Sept. IS. 1919 Investigations on Mosaic Disease of the Irish Potato 263 



the insects could crawl most easily to the new host; (2) by introducing 

 aphids when the new host was young, 3 to 13 inches tall; and (3) by 

 introducing a rather large number, 40 to 220 by estimate. Cylindrical 

 cages consisting of coarse wire screening covered with fine cloth gauze 

 (5, PI. 6 B) were used to confine the aphids to the individual treated 

 plants. These effectually served their purpose. For the three treat- 

 ments with aphids — from mosaic potato, healthy potato, and radish — 

 the height of the tallest shoot of the new host when the aphids were 

 introduced was on the average 6, 5, and 7 inches, respectively. The 

 number of aphvds introduced was on the average respectively 130, 80, 

 and 120, while the average number of days the insects remained was 

 respectively 7, 14, and 9. After feeding on the new host for a week or 

 longer, the ap'iids were killed by nicotine fumigation intense enough to 

 cause the marjins and tips of some leaflets to become yellow and later 

 to die. This yellowing occurred on both the aphid-infested and aphid- 

 free plants and was in no way similar to mosaic mottling. It did not 

 occur on the leaves which were the first to show mosaic symptoms. 

 Frequent cyanid and nicotine fumigation of the uncaged plants was 

 practiced. No white flies (Aleyrodes vaporariorum Westw.) and very 

 few dispersed aphids were found at any time in the room occupied by 

 the plants included in this experiment. No other species of aphids was 

 found in the greenhouse. Thrips fed somewhat upon all the plants, 

 both those within cages and those uninclosed. 



The aphids were introduced in December and January. Symptoms 

 of mosaic were first seen in 1 8 to 3 1 days and then consisted of the mot- 

 tling characteristic of "slight" mosaic, but the mottling soon became 

 more pronounced and sometimes was accompanied by considerable 

 wrinkling. The average number of days that elapsed between the 

 introduction of the aphids and the time when mosaic symptoms were 

 first ascertained was 26. It might have been shorter if the plants had 

 been examined daily instead of semiweekly. The average height of 

 the tallest shoot at the time when the symptoms were first ascertained 

 was 20 inches. The symptoms appeared first in the one, two, or three 

 topmost leaves of an affected shoot, which, if already formed, w-ere still 

 very small at the time the aphids were feeding on the plant. 



The fact that a large percentage of the plants treated with aphids 

 from mosaic potatoes showed mosaic while the others, either untreated 

 or treated with nonvirulent aphids, all remained healthy, can be attri- 

 buted only to aphid transmission. A.s pointed out before, the group of 

 plants that showed mosaic came from the same tubers as the healthy 

 controls. Moreover, special precautions were followed because of the 

 previous tendency to regard mosaic as a physiological disease and there- 

 fore to neglect some operations normally followed in pathological work. 

 All the plants were grown in the same greenhouse room and were 

 arranged so that those with each type of treatment were distributed 

 122502°— 19 2 



