Part B - Bacterial

Question 1

Explain why plant leaf lesions caused by bacteria often appear angular in shape.

Answer 1

Because the bacteria spread in a straight line from the point of entry.
The growth of the lesion is also limited by the leaf veins. The leaf veins are made of tough, woody tissue that the bacteria cannot penetrate. This is why the lesion often stops growing at the edge of a leaf vein.

Question 2

You are inspecting two Petri dishes that were used in an attempt to isolate a suspected bacterial pathogen.
One has many colony types, and the other has only one kind of colony. Which plate is more likely to contain the pathogen, and why?

Answer 2

The one with just one, as it is the dominant pathogen. Usually found after a few series of inoculations to isolate pathogen.

Question 3

Describe a single bacterial disease of plants, including the names of the host, the pathogen and the disease caused. Include in your answer the significance of the disease and the major management strategies that would be used for its effective control.

Answer 3

Host: Tomato
Pathogen: Xanthomonas campestris pv. vesicatoria
Disease: Bacterial spot

Signs and symptoms: Bacterium produces a toxin that causes the leaves to develop small, water-soaked spots. As the disease progresses, the spots enlarge and turn brown. The leaves may also curl and fall off. The fruit may also be affected, developing small, sunken spots.

Significance: Bacterial spot can cause significant yield losses in tomato crops. It can also reduce the quality of the fruit, making it unmarketable.

Management strategies:
Using resistant varieties
Rotating crops
Removing diseased plants
Applying pesticides
Practising good sanitation

Question 4

You are provided with a plant exhibiting a lesion consistent with a bacterial infection and you wish to isolate the causal agent. What steps do you take, and what is the best part of the lesion to use to attempt isolation? Use a drawing to illustrate your answer.

Answer 4

1. Prepare the sample: Use a scalpel to cut a small piece of tissue from
   the lesion. Transfer the tissue to a sterile petri dish.
2. Dilute the sample: Add a small amount of sterile water to the tissue
   and mix it gently. This will help to dilute the bacteria and make it
   easier to isolate.
3. Streak the sample: Use an inoculating loop to streak the diluted
   sample across the surface of a nutrient agar plate. This will spread
   the bacteria out evenly and allow them to grow into individual
   colonies.
4. Incubate the plate: Incubate the plate at a warm temperature, such
   as 37 degrees Celsius. This will allow the bacteria to grow and form
   colonies.
5. Select a colony: After a few days, select a single colony that appears
   to be pure.
6. Identify the bacteria: Use a variety of methods to identify the
   bacteria, such as biochemical tests and DNA sequencing.

The best part of the lesion to use to attempt isolation is the area that is most discoloured and damaged