Apr. 15, 191 5 Phoma Destructiva 1 1 



first infection, dark streaks appearing along the petioles and spots upon 

 the leaf surface. Within 12 days infection had become general, spots 

 having been produced upon the youngest leaves as well as upon the 

 oldest. The spots varied from pinpoint in size to areas 5 to 6 mm. in 

 diameter. Frequently these spots united, forming irregular blotches, 

 the tissue within which became dark-colored and shrunken, until finally 

 the whole leaf yellowed and fell from the stem. Diseased leaves picked 

 from plants under bell jars and from plants in the infection cages were 

 brought to the laboratory for microscopical examination. Mycelium, 

 pycnidia, and spores of the Phoma were found, and by means of agar 

 plates the fungus was recovered in pure culture. The inoculated plants 

 which were kept under bell jars for only 48 hours and then exposed to 

 ordinary greenhouse conditions of temperature and moisture showed 

 some spotting on the lower leaves. These spots, however, increased only 

 slightly, and after three weeks the plants appeared to have thrown off 

 the disease and were making good growth. A few of the leaves taken 

 from a plant in the infection cage were photographed on March 20, 191 4 

 (PI. V). The average maximum temperature of air in the infection 

 cage during 10 days (March 10-20, 1914) was 7.5° C. higher than that of 

 the room, while the average minimum temperature of the cage was 4.1 ° C. 

 lower than the room temperature. 



Potato tubers (Solanum tuberosum). — Potato tubers (Irish Cobbler 

 variety) were thoroughly washed, soaked in a solution of mercuric chlo- 

 rid (i to 1,000) for two hours, and rinsed in distilled water. Stab inocu- 

 lations were then made into the tubers from an oat-agar culture of the 

 Phoma fungus, and the potatoes were placed in zinc boxes. The average 

 temperature within the boxes was 25° C. during a period of four weeks, 

 and the humidity was nearly 100 per cent, the boxes being kept wet with 

 moist cotton and filter paper. At the end of eight days a slight darken- 

 ing of the tissue was noticed about the needle stabs, but at the end of 

 four weeks no definite spots had been produced. Four other inoculation 

 experiments with potato tubers were tried, three in moist chambers at 

 temperatures ranging from 14° to 29° C. with a low humidity and one at 

 a temperature of 35° with high humidity. Under these conditions no 

 infection occurred. 



Sugar-beet plants (Beta vulgaris). — Young sugar-beet plants grow- 

 ing in pots at the greenhouse were inoculated on January 28, 1914. The 

 plants, which were 10 to 16 cm. high, were sprayed with a spore suspen- 

 sion of the Phoma fungus and placed under bell jars. Control plants 

 were used. The plants were examined at intervals of several days, but 

 showed no spotting in a period of three weeks. 



Half-grown sugar beets growing in an open bed in the greenhouse were 

 sprayed with a spore suspension of the fungus on February 4, 191 4. 

 These plants were not covered. In four weeks no spotting or other 

 evidence of infection had taken place. 



