594 Journal of Agricultural Research voi. xv, No. n 



umber. Its lesions are often depressed with raised margins, and the 

 pycnidia in macroscopical appearance are darker than the fruiting" 

 bodies of the brown canker (PI. 46, A). 



ISOLATION OF THE CAUSAL ORGANISM 



Following Keitt's method ^ for obtaining pure-culture strains, the 

 writer made single pycnospore and single ascus isolations. It was 

 thought advisable to isolate a single ascus rather than an ascospore, 

 as the ascospores, especially when immature, appear much the same in 

 in size and color as the pycnospores. From these isolations of the fungus 

 transfers were made to various media. From the imperfect-stage strain 

 only pycnidia developed, while from the perfect-stage strain growing on 

 sterilized rose canes the formation of pycnidia was followed by the de- 

 velopment of a perfect stage similar to that found in nature. The 

 growth from the pycnidial stage was labeled "A," and that from 

 the perithecial stage "B." Cultures of the two stages were used in 

 inoculation experiments. 



INOCULATION EXPERIMENTS 



Inoculation tests were made on rose plants under ordinary greenhouse 

 conditions and on cut rose canes kept in a moist atmosphere under bell 

 jars in the laboratory. The inoculations were made by smearing spores 

 or a portion of the culture over an incision in the cane made with a sterile 

 scalpel. The incision was then covered with sterile cotton moistened 

 with sterile water. 



Inoculatioiis with stage A in the greenhouse produced practically 

 no infection. On one plant, which was inoculated on May 28, 1917, 

 and placed under a bell jar, and which had gradually lost most of its 

 leaves from mildew, the disease developed after a long period, producing 

 the pycnidial stage of the fungus. Freshly cut rose canes placed under 

 bell jars became infected in a few days. On stems inoculated on Febru- 

 ary 8 the disease had progressed upward 4 inches by February 20; and 

 by February 23 it had progressed to the tip of the cane and had passed 

 downward from the point of inoculation a distance of 4 inches, darken- 

 ing and killing the stem. The cane was somewhat lighter in color for 

 about I inch above and % inch below the point of inoculation. In this 

 region were developed the characteristic pycnidia from which spore 

 tendrils were exuded. The controls subjected to similar treatment 

 remained fresh and healthy, and there was no discoloration of the tissue 

 near the point of inoculation. 



Inoculations were made with B in the greenhouse on October 20. 

 In four days there was slight infection. The plants were kept under 

 observation until January, but the area of infection remained very 



1 KBITT, G. \y. SIMPLE TECHNIQUE FOR ISOLATING StNGLE-SPORE STRAINS OF CERTAIN TYPES OF FUNGI. 

 In Phytopathology, v. s, no. s. P- 266-269, i fig. 1915. 



