12 Walter H. Burkholder 



Twice these beans were inoculated by spraying with a water suspension of 

 Phyt. phaseoli, but the disease progressed very slowly. On a southern 

 slope in the same field, and only 300 feet distant, a variety test of beans 

 was being conducted. In this plot the blight broke out and spread rapidly 

 from plant to plant, causing considerable damage. The difference in 

 temperature in the two plots of beans was held accountable for the difference 

 in the severity of infection in the two places. The pathogene progresses 

 more rapidly in the plant tissue at high temperature than at low. 



With the infection of the seeds of diseased plants, the life history of 

 Phyt. phaseoli is completed. The organism gains entrance to the seed by 

 passage through the vascular system or through local lesions on the pod. 

 Seeds that are but slightly infected are more likely to introduce the organ- 

 ism into the field in the following season than are severely infected seeds. 

 The latter are usually removed during the process of cleaning and picking 

 in the seed houses, or, if planted, they may decay. 



SYMPTOMS 



General 



Generalized symptoms may occur on the bean plant when the vascular 

 system of the stem has become invaded by the pathogene. The severity 

 of the symptoms varies with the proportion of the vascular system that is 

 invaded. In light infections a stunting of the plant often will occur and 

 will scarcely be noticeable except in comparison with normal individuals. 

 On such lightly infected plants, an incipient wilting may take place during 

 hot, dry days, but a recovery is always made in the evenings. 



When the bacteria begin to invade the entire system a true wilt usually 

 results. Seedling wilts are not infrequent, and at times this symptom 

 arises very quickly. In older plants a leaf, a branch, or the entire plant 

 may wilt (Plate V, D and E), and the progress of the wilt is usually in 

 this order. 



When seeds which are fairly severely infected, but not sufficiently so to 

 injure the germination, are planted, a seedling may be killed as it emerges 

 from the ground, or the bacteria may attack the growing tip of the plant 

 and destroy it, leaving only the cotyledons. The appearance of such 

 seedlings has given rise to the name snakehead. It should be stated here, 

 however, that certain insects, fungi, bacteria, and seeds broken by threshers 

 may cause a similar condition. 



On the leaf 



In the most typical leaf spots of beans due to Phytomonas phaseoli, 

 shown in figure 1, the lesions may first appear as minute water-soaked 

 spots or light green wilted areas, which, as they enlarge, dry out and 

 become brown and brittle. About the edges of such lesions is a yellow 



