MARKET DISEASES OF FRUITS AND VEGETABLES 11 



Although it may extend well into the head, usually the youngest 

 leaves in the center are not affected. 



The first symptom is the appearance of small, yellowish, translucent 

 areas or spots near the leaf margins. As these enlarge and become 

 more numerous the tissues near the edge of the leaf wilt, turn brown, 

 and die, thus forming an irregular brown border along the edge of 

 the leaf (pi. 6). Darkening of the veins in the affected areas usually 

 occurs. The diseased tissues may dry out somewhat and curl up 

 or they may remain moist. They frequently are later infected with 

 soft rot bacteria (p. 8) and become slimy. 



Tipburn is apparently brought on by a too-rapid loss of water 

 from the leaves. However, just what combination of environmental 

 factors is responsible for this is not fully understood. The disease 

 is known to be much more prevalent at high temperatures. Hot, dry, 

 sunny weather following cloudy, rainy periods is commonly believed 

 to favor the appearance of the trouble. Losses are greatest in lettuce 

 that is nearing maturity. There is some evidence that tipburn may 

 increase in severity during the transit and market period. 



Maintenance of a uniform supply of soil moisture, the use of good 

 cultural methods, and care in the application of fertilizer are all rec- 

 ommended as control measures. In California, planting is so ar- 

 ranged as to avoid the maturing of crops during the hottest weather; 

 during such periods lettuce is grown in the cooler coastal districts 

 rather than in the hotter valleys of the interior. Some progress has 

 been made in developing partially resistant varieties. 



(See 55, 82, 99, 118, 122, 128, 129, 143, 1U.) 



Watery Soft Rot (Sclerotinia Rot) 



{Sclerotinia sclerotiorum (Lib.) DBy. and S. minor Jagger) 



OCCURRENCE, SYMPTOMS, AND EFFECTS 



Watery soft rot is commonly found affecting a wide variety of 

 vegetables under transit, market, and storage conditions. It is fre- 

 quently of market importance on lettuce, endive, and escarole. 

 Formerly, when lettuce was produced extensively under glass, losses 

 from sclerotinia rot, or lettuce drop, or wilt, as it is also called, were 

 very serious. Under field conditions lettuce drop at times causes im- 

 portant losses wherever the crop is grown. 



As is indicated by the name, the chief symptom is the development 

 of a soft, watery decay that usually is first evident on the stem and 

 lower leaves near the soil surface. From there it may progress down- 

 ward, killing the roots or upward into the center of the plant, affect- 

 ing the bases of the leaves. As this takes place, the leaves progres- 

 sively wilt and collapse. The outer part of the plant shrivels and 

 dries ; the central part may be converted into a soft, wet, rotten mass. 



On the market the decay may be found anywhere on the head, 

 although it usually occurs at the basal part. Affected tissues are 

 water-soaked, light or pinkish brown, and without a characteristic 

 odor. In advanced stages they collapse to form a wet, leaking decay. 

 The presence of the cottony, white mycelium of the pathogen aids in 

 identifying the rot. Another characteristic sign is the scattered de- 



