158 Diseases of Econo7nic Plants 



Downy-mildew ^^^' ^^-^ (Phytophthora phaseoli Thaxt.). ^ 

 Downy-mildew is a northern disease regarded by Halsted 

 as the most serious disease of the lima bean. It was first 

 recorded by Thaxter in 1889. 



Upon the pods growths of dense, woolly-white, irregular 

 patches appear. These consist of myriads of spore-bearing 

 hyphse of the causal fungus, laden with their spores. The 

 mildew also attacks young shoots, petioles, flower clusters, 

 and leaves, causing them to develop in dwarfed, imperfect 

 fashion and destroying their value. The spores are largely car- 

 ried by nectar-seeking insects, which accounts for the preva- 

 lence of flower infection. Spores are also carried by wind. 



Even in a season favorable to the development of the 

 disease, thoi-ough use of Bordeaux mixture, three sprayings, 

 will insure the crop. The destruction of infected trash is 

 advisable. 



Southern-blight {Sclerotium rolfsii). — The symptoms 

 are wilting of the leaves, temporary recovery, yellowing, 

 dying, and eventually dropping. The fungus usually at- 

 tacks the plant just below the surface of the ground, in- 

 vades the cambium, and destroys it. For further details, see 

 pepper. 



Pod-blight ^'^^ (Diaporthe phaseolorum (C. & E.) Sacc, 

 Phomopsis). — This destructive lima-bean blight was first 

 recognized in New Jersey in 1891, and is now known gen- 

 erally in the East, producing upon pods and leaves large, 

 brown patches, with the pycnidia arranged in concentric 

 circles. When the disease is prevalent, the pods fail to 

 mature their seeds. 



Clean seed should be used and the plants sprayed with 

 Bordeaux mixture. 



Drop ^•"' (Sclerotinia lihertiana) . — This rot is reported to 

 cause a reduction of 30 per cent in yield in some fields. Loss 

 of nearly $3,000 is reported on one 30-acre field in Virginia. 

 It may develop rapidly in transit, and thus cause secondary 

 freight losses. Field refuse should be burned, and crop 

 rotation practiced. See also lettuce, 



