September 22, 1922] 



SCIENCE 



343 



planation of the widespread occurrence of the 

 disease in certain seasons seems to be that 

 occasional plants which show mosaic early in 

 the season serve as the sources of infectious 

 material which is carried throughout the field 

 by vjrus-earrying insects. The probability of 

 this being the correct explanation is strength- 

 ened by the known facts concerning the spread 

 by insects of mosaic diseases of other plants. 



Indirect proof of the relation of insects to 

 the dissemination of bean mosaic was obtained 

 during the summer of 1921. Plantings in late 

 May of seed collected from plants showing 

 mosaic the previous season were made in rows 

 four feet apart. Another plot was planted 

 at the same time seventy-five feet away with 

 seed from plants grown for two previous sea- 

 sons under insect-proof cages. These plants 

 had shown no symptoms of mosaic. By the 

 middle of July, practically all of the plants in 

 the mosaic seed plot were showing symptoms 

 of the disease. None of the plants in the clean 

 seed plot had developed any signs of mosaic 

 up to the time of the appearance of the second 

 pair of leaves. Weekly inspection was made 

 and the first case of mosaic was found in one 

 plant of the Long White variety on June 12. 

 A count was made each week of the new plants 

 which had developed mosaic and on September 

 17, when the results were summarized, 19.5 

 per cent, of the total number of plants in the 

 plot, which included twenty varieties, were 

 affected with the disease. On July 15, clean 

 seed from the same source as the above was 

 planted in hills between the rows of diseased 

 plants in the mosaic plot, half of the seed 

 under insect-proof cages. By September 1, 

 every plant outside of the cages was affected 

 with mosaic, while not a single diseased plant 

 appeared in the cages up to the time the plants 

 were killed by frost. The diseased and healthy 

 plants at all times during their growth were 

 sufficiently far apart so that they did not come 

 in contact with one another. This experiment 

 indicated that insects are directly responsible 

 for the dissemination of the disease. 



In May, 1922, a planting of beans in the 

 greenhouse was found to be infested with 

 aphids. Several of the plants had early 

 shown symptoms of mosaic. Growing some 



distance from the aphid-infested plants were a 

 group of bean plants in water cultures. Prac- 

 tically every plant in these cultures developed 

 a severe case of mosaic, and examination 

 showed that the aphids were also abundant on 

 them. Golden Wax and Green Pod Stringless 

 beans were planted in pots in the greenhouse 

 and the pots placed in cages. Brittle Wax 

 beans were planted in the field and three cages 

 placed over a portion of the row. In addition, 

 a twenty-foot row of the same variety was 

 planted and left uncaged for observation as to 

 freedom of the seed from mosaic. As soon as 

 the greenhouse and field plants had developed 

 the first pair of leaves, aphids from the mosaic 

 plants were transferred by means of a camel 

 hair brush to watch glasses, and then placed 

 on the leaves of three plants in one of the field 

 cages, and on five plants of Golden Wax and 

 three of Green Pod Stringless in cages in the 

 greenhouse. Two cages of plants in the field 

 as well as the additional twenty foot row were 

 held for field checks. A large number of 

 plants in pots were held in separate cages in 

 the greenhouse as cheeks. The plants were 

 inspected daily. After five days, three plants 

 of Golden Wax and one of Green Pod String- 

 less showed apparent symptoms of mosaic as 

 water-soaked areas along the veins which 

 gradually involved the entire leaf. In addition 

 there was a distinct down-curling of the leaves, 

 usually characteristic of mosaic. This was con- 

 trary to usual observations which have been 

 that when mosaic is transferred to healthy 

 plants by aphids, symptoms of the disease do 

 not appear on the inoculated plant until the 

 new leaves have developed. However, with 

 both of the varieties used the mottling was 

 very distinct on the original leaves. Contrary 

 observations on other plants have probably 

 been due to the fact that it is difficult to detect 

 mosaic S3nQiptoms on old leaves. After twelve 

 days new leaves had appeared and these showed 

 marked mosaic S3Tnptoms. These plants were 

 held for eighteen days and the succeeding 

 leaves continued to present characteristic mo- 

 saic markings. None of the plants in the cheek 

 cages had developed any signs of mosaic one 

 month after the beginning of the experiment, 

 when they were discarded. 



