MARKET DISEASES OF FRUITS AND VEGETABLES 15 



CONTROL MEASURES 



The best control for phytophthora rots is obtained by making sure 

 that only healthy roots from fields free from disease are used when new 

 plantings are made. Roots showing any internal discolorations should 

 not be planted, and soil known to be infested should be avoided. 



The spraying of young plants and the soil about the crown of the 

 root with bordeaux mixture (4-4-50) before the disease appears has 

 been recommended as one means of control. 



Although no data are available regarding the development of decay 

 during transit, the possibility of such decay occurring makes it un- 

 desirable to pack for shipment any rhubarb showing any evidence of 

 infection. • 



(See 5, 31, 95.) 



Stem Spot (Leaf Spot) 

 (PhyUosticta straminella Bres.) 



This stem and leaf spot disease is sometimes of market importance 

 on Illinois and California rhubarb. The leaf spot stage is found on the 

 young as well as the old leaves. Small, greenish-yellow spots on the 

 upper surfaces of the leaves indicate the first stages of infection. As 

 these spots enlarge they become tan, with wine-red borders. Even- 

 tually portions of the dead tissues drop out, leaving ragged holes. 

 When the stems are infected the most serious loss generally occurs in 

 the first cuttings. Stem lesions are characterized by small, oval to 

 oblong, reddish-brown spots. With age some of these may become 

 elongated and extend one-half inch or more lengthwise of the stem. 

 In both the leaf and stem spots small, black fruiting bodies (pycnidia) 

 eventually appear in the dead tissue. The presence of these black 

 pycnidia generally serves to identify this disease. In some instances 

 Botrytis sp. and secondary fungi, such as Rhizopus, Cladosporium, 

 and PenicilUum, invade stem spot lesions. 



Experimental work has shown that infection may occur in healthy, 

 uninjured tissue wherever the fungus comes in contact with moist 

 stems. However, it appears that the development of lesions takes 

 place so slowly that no new spots would develop during transit. 



In most localities this disease probably can be controlled by re- 

 moving the infected leaves as soon as the disease is detected and by 

 destroying dead leaves and stems at the end of the season so that the 

 fungus will not be harbored hi the plant refuse. 



(See 125, 131.) 



SPINACH 



Spinach for the market consists of the leaves and crown of the plant 

 (Spinacia oleracea L.) . It should be of good green color, fresh, crisp, 

 and free from blemishes and decay. 



The more important diseases that affect spinach are anthracnose 

 (C olletotrichwm spinaciae Ell. and Halst.), bacterial soft rot, damp- 

 ing-off (Pythium ultimum), downy mildew, heterosporium leaf spot, 

 other leaf spots (Cercospora beticola, 0. flagelliformis Ell. and Halst., 

 and PhyUosticta chenopodii Sacc.) , rust (Puccinia aristidae Tracy) , 

 white rust, wilt (Fusarium spp.), and the virus diseases cucumber 

 mosaic (blight or yellows) , beet mosaic, curly top, and spotted wilt. 



