First experiments were undertaken in the greenhouse, where minute 

 amounts of streptomycin w^ere applied to the stems of bean plants before 

 inoculation of the leaves with the causal bacterium. The stems absorbed 

 the antibiotic and carried it up to the leaves in sufficient amounts to protect 

 them from infection. 



In subsequent field tests the disease was effectively controlled with 

 varying amounts of streptomycin, and in 1954 it was largely eradicated 

 from treated plants with an Agri-mycin spray. In plots treated 3 times at 

 weekly intervals with spray containing ZOO p. p.m. of streptomycin, only 

 0.8 percent of the plants were infected, as compared with 19 percent of 

 the untreated plants. The first spray was applied when the first signs of 

 halo blight appeared. Plots treated similarly with an Agri-mycin spray 

 containing 500 p. p.m. of streptomycin, beginning when the symptoms were 

 nnore advanced, showed no active infections shortly after treatment. In the 

 untreated plots, 26.5 percent of the plants were infected. 



Common Blight 



Caused by the bacterium, Xanthomonas phase oli , common blight differs 

 little from halo blight in observable symptoms. 



In greenhouse tests at the Agricultural Research Center, streptonnycin, 

 applied at a concentration of 1,000 p. p.m., protected bean plants from 

 common blight. Field plots that were thoroughly sprayed 3 tinnes with the 

 same concentration developed sonne diseased plants late in the season. 



DoNvny Mildew 



Downy mildew of lima beans is caused by a fungus, Phytophthora 

 phaseoli . It has caused considerable damage in some of the Middle Atlantic 

 and North Atlantic States, but it is of minor inaportance elsewhere. 



The disease is recognized by the white cottony growth that forms in 

 large, irregular patches on the pods. When young pods are attacked and 

 when older ones are covered with the fungus growth, they shrivel, wilt, 

 and die . 



Clean cultivation, crop rotation, and the planting of seed from an 

 uninfested crop are recommended. Dusting with a copper dust at weekly 

 intervals during periods of wet weather has been helpful in control of the 

 fungus but does not give full protection. 



In ARS greenhouse tests excellent control of downy mildew was ob- 

 tained with commercial streptomycin formulations. Almost complete 

 protection resulted when lima bean seedlings were sprayed with these 

 formulations at concentrations of 100 p. p.m. of streptomycin and were 

 inoculated 12 to 24 hours later with spores of the downy mildew fungus. 

 Almost complete control also resulted from an Agri-mycin spray con- 

 taining only 50 p. p.m. of streptomycin and the same amount of a neutral 

 copper fungicide. 



A dust containing 1,000 p. p.m. of streptomycin gave the same protec- 

 tion against downy mildew as sprays containing 100 p. p.m. On an acre 

 basis, dust would be cheaper per application than the spray, despite the 

 higher concentration. To be effective, however, the dust has to be applied 

 when moisture is on the plants, since the antibiotic is absorbed by the 

 leaves or stems only if moisture is present. 



