INOCULATIONS WITH BACTERIA CAUSING PLANT DISEASE x^-? 



ration before as well as after inoculation. The length of time varies 

 with the host plant and the parasite. A saturated atmosphere for 

 6 to 36 hours in both instances favors infection with many leaf para- 

 sites. Various kinds of moist chambers, e. g., that described by 

 Keitt et al. (1937), can be used in the greenhouse. Small outdoor 

 plantings can be covered for a short time with a cloth tent (Keitt, 

 1918) and water sprayed over the exterior. The amount of moisture 

 in the air apparently influences the inter-cellular humidity and, corre- 

 spondingly, the susceptibility of the host. (2) If the plant parts are 

 diflicult to wet because of a waxy covering, the surface can be gently 

 rubbed with a moist cloth. For work on a large scale, the suspension 

 of the organism can be made in a solution of a spreader (e. g., castile 

 soap, 1:1000) to reduce surface tension. The concentration is arbi- 

 trary and is varied according to requirements. Some spreaders, 

 however, are toxic for certain pathogens. (3) A reduced oxygen 

 supply may be important if the pathogen is a facultative anaerobe. 

 For example, the protective wound-cork formation in potato tubers 

 requires abundant oxygen, while certain bacterial pathogens, such as 

 Erwinia carotovora (Jones) Holland, grow well with little oxygen. (4) 

 Water pressure, suction, prolonged spraying, and other means can 

 be used to saturate the intercellular spaces below the stomata and 

 thus to improve the penetration of bacterial suspensions into these 

 regions. This is particularly important with a pathogen, like that 

 causing black fire of tobacco (Johnson, 1937), which is often not 

 aggressive. With this method it is possible to induce necrotic 

 areas on plants not ordinarily considered hosts of the microorganism 

 used. Since bacteria that are usually considered as saprophytes 

 have caused damage under these circumstances, care is necessary 

 while interpreting such results. For example, such saprophytes 

 would hardly fulfill the first of Koch's postulates, as given earlier. 



WOUND INOCULATION 



Suspensions of bacteria, small portions of culture, or of diseased 

 tissue can be introduced into healthy plants through wounds when 

 they do not readily gain entrance through natural openings or when 

 heavier or more rapid infection is desired. The simplest procedure is 

 to smear the point of a dissecting needle with the bacterial mass and to 

 insert the needle into the plant tissue. If large numbers of inocula- 

 tions are to be made, various instruments are useful. For example, 

 an inoculator is described in detail by Ivanoff (1934). It consists of 

 a hypodermic needle (size varied according to needs) with end closed 

 and smooth-walled opening made on one side, of a suitable chamber 

 to hold a bacterial suspension, and of a valve to regulate flow. This 

 needle with a side opening may be used with an ordinary syringe. 

 The common type of needle clogs too easily to be practical. 



Known small numbers of bacteria may be introduced into micro- 

 wounds by means of a micromanipulator. Such wounds may re- 

 semble those made by insects (Hildebrand, 1942). 



