MARKET DISEASES OF FRUITS AND VEGETABLES 19 



the disease. Accompanying the white sori is a slight yellowing of 

 the adjacent areas that is apparent on both surfaces of the leaf. The 

 yellowed areas are indefinite in outline and when seen from the upper 

 side are somewhat similar to those on leaves affected with downy mil- 

 dew. White rust usually appears first near the borders of the outer- 

 most leaves of the plant (pi. 10, B). Later the under surfaces of 

 all leaves may bear a few to many sori. Occasionally a few sori 

 are found on the upper surface. Leaves that are severely infected 

 may show brown, necrotic spots and the entire leaf may later turn 

 brown. 



There have been no adequate measures developed for controlling 

 white rust. 



(See 58,155.) 



SWISS CHARD 



Swiss chard (Beta vulgaris var. cicla L.), a close relative of beets, 

 is grown for its stalks and leaves, which are used as greens. Bac- 

 terial soft rot (p. 2) and cercospora leaf spot (p. 3) are the most 

 common diseases of this crop in transit and on the market. 



SWEETPOTATOES 



The sweetpotato of commerce consists of the thickened roots of the 

 sweetpotato plant (Ipomoea batatas (L.) Lam.). There are a great 

 number of varieties but only about a dozen are important for food in 

 the United States. These can be divided into two general types, viz, 

 the dry mealy kinds, such as the Yellow Jersey, and the moist fleshy 

 ones, such as the Nancy Hall. The former are preferred generally in 

 the northern markets and the latter in the markets of the South. 

 Regardless of variety the roots should be firm, smooth, well-shaped, 

 and free from blemishes and decay. 



Many diseases which attack plants in the field not only lead to 

 reduction in yield but also may affect the quality of the crop by causing 

 production of poorly shaped, poorly colored, rough, undersized, or 

 blemished potatoes. Some of the diseases which affect the growing 

 plants also cause blemishes and decay of the roots during storage and 

 marketing. 



The most important diseases which affect the growing plant are black 

 rot, foot rot, mottle necrosis, Texas root rot (Phymatotrichum omniv- 

 orum (Shear) Dugg.), sclerotium rot (Sclerotium rolfsii Sacc), soil 

 rot, and stem rot, or wilt. Scurf and rhizoctonia rot (Corticium 

 solani) affect the skin of the roots, making them unsightly and im- 

 pairing their keeping qualities. 



The following diseases lead to losses of sweetpotatoes in storage and 

 on the market by causing blemishes and decay : Alternaria rot (Alter- 

 naria sp.), black rot, blue mold rot, charcoal rot, dry rot, epicoccum 

 rot (Epicoccum sp.), end rots, foot rot, gray mold rot, Java black rot, 

 mucor rot, rhizopus soft rot, sclerotinia rot (Sclerotinia sclerotiorum) , 

 scurf, soil rot, surface rot, and trichoderma rot (Trie ho derma koningi 

 Oud.). Many of the storage diseases are less serious if sweetpotatoes 

 are cured for 10 days to 2 weeks at 85° F. and a relative humidity of 85 

 to 90 percent and subsequently stored at 50° to 55° and a relative 

 humidity of 80 to 85 percent. 



