The Bacterial Diseases of the Bean 47 



impossible to differentiate from those caused by the other bacterial or 

 fungous diseases that affect the bean seed. 



Phyt. medicaginis var. phaseolicola may infect the seed by working its 

 way through the pod tissue from local infections on the outside, or it may 

 enter the seed through the vascular tissue. In the latter case the bacteria 

 spread out underneath the seed coat. Very light infections of either type 

 are of the greatest importance, since they pass unobserved by the pickers 

 in the seed houses and the beans may be used for seed. A few infected 

 plants in a field are all that is needed for the disease to become epiphytotic 

 when the conditions are favorable. 



INOCULATION EXPERIMENTS 



Many cross inoculations have been conducted with Phytomonas medi- 

 caginis var. phaseolicola on about a dozen legumes and a few non-legumi- 

 nous plants. In these experiments the organism has shown itself to be 

 rather restricted in its pathogenicity. Besides the common bean (Phaseo- 

 lus vulgaris), only two members of the genus Phaseolus — the scarlet 

 runner bean (P. coccineus) and the sieva bean (P. lunatus) — proved to 

 be susceptible. All species outside of this genus used in the cross-inocula- 

 tion experiments were found to be immune. This is a host range more 

 limited than that of Phyt. flaccumfaciens or even that of Phyt. phaseoli. 



Negative results were obtained with the following species of legumes: 

 adzuki bean (P. angularis), mung bean (P. aureus Roxb.), cowpea (Vigna 

 sinensis), yard-long bean (V. sesquipedalis), soybean (Glycine max), broad 

 bean (Vicia faba), hyacinth bean (Dolichos lablab), white-flowering lupine 

 (Lupinus polyphyllus) , kudzu vine (Pueraria hirsuta), field pea (Pisum 

 sativum var. arvense), red clover (Trifolium pratense L.), alfalfa (Medicago 

 saliva), and sweet clover (Melilotus alba). Attempts were made, but 

 without success, to infect the following non-leguminous plants: potato 

 (Solanum tuberosum L.), cotton (Gossypium hirsutum L.), sunflower 

 (Helianthvs annuus L.), tomato (Lycopersicon esculentum Mill.), and 

 tobacco (Nicotiana tabacum L.). 



ETIOLOGY 



Morphology 



Phytomonas medicaginis var. phaseolicola is a fairly large rod with 

 rounded ends, occurring singly or in pairs. Single cells are sometimes 

 found which are slightly curved, and filaments are not uncommon on 

 certain media. The cells stain well with carbol fuchsin, gentian violet, 

 and Loeffler's methylene blue. Granules always may be demonstrated, 

 and in some cases they are bipolar. On beef-extract agar (pH 6.9), and 

 grown at a temperature of 27° C, single cells of a culture twenty -four 

 hours old measured as follows: 1.9/* (1.35 to 3.6/x) by 1.0/t (0.9 to 1.2/0- 



The pathogene is actively motile and a single polar flagellum has been 



