298 



Bulletin 239. 



III. BEAN RUST 



The true rust of beans is, like the anthracnose, a fungous disease. It 

 dififers from the anthracnose, however, in many important respects. The 

 most important difference from the grower's point of view is that it is a 

 much less common and destructive disease. It occurs usually only on 

 the leaves of the bean, rarely on the stems and pods. Except in very 

 severe cases, it does not materially injure the leaves. 



- Epidermis 

 ' ' of Leaf 



"^-;; spores 



— Leaf Tissue 



YOLiaq Spore 



Fig. 1 15. — Diagrammatic section through a bean leaf affected with "winter 

 stage " of the "Rust." The spores are formed like the summer spores 

 on the ends of mycelial threads just beneath the epidermis. The spores 

 are, however, smooth and very dark brown or nearly black. Their walls 

 are thick and their contents oily, ivhich enables them to withstand much 

 cold and drying. 



The rust fung'us (Uromyccs oppciidicitlatiis) sends its mycelial 

 threads into the tissues of the bean leaf, there to secure nourish- 

 ment for its own growth and development. From the ends of the 

 threads that come to the surface at certain spots, spores are formed 

 (Fig. 114). These are of two kinds: the summer spores, brown and 

 forming powdery specks (Fig. 113 A) on the under side of the leaf which 

 readily rub off on the hands as a rusty brown powder ; the winter spores, 

 black and produced in small compact warts on the under side of the leaf 

 or sometimes also on the upper side (Fig. 113 B and C). The spots on the 

 upper side are commonly surrounded by a yellow border. The summer 

 spores appear rather early in the season and arc produced in abundance. 



