2023TrialShort answer

Q21 (b)

4 marks

Question

A plant pathologist observed brown spots developing on the leaves of a lemon tree and wanted to determine the cause of the disease. The pathologist suspected the tree was suffering from a bacterial disease, caused by infection with Xanthomonas citri.
Describe the steps a pathologist could use to test the hypothesis that the suspected bacterium is the cause of the disease.

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Solution

Criteria	Marks
• Clearly outlines a method using Koch’s postulates, including four steps	4
• Outlines a method using Koch’s postulates, including three steps	3
• Outlines a method using Koch’s postulates, including two steps	2
• Provides some relevant information	1

Sample Answer

Isolate the suspected pathogen from the diseased area of the leaf.
Grow the supected pathogen in pure culture.
Infect disease-free leaves with the suspected pathogen from the cutlure. The infected leaves should show the same symptoms as the original diseased leaves.
Isolate the suspected pathogen from the infected leaves and grow it in culture. It should be identical to the pathogen in the first culture.

Supplied marking guidance · human reviewed

Compare this with your working, not only your final sentence.

Syllabus alignment

Module 7 · Infectious Disease

Causes of Infectious DiseaseExplain disease causation and transmission through the work of Koch and Pasteur.
Causes of Infectious DiseaseDesign and conduct microbial testing of water or food samples.

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