MARKET DISEASES OF FRUITS AND VEGETABLES 29 



Nothing is known regarding the cause of the injury. No organ- 

 isms are involved. It may be a matter of the death of cells that for 

 some reason are more sensitive to external factors than their im- 

 mediate neighbors or that have a higher metabolic rate and conse- 

 quently break down more rapidly. Moist, warm beans held in a 

 confined space have been observed to develop somewhat similar dis- 

 color ations thus suggesting that russeting may be due in part to 

 respiratory trouble. Carloads of green beans have been known to 

 develop serious russeting during long transit periods. 



A somewhat similar type of surface discoloration of bean pods 

 is sometimes associated with mosaic. 



RUST 



{Uromyces pfiaseoli typica Arthur) 



Bean rust is world-wide in distribution and to some extent is found 

 each year in most bean-growing localities of the United States. It 

 may be very serious in localized areas, sometimes causing total loss 

 of the crop, but usually it is of minor importance in the field as well 

 as on the market. 



There is a considerable range in susceptibility to rust among varie- 

 ties of field and garden beans. Some of the pole beans and dwarf 

 field beans may be very severely affected. Dwarf garden varieties 

 of beans are sometimes affected late in the season under cool, moist 

 conditions, but ordinarily only slight loss is sustained. 



Rust causes most severe injury to the leaves of the bean plant, al- 

 though the stems and sometimes the pods are affected. The first 

 symptoms of infection are small dark-green or brownish raised pus- 

 tules commonly found on the under side of the leaves. These pustules 

 soon turn reddish brown and break open releasing innumerable spores 

 as a rustlike powder. Later in the season black powdery spore masses 

 are produced in pustules similar to the rust-colored ones. In severe 

 infections the leaves may die or the green leaf tissue may be so 

 reduced that the plant is not able to manufacture food enough 

 to set and develop pods. 



When bean pods become infected with rust small brown spots 

 are produced similar to those formed on the leaves. In advanced 

 stages these spots appear as rusty-brown pustules, which vary from 

 one-sixteenth to one-eighth inch in diameter and show definite slits 

 and feathering of the broken epidermis over the rusty spores within. 

 Although these pustules do not develop into decay they sometimes 

 blemish the stock sufficiently to reduce its market value. 



The causal fungus, Uromyces phaseoli typica, overwinters in bean 

 trash, and this serves as a source of inoculum the next season. 



The minimum temperature for spore (uredospore) germination 

 is 35° F., the optimum 58°, and the maximum 94°. " The optimum for 

 infection is about 63°. Regardless of temperature conditions, no 

 infections occur unless the relative humidity is very high (95 percent). 

 A period of high humidity lasting for at least 8 hours is necessary for 

 infections to take place. 



Varietal resistance to bean rust is so marked and has been so 

 well demonstrated in various localities that an effective control is 

 now within reach of all bean growers. The choice of varieties. 



