MARKET DISEASES OF FRUITS AND VEGETABLES 9 



by slow browning and dying of the leaves and eventually in many 

 cases by the early death of the plant. 



The presence of brown, somewhat sunken, irregularly scattered 

 blotches and streaks in the leaves is the usual indication of the disease 

 on plants that are attacked after the heads have begun to form (pi. 5, 

 G and D). Usually these lesions occur along the midribs and larger 

 veins, with brown streaks occasionally extending into the vascular 

 tissues of the stem. Few or many of the leaves may be affected. The 

 lesions are usually more conspicuous on some of the outer leaves, be- 

 come less so toward the center of the head, and are not found on the 

 small heart leaves. Occasionally heads which become diseased about 

 harvesttime appear normal externally and may be packed uninten- 

 tionally for market. The symptoms on mature heads are very sim- 

 ilar to those of spotted wilt (p. 10) . 



(See 60,61, 64, 142.) 



Downy Mildew 

 (Bremia lactucae Regel) 



Downy mildew is of wide distribution on lettuce both under glass 

 and in the field. It is regularly of some importance in the commercial 

 lettuce-growing districts of coastal California and occasionally in 

 Florida. Endive and escarole are also susceptible. 



The first symptoms are light-green or yellowish spots scattered 

 over the upper surface of the older leaves. On the lower surface of 

 the leaves, beneath the spots, the pathogen develops as an incon- 

 spicuous white mold, which is barely visible when moist (pi. 5, A and 

 B) . Here are borne the spores of the fungus, which serve to spread the 

 disease to other leaves and plants. Through the development of new 

 spots and the enlargement of old ones considerable areas of the leaf 

 may become infected, turn brown, and die. These tissues are then 

 readily invaded by bacteria and fungi which cause secondary decays. 

 At times when downy mildew is prevalent in the field it constitutes 

 a serious transit and market problem. However, in recent years it 

 has been of only minor importance on the market although it is oc- 

 casionally found in small amounts on California lettuce and Florida 

 escarole. 



A number of strains of Imperial lettuce which are resistant to brown 

 blight and which were resistant to those strains of the downy mildew 

 pathogen that were prevalent at the time of their introduction 

 have been developed. There has recently appeared, however, a new 

 physiologic race of the downy mildew pathogen that will attack all 

 the hitherto resistant lettuce strains. 



(See 25, 63, 6^,84,94,96.) 



Gray Mold Rot 



(Botrytis sp.) 



Gray mold rot is occasionally of some importance on lettuce both 

 on the market and in the field. It was formerly a serious disease of 

 greenhouse lettuce. 



