MARKET DISEASES OF FRUITS AND VEGETABLES 7 



humid conditions are carried by air currents to growing plants in 

 the field. Although wounds and other breaks in the epidermis facili- 

 tate infection the mycelium can readily penetrate moist unbroken 

 tissues. Dead or dying tissues or those weakened by freezing or other 

 unfavorable conditions are more readily infected than vigorous, 

 healthy ones. 



The pathogen has a wide temperature range, from about 28° to 90° 

 F., for growth, sporulation, and spore germination. Optimum tem- 

 peratures are approximately 75° to 77°. Although the fungus makes 

 only slight growth at 28° to 32°, it is able to grow sufficiently at these 

 low temperatures to become established in the plant tissues. It is 

 because of this that vegetables which appear sound when removed from 

 refrigerator cars or from cold storage may develop gray mold rot very 

 rapidly when exposed to higher temperatures. 



Moisture is much more of a limiting factor than is temperature for 

 growth of the fungus, germination of the spores, and infection of 

 plant tissues. The disease is therefore particularly important under 

 conditions of moderate temperature and high humidity. Such con- 

 ditions exist along the coastal districts of central California where 

 globe artichokes are extensively grown. 



CONTROL MEASURES 



The only satisfactory control measure for reducing losses from gray 

 mold rot is maintaining as low refrigeration temperatures as prac- 

 ticable during the transit and marketing periods. At the same time 

 field sanitation practices and care in harvesting and packing are es- 

 sential. Where artichokes are held on the market it is advisable to 

 maintain a low temperature and as low humidity as possible. 



(See 86, 117.) 



LETTUCE 



Lettuce for the market consists of the leaves or leaf heads of the 

 lettuce plant (Lactuca sativa L.). Other common salad vegetables 

 botanically related to lettuce are endive, escarole, and chicory. 



There are four types of lettuce, namely, the crisphead varieties; 

 the butterhead varieties ; the cos, or romaine, varieties ; and the loose- 

 leaf, or bunching, varieties. With each type practically all the leafy 

 part of the plant is marketed. Consequently leaf diseases, whether 

 resulting in blemishes or decay, are factors directly affecting the 

 marketability of the crop. Even in the crisphead types, lesions on the 

 loose outer leaves may be important on the market for they may lead 

 to secondary decays which spread to the inner leaves. Diseases of 

 the roots, stems, or leaves may be indirectly important on the market 

 through stunting of plants and production of loose heads or leaves that 

 are poorly colored, bitter, tough, or wilted. 



Lettuce is subject to many diseases the following of which are the 

 more important: Anthracnose 4 (Marssonin-a panattoniana (Berl.) 

 Magn.), bacterial rots, bottom rot (Corticium solani), brown blight, 

 damping-off (Pythium ultimum Trow), downy mildew, gray mold rot, 

 leaf spots (Oercospora longissima Sacc. and Septoria lactucae Pk.), 



4 Gardner, M. W., and Kendrick, J. B. lettuce anthracnose in California. U. S. 

 Bur. Plant Indus. Plant Dis. RDtr. 22 : 125. 1938. [Processed.] 



