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



Affected tissues are water-soaked, somewhat grayish-green or 

 brownish, without definite odor, and semiwatery in consistency. The 

 characteristic grayish-brown mycelium and spore masses of the path- 

 ogen that later develop over the lesion aid in identifying the decay. 

 In the field or greenhouse the disease resembles watery soft rot 



(p. ii). 



The destruction of diseased plants and plant trash in the field and 

 around the packing house and the use of low temperatures during 

 the transit and marketing periods are the chief measures for control- 

 ling the disease. 



(See also Globe Artichokes. Gray Mold Kot, p. 6.) 



(See 101, 127, 128, 150.) 



Spotted Wilt 



Spotted wilt of lettuce has become increasingly important in some 

 of the coastal districts of California, particularly on the late-summer 

 and early-fall crops. It has not been found either in the Imperial 

 Valley of California or in the lettuce-producing districts of Arizona. 



The disease is caused by a virus that is transmitted from plant to 

 plant through the feeding activities of the onion thrips {Thrips tabaci 

 Lind.) and the flower thrips (FranMiniella tritici (Fitch)). The 

 virus has a wide range of suscepts among which are included weeds, 

 winter-crop plants, ornamentals, and a number of different vege- 

 tables. The disease has been reported on endive and spinach as well 

 as on all varieties of lettuce tested. It has not been observed on 

 globe artichokes, beets, rhubarb, or Swiss chard. 



The symptoms of spotted wilt on lettuce are slight yellowing, 

 petiole curvature, and marginal wilting of the leaves. Plants in- 

 fected while young flatten out and soon die. The most conspicuous 

 symptom on plants of all ages is the development on the leaves of 

 brown, sunken, necrotic streaks and spots that are particularly preva- 

 lent on the lower part of the midrib (pi. 7, A). Any of the leaves, 

 including the heart leaves, may be affected (pi. 7, B). There is thus 

 considerable resemblance between the symptoms of spotted wilt and 

 brown blight (pi. 5, C and D). In the case of spotted wilt, how- 

 ever, the plant is usually attacked on only one side and there is a 

 pronounced curvature of affected leaves. Another important differ- 

 entiating symptom is the diffuse brown discoloration of the pith and 

 cortex that is evident upon making a fresh cut through the stem. 

 This is usually more pronounced on one side of the plant. 



Affected heads are occasionally seen on the market, where at times 

 the disease may have been mistaken for brown blight. 



(See 27, 28, 29, S3, lift, 146.) 



TlPBURN 



Tipburn is a physiologic disease that is found wherever lettuce is 

 grown. It is of considerable importance on the market. 



As suggested by the name, the disease is characterized by a burning 

 or dying of the leaf tissues. Tipburn ordinarily does not affect the 

 older outer leaves but is confined to the more actively growing inner 

 ones. Thus it is not always apparent from the outside of the head. 



