THE BACTERIAL BLIGHT OF BEANS: .BACTERIUM 

 PHASEOLI ERW. SM. 



By Ethel M. Doidge, M.A., D.Sc, F.L.S. 



{Read, July lo, 1918.) 



From time to time complaints have been received of the 

 discoloration and malformation of beans cultivated as a field 

 crop, and in every case bacteria were found in the tissues ; but 

 only recently has an opportunity occurred for isolating and 

 identifying the causal organism. 



The exceptionally heavy and continuous rains last summer 

 were favourable to the development of all kinds of fungous and 

 bacterial parasites, and not only were numerous samples of 

 diseased beans sent for examination, but plots of French beans 

 {Phaseolus vulgaris) and Lima beans {Phaseolus lunatiis) 

 planted for experimental purposes became infected, and so a 

 favourable opix>rtunity was presented for studying the disease. 

 The prevalence of the bean blight is probably not only due to the 

 wet season, but also to the fact that imported seed has been diffi- 

 cult to obtain, and that the local seed is not carefully selectted. 

 A local firm of seedsmen in applying for a recommendation for 

 a permit to import bean seeds, submitted for examination a 

 sample of seed which had been offered them by a Transvaal 

 grower, and which was infected to the extent of 75 per cent, with 

 Bacterium PhaseoU. 



Description of the Disease. 



The bacterial blight attacks the ordinary field and garden 

 varieties and also Lima beans, and its effects are most conspicu- 

 ous on the leaves, pods and seed. 



Like most leaf spot diseases, the first evidence of infection 

 is the development of minute water-soaked areas in the leaf 

 tissues ; these increase in size, and turn dark brown or black ; 

 and the leaf blade being thin, and consisting of only a few layers 

 of cells, the affected areas appear translucent if the leaf is held 

 up to the light. Under favourable conditions the spots formed 

 are so numerous that they coalesce, so that large areas of the leaf 

 turn yellow and the leaves fall prematurely. On other occasions 

 the area of infection spreads very rapidly, forming large brown 

 patches which are soft and limp in wet weather, but in any 

 weather become papery and brittle. On a badly blighted patch, 

 the leaves become dry and curled as if scorched. 



The pods may also become infected; if they are attacked in 

 the very early stages they shrivel and die, but on the older pods 

 the disease produces spreading watery spots which finally become 

 discoloured, but are never sunken like those caused bv the 

 anthracnose fungus (Collefofriclium lindemuthianum). From 



