20 MISC. PUBLICATION 4 4 0, U. S. DEPARTMENT OF AGRICULTURE 



pods together into nests. The abundant white cottony mycelium and 

 the leaky condition of the pods have given rise to the common name 

 "cottony leak" to describe this disease on the market. Cottony leak 

 appears very similar in many respects to watery soft rot on beans. 

 However, the cottony leak organism usually produces a greater abun- 

 dance of white cottony mycelium, but it never produces the black 

 sclerotia that sometimes characterize the advanced stages of watery 

 soft rot. Furthermore, it grows more rapidly at high temperatures, 

 whereas the watery soft rot organism develops best at cool to mod- 

 erate temperatures. 



Although species of Pythium have long been known to cause root 

 rots and damping-off of seedlings, apparently it was not until 1930 

 that a serious wilt of beans caused by this type of organism was re- 

 ported and then from localities as widely separated as Colorado and 

 Virginia. It was found that the organism infects the stems of the 

 bean plants at the soil line and causes a water-soaking of the tissues, 

 which often progresses upward into the branches and lower leaves. 

 The affected tissues are not discolored but are water-soaked and sof- 

 tened so that they slip off of the more woody vascular tissues. If the 

 temperature and moisture conditions are favorable, some plants are 

 wilted within a few days after infection takes place. At tempera- 

 tures near 90° to 100° F. the wilt progresses very rapidly. Plants 

 severely affected rarely produce any marketable pods. 



All varieties of snap beans appear to be susceptible to the cottony 

 leak organism. No control measures are known except to discard 

 at the packing shed all beans that show water-soaked lesions. Beans 

 harvested from fields known to be infested should be surface-dried 

 and cooled to a temperature of 50° F. or below as soon as possible, 

 in order to prevent the decay from spreading from one pod to another 

 during transit. 



(See 98, 99.) 



DOWNY MILDEW 

 (Phytophthora phaseoli Thax.) 



Downy mildew is one of the important diseases of lima beans, af- 

 fecting both the bush and pole varieties. It was first described in 

 1889, and since that time has been reported as causing more or less 

 serious damage each year. Downy mildew is prevalent mostly in the 

 Northeastern States. Reports from Connecticut, New York. Dela- 

 ware, New Jersey, Pennsylvania, Virginia. West Virginia, and Ohio 

 have cited losses due to this disease during recent years. 



The fungus is most often observed on the pods, but it may also 

 attack other parts of the plant. It appears as white, woolly masses, 

 consisting of dense patches of mycelium and conidiophores, which 

 usually are first noticed on one side of the pod, and under favorable 

 conditions spread to all parts of the pod (pi. 8, A). Injury to the 

 pod usually is limited to the area covered by the fungus. A purplish 

 border sometimes separates the healthy green tissue from the diseased 

 area. The fungus eventually may work through the pod and infect 

 the seeds. Severely affected pods wilt and shrivel. Young pods may 

 be killed outright or injured so that they do not mature. When af- 

 fected pods do mature their appearance destroys their market value. 



