined under the microscope 
revealed a distinct difference in the fineness of the mycelium. This dif¬ 
ference in the diameter of R5 and the remaining strains was very evident 
