476 Bacterial Disease of Pisum sativum 



and the expense. Care should be taken to thoroughly clean all imple- 

 ments used on infected land so as not to spread infection. 



Summary. 



Ps. seminum is a motile sporing facultatively anaerobic bacillus 

 capable of growth on acid, neutral and alkahne media. 



The chief characteristics of the disease caused by this organism 

 is that it is present in the interior of the seed, forming a discoloured area 

 in the centre of each cotyledon. It is not possible to tell from the 

 exterior of the seed whether it is, or is not, infected. 



■ External infection takes place through the young tissues, and, as 

 far as is known, the organism is unable to spread through matured 

 tissues. 



Infection occurs considerably in advance of discoloration in the 

 tissues. 



The organism occurs throughout the plant, but has never been 

 observed in the vessels. 



It has been isolated from the interior of the seed, both dry and green, 

 the stem, the embossed pod and the layer of thin-celled tissue which 

 forms the innermost lining of the pod. 



The organism has never been seen passing through the micropyle 

 of the seed, although on one occasion rods were observed lying in the 

 spaces between the cotyledons and the young embryo. 



The organism passes up the plant in the motile rod stage. 



The macroscopic appearance of the colonies on artificial media is 

 very variable. 



The organism develops rapidly at a temperature of 25° C. but 

 development is slow below 20° C. 



The motile stage only lasts a short time unless there is a consider- 

 able amount of moisture present. This is followed by the beaded 

 stage, and finally spores are developed inside the beaded stage, and 

 the spores eventually lie embedded in a pseudo-zooglaeal matrix. 



All varieties of culinary peas so far examined are more or less 

 susceptible. 



Germination is not arrested, but in bad cases the development of 

 the plant is retarded owing to the dying back of the main shoot and 

 the development of laterals to carry on the growth of the plant. 



