738 Journal of Agricultural Research voi.xx, No. 10 



Attempts to islolate fungi or bacteria from the apparently healthy 

 plants were unsuccessful, at least in so far as it has not been possible to 

 isolate any organisms capable of producing the disease on reinoculation. 

 Furthermore, examination of the etiolated areas of the diseased plants 

 does not disclose the presence of any recognizable parasite. 



Variations parallel in every respect to those described above have 

 been observed frequently by the writer in the mosaic disease of beans. 



Approximately 75 per cent, or more, of the plants in these fields be- 

 came diseased. Frequent observations showed that aphids (Myzus 

 persicae Sulz.) were abundant on the lettuce during the time the disease 

 was developing. Similar conditions were observed in April, 19 19, 

 when the writer found what appeared to be the same disease in destruc- 

 tive amounts in several fields of head lettuce at Beaufort, S. C, which 

 were at that time nearly ready for harvest. Several growers stated that 

 aphids had been abundant in these fields a few weeks earlier. A disease 

 that seemed identical has also been observed every season for several 

 years in numerous localities in New York State, usually, however, affect- 

 ing only occasional plants and causing only minor losses. During four 

 seasons (1914-1917) it occurred in practically all fields of lettuce in the 

 vicinity of Rochester, N. Y., where aphids and other insects were usually 

 more or less abundant, while on the same farms lettuce grown during 

 the winter in the greenhouses, where aphids and other insects were held 

 at a minimum by fumigation, was usually entirely free from the disease. 



In order to follow up experimentally these observations, which sug- 

 gested a relation between the mosaic disease and aphids, several insect 

 cages were constructed of cheesecloth, which were large enough to per- 

 mit the growing of several lettuce plants under each. Lettuce of both 

 the Big Boston and Paris White Cos varieties was grown from seed 

 under the cages in the field at Sanford, Fla., during the winter season of 

 1920, particular care being exercised to prevent any aphids from reaching 

 the plants except when intentionally placed on them. Myzus persicae 

 Sulz. was used in all the experiments. 



On February 10 two aphids collected from several mosaic lettuce 

 plants were placed on each of 25 small healthy lettuce plants under an 

 insect cage. When these were examined, on March 8, there were 7 

 mosaic and 5 healthy Paris White Cos plants and 5 mosaic and 8 healthy 

 Big Boston plants. Twenty-five plants grown under an adjacent control 

 cage, under conditions comparable in every respect except that no aphids 

 had been placed on them, were all healthy, with the exception of one 

 mosaic plant. The plants were still small, having made slow growth 

 on account of cool weather. There were no aphids in the control cage. 

 In the aphid cage there were at least a few aphids on each plant, but they 

 were apparently not numerous enough to interfere materially with 

 normal growth. 



