8 MISC. PUBLICATION 541, U. S. DEPT. OF AGRICULTURE 



powdery mildew (Erysiphe cichoracearvm) , stunt (Pythium sp.), tip- 

 burn, watery soft rot, and the virus diseases big vein, mosaic, spotted 

 wilt, and yellows. 



The diseases most frequently found affecting lettuce in transit and 

 on the market are bacterial soft rot, downy mildew, gray mold rot, 

 spotted wilt, tipburn, and watery soft rot. 



(See 9, 10, IS, 62, 69, 87, 100, 124, 146, 147, 154.) 



Bacterial Soft Rot 

 {Erwinia. carotovora (Jones) Holland and other organisms) 



Bacterial soft rot is the most important market disease of lettuce. 

 It is readily recognized by the soft, mushy consistency of the rotted 

 tissues (pi. 4). The decay is at times confined to the outer leaves, any 

 part of which may be affected. It commonly follows tipburn, brown 

 blight, and spotted wilt lesions and is thus noted on some of the inner 

 leaves. 



Little has been done in the way of identifying the specific pathogens 

 involved in the market decay of lettuce. The common bacterial soft 

 rot organism (Erwinia carotovora) is capable of rotting lettuce as well 

 as a number of other vegetable commodities. Whether Er. carotovora 

 is responsible for most of the market decay of lettuce or other bacterial 

 pathogens are of the greater importance has not been determined. 

 Among the latter are Pseudomonas viriolilivida (N. A. Brown) Stapp, 

 at times the cause of a destructive field disease; Bacterium vitians 

 N. A. Brown, which produces a field wilt and head decay; and Ps. 

 intybi. 



The shipping of lettuce free from lesions of tipburn and other dis- 

 eases and the use of low refrigeration temperatures during the transit 

 period are important means for controlling bacterial soft rot on the 

 market. 



(See also Spinach. Bacterial Soft Eot, p. 16.) 



(See IS, 14, 83, 129, 130.) 



Brown Blight 



Brown blight was at one time very important in the lettuce-growing 

 districts of Arizona and California, where it was particularly destruc- 

 tive in the Imperial Valley. The disease has been so effectively con- 

 trolled in recent years through the development and the widespread 

 planting of the resistant Imperial strains that it is seldom seen in the 

 field. Therefore it is unlikely that many affected heads will be found 

 on the market, The probability that other market troubles are being 

 confused with brown blight makes it desirable, however, to include 

 a description of the disease in the present publication. 



The cause of brown blight has not been determined, but the disease 

 is known to be soil-borne. Once it has appeared in a field it increases 

 rapidly with continued cropping. 



On plants attacked while small the first symptom of the disease is 

 the formation of yellow spots on the expanding leaves. All subse- 

 quently developed leaves likewise become yellowed, stunted, and flat- 

 tened, thus giving the plant a rosette appearance. This is followed 



