EFFECT OF TEMPERATURE ON GERMINATION AND 

 GROWTH OF THE COMMON POTATO-SCAB ORGANISM 



By MicHAEt Shapovalov, 

 Assistant Plant Pathologist, Maine Agricultural Experiment Station 



INTRODUCTION 



The causal organism of the common potato scab which has been 

 known to phytopathologists since 1892 as Oospora scabies Thaxter was 

 recently pronounced by Lutman and Cunningham ^ as identical with 

 Actinomyces chromogenus Gasperini, which was described in 1891. The 

 writer's studies were conducted upon several strongly pathogenic strains 

 isolated from diseased specimens received from Maine, Vermont, and 

 Wisconsin.^ 



All these strains fruit abundantly on the so-called Thaxter's potato 

 agar. The gray film which almost invariably occurs on the scabby 

 spots of naturally or artificially infected tubers when first removed 

 from the soil is made up of the same elements which constitute the 

 fruiting stage in artificial cultures. These elements, called "gonidia," 

 are short, cylindrical segments of aerial filaments and when mature — 

 i. e., when the aerial growth turns from white to dark gray— were 

 employed in making the germination studies here described. They are 

 1.5 to 2// long and 0.8 to i/^ broad, with truncate ends. These bodies, 

 after having been sown in agar and shortly before germination, become 

 somewhat broader and rounder, sometimes oval or nearly spherical. 

 Germ tubes may be produced at either or both ends. 



EXPERIMENTAL METHODS EMPLOYED 



In making the germination tests the ordinary agar hanging-block 

 used in studying the growth of bacteria was employed. A straight 

 transfer needle was rubbed against the surface growth of cultures bear- 

 ing mature gonidia and then gently drawn across the surface of solidified 

 agar in Petri dishes. The agar blocks for germination studies were then 

 removed from along this inoculated streak and mounted in moist cells 



1 Lutman, B. F., and Cunningham, G. C. Potato scab. Vt. Agr. Exp. Sta. Bui. 184, 64 p., 7 fig., la pL 

 1914. 



2 The foUowing method was used to obtain pure cultures from these and numerous other specmiens: 

 Both the operator's hands and the diseased tuber are thoroughly washed. Then the latter is rinsed in 

 hydrogen peroxid and dried with sterilized absorbent paper. Next the corky covering of a scabby spot 

 is lifted o£F by insertmg the point of a flamed scalpel under one side of it. The layer of parcnch>Tna under- 

 neath is greenish yeUow in color, owing to the action of the parasite. The discolored area thus exposed 

 is then gently scraped with a flamed knife and a small quantity (about i c. c.) of the pulp transferred 

 to tubes containing 2 or 3 c. c. of steriUzed, distilled water. One or more 2 mm. loops of this dilution arc 

 transferred to tubes containing 10 c. c. of melted beef agar and the plates poured in the usual way. 



Journal of Agricultural Research. Vol. IV. No. : 



Dept. of Agriculture, Washington, D. C. ^^^ *S' '^'S 



Mame — ^4 



(129) 



