June 5, 1916 Silver-Scurf of Irish Potato 347 



mother tubers taken on August 2, 1914, the date of the last examination, 

 from an oat field at Houlton, in which they undoubtedly over-wintered 

 in the soil. Many of these volunteer plants occurred in fields in which 

 rotation had not been practiced, the deep snows which covered the 

 ground the previous winter having protected the tubers. 



Whether the fungus may live over in the soil from which the tuber 

 host has been removed is not yet known, but that it may do so is not 

 improbable, in view of what occurs in the case of fungi having a similar 

 life history. Investigations to determine this point are now in progress. 



Several series of experiments were undertaken to ascertain how readily 

 5. atrovirens spreads from infected to healthy tubers and whether infec- 

 tion in this way might occur during the entire storage season. Inverted 

 bell jars were used in these experiments to secure moist chambers which 

 would hold a sufficient number of tubers for a satisfactory test and at the 

 same time retain uniform moisture conditions. A wire rack of X-inch 

 mesh was placed in each jar to support the potatoes and to prevent con- 

 tact w^ith the water in the jars, the inside of each jar was lined with blot- 

 ting paper to conserve the moisture and prevent the entrance of excessive 

 light, and the mouth was covered with window glass. Four varieties of 

 potatoes were used: Rural New Yorker, Green Mountain, Irish Cobbler, 

 and Bliss Triumph. A spore suspension of conidia which had been 

 grown in pure culture on sweet-clover stems for four weeks was sprayed 

 on the tubers with an atomizer, and for several days thereafter water 

 was sprayed into the jars with the atomizer to keep the air saturated. 

 A similar lot of healthy tubers was arranged as a check. 



The first series was begun at Houlton, Me., on March 26, 1914; and 

 within three weeks the entire surface of the inoculated tubers was covered 

 with dark-brown conidiophores and conidia, while the checks were free 

 from infection. Additional tests were made at Caribou, Me., on July 20, 

 1 914; Washington, D. C, in December, 19 14; Madison, Wis., on March 25, 

 1915; and Presque Isle, Me., on August 2, 1915. In each case infection 

 occurred wdthin three weeks after inoculation. 



Similar infection experiments were conducted upon young tubers just 

 harvested, as well as upon tubers still attached to the vines. In case of 

 the tubers attached to the vines the soil was removed and a spore sus- 

 pension was applied with an atomizer, whereupon the tubers were again 

 covered with earth. Checks also were made. In each of these tests 

 infection appeared upon tubers varying in diameter from i and 2 cm. 

 to full-grown tubers (PI. XLVII). Checks showed no infection. 



From these results it is apparent that infection from S. atrovirens may 

 take place at any stage in the development of the tubers and at any time 

 throughout the storage season. 



