You can do it in two ways- infiltration through a syringe or make a cut with impregnated scissors i.e. you deep the scissors in bacteria solution and give a cut on the leaf surface.

I think you could simply let the leaves soaking into a bacterial solution, like saline solution at 0,5 McFarland. So you can estimate even the bacterial concentration at the beginning.

Thank you very much for your answer.

After inoculation, if blight or local lesion is developed smash some affected tissues from the advanced region, put them in 2-3 drops of water in a test tube. If water forms cloud, you got the bacteria. Loop out some and streak against a culture medium, in24 hrs colony will be seen.

There are two simple methods which do not involve damaging any of the actual foliage.  Either inoculate the water which would be exposed to the roots or infuse the organism into soil in which the plant is growing.

On another subject if your experiments are in the area of food safety you may wish to consider a different strain.  Typically this type of research would focus on E.coli 0157 H7. 

Here in Italy we have the 50% of O:26 (mostly underestimated) not O:157 (wich is usually less then 15%). But maybe she's focusing on the German epidemic strain of 2011...

Thanks everyone!  

I had the similar question as Dr. Azimun Nahar: how to inoculate pathogens onto foods.  And then checking the efficacy of antimicrobial compounds or mixture of lactic acid bacteria against pathogens on foods.  

You can place whole leaflets on wet Whatman filter paper and innoculate each leaflet with a suspension of your organism. Wrap the plates with parafilm and incubate to observe growth. You may need to add sterile water to the plates to prevent wilting of the leaves. Check Torregrosa et al. 2004. Molecular Plant Plant-Microbe Interact 17, 909-920

@ Philippa, Thank you very much for your kind answer.

Hi, I just found this article from the FDA, US, and it can be useful!

Analysis and Evaluation of Preventive Control Measures for the Control and Reduction/Elimination of Microbial Hazards on Fresh and Fresh-Cut Produce

http://www.fda.gov/Food/FoodScienceResearch/SafePracticesforFoodProcesses/ucm091260.htm

@Chiang. Thank you very much for your kind information.