MARKET DISEASES OF FRUITS AND VEGETABLES 13 



few diseases which affect the leaves of the plant in the field and some 

 that attack the leafstalk directly. The leaf spots (Ascochyta rhei 

 Ell. and Ev., Cercospora sp.) and rust (Puccinia phragrrdtis 

 (Schum.) Korn.) are relatively unimportant compared with anthrac- 

 nose, crown and foot rots, rhizoctonia rot (Cortidum solani), and 

 stem spot which affect the leafstalks. 



Bacterial soft rot and gray mold rot are the most important diseases 

 of rhubarb on the market. 



(See 4,7,16.) 



Anthracnose 



(Colletotrichum erumpens Sacc.) 



Anthracnose has been found on rhubarb in the field in several 

 localities and on the market, where it is especially damaging because 

 the lesions detract from the appearance of the product and sometimes 

 open the way for secondary decay-producing organisms. 



Anthracnose is characterized by soft, watery, and translucent spots 

 on the leafstalks, which are the only part of the plant affected. The 

 lesions are usually oval in outline, with the long axis lengthwise of 

 the stalk. When a lesion attains a diameter of about one-half inch 

 very small, black specks (spore-producing acervuli) appear in great 

 numbers in the center of the spot. In advanced stages the whole 

 stalk is soft and rotten and covered with acervuli. This complete 

 decay is most often found in old, wilted stalks in the fields. Only 

 the small spots which escape the notice of the packer are found on 

 the market. 



The pathogen produces innumerable spores in the acervuli. These 

 spores are spattered by raindrops or are carried by insects from one 

 stalk to another. In moist, rather warm weather infection and 

 development of decay may take place in a short time. Wounds are 

 not necessary for infection; consequently there is a possibility of 

 the disease spreading by contact if infected stalks are packed with 

 healthy ones. 



No control methods have been developed. 



(See 181.) 



Bacterial Soft Rot 

 (See Spinach, Bacterial Soft Rot, p. 16.) 



Gray Mold Rot 



(Botrytis sp.) 



Gray mold rot is the most serious transit and market disease of 

 rhubarb. Although the causal fungus is practically ever present 

 wherever this crop is grown, it seldom affects vigorously growing 

 plants in the field. However, injured plants and old leaves become 

 infected readily under humid conditions. The causal fungus pro- 

 duces great numbers of spores, which contaminate the leafstalks 

 during harvesting and packing. 



On the market gray mold rot in the early stage appears as small 

 red spots on the sides of the leafstalk (pi. 8, A) and as water-soaked 

 brown areas at the base of the stalk or in injured tissues elsewhere. 



