Mayi3. i9i8 Sonte Bacterial Diseases of Lettuce 371 



kind they could obtain, and it was all bought from the same source. 

 The grower who had the highest percentage of disease had grown sorghum 

 and cowpeas on his land and had plowed them under two to four weeks 

 before the lettuce was planted there. The sorghum was not decomposed 

 in November, so that very likely it made a splendid medium for the 

 bacteria to live and thrive in. They would then be ready to attack the 

 lettuce when it became weakened after the frost and could no longer 

 resist their entrance. 



There was one farm, which was inaccessible at the time of inspection 

 because of heavy rains, on which the neighboring farmers said there was 

 no disease. The growers claimed, too, that the same cultural and soil 

 conditions obtained on this farm as on the others. 



Two different bacteria were isolated from the Virginia lettuce plants, 

 and even from the same plant but from different spots. One organism, 

 which formed a distinct yellow growth on potato, proved to be identical 

 with that isolated from the South Carolina lettuce (PI. E, fig. 3). The 

 other organism proved to be the same as one already described as causing 

 a serious disease of lettuce in Louisiana in 1915.(5) This second organism 

 (Bacterium viridilividum) forms, or may form, an evanescent blue-green 

 growth on potato (PI. E, 1). Both organisms were inoculated into 

 lettuce plants, and both produced disease and later were reisolated. 



THE LOUISIANA LETTUCE DISEASE 



The Louisiana outbreak was at Nairn, Plaquemines County, La., 

 during the winter of 1914-1915. About 200 acres of lettuce plants were 

 infected, and the crop was almost a total loss. The plants were nearly 

 mature when the infection overtook them. The outer leaves of the heads 

 were the ones most affected, being either spotted or darkened throughout. 

 The disease did not start in the stem or roots, for the center of the heads 

 were sound and interior parts were rotted only when the disease spread 

 in toward the center from the outer leaves. There had been excessive 

 rainfall in this region for three months, and the unfavorable weather was 

 thought to be the cause of the disease. The lowland plants were affected 

 more severely than those on the high lands. A bacterium was isolated from 

 the spots on the leaves, and by repeated inoculations with it into healthy 

 lettuce plants it was proved to be the organism causing the disease. 



As infection started in the outer leaves, it is reasonable to suppose 

 that pathogenic organisms were washed up from the soil on the leaves, 

 and when the plants became weakened through unfavorable weather 

 conditions these organisms established themselves and the plants be- 

 came diseased. The name ''Bacterium viridilividum" was given this 

 organism, and a report made of the disease by the writer {8). Illustra- 

 tions of the Louisiana disease are included in this paper for compar- 

 ison, as none were published in the earlier paper (PI. E, 2; PI. 32, A, B; 

 PI. 35, A, B). 



