Plant Disease (AQA GCSE Biology)

Plants can be infected by a range of pathogens.

Aphids are small insects that feed on plant sugars and carry pathogens.

Figure 1 shows an aphid feeding on a plant

Figure 1

Plants infested with aphids show signs of reduced growth.

Suggest and explain one reason why.

Newly-hatched aphids do not have wings.

After a number of generations, a number of aphids hatch with wings.

A mutation in one population of aphids results in the wings failing to develop.

Suggest the impact of this on the aphid and other organisms. 

It is common for plants infested with aphids to show signs of magnesium deficiency.

Describe and explain the symptoms displayed by a plant with a magnesium deficiency.

Plants have methods of protecting themselves from attack.

One such method is the production of oils by cells on the surface of leaves. 

Suggest how the production of oil might protect a plant from being attacked. 

Higher Tier Only

A gardener notices that the leaves of one of his rose plants are covered in black and purple spots.

Name the disease affecting the gardener's rose plant.

Higher Tier Only

Describe the cause of this disease and explain how the plant is affected by it.

Higher Tier Only

Figure 3 shows the arrangement of rose plants in the gardener's garden.

Figure 3

The infected plant is labelled I.

The gardener decides to move the plants labelled A and B to the other side of the garden (the area labelled X) to try and prevent the infection from spreading.

He decides not to use a fungicide.

Evaluate the effectiveness of the gardener's decisions in controlling the spread of the infection amongst the plants. 

Witch hazel is a plant that produces oil as part of its defence against pathogens.

Many skincare companies extract the oil to use in consumer products. 

The oil prevents bacteria from attacking the plant but is safe to use in skincare products. 

Suggest and explain a reason why.

Scientists working for a skincare company investigated how effective three different plant oils were at preventing the growth of one strain of E. coli bacteria.

Figure 4 shows their results.

Figure 4

Evaluate the effectiveness of the three plant oils.

Higher Tier Only

A number of plants in a garden are infected with a pathogen.

Give two ways of identifying the pathogen that is infecting the plants in the garden. 

A number of mineral ions are needed by plants.

Plants that are deficient in mineral ions will not be healthy.

Name two essential mineral ions and describe the appearance of plants with deficiencies in these ions.

Other minerals are essential for optimum plant growth.

NPK fertilisers are three-part fertilisers containing nitrogen (N), phosphate (P) and potassium (K) in varying proportions. 

Plants deficient in potassium have stunted roots.

Plants deficient in phosphate ions cannot form flowers and fruits.

The percentages (%) of minerals in three commercially-available NPK fertilisers were compared. The gardener used this data to decide which fertiliser to buy. 

The data are shown in Table 1.

Table 1

The gardener decided to buy Fertiliser 2.

Explain why he chose Fertiliser 2.

A common plant disease is caused by the tobacco mosaic virus (TMV).

Describe how a gardener would be able to recognise that their plants were infected with TMV.

Suggest how monoclonal antibodies could be used to confirm that TMV was the cause of the disease in the gardener's plants.

Suggest why the gardener washes all of their tools in disinfectant after using them on TMV-infected plants.

TMV can cause plants to produce less chlorophyll.

Explain why plants with TMV exhibit stunted growth. 

Black Sigatoka is a type of plant disease caused by the fungal pathogen Mycosphaerella fijiensis.

It mainly infects banana plants and one of the main symptoms is that the leaves appear to have black streaks across them.

The fungal spores spread in the wind and in water droplets from plant to plant. They enter via the leaves.

Using Figure 1, identify the area of the leaf that allows the entry of pathogens. 

Figure 1

The black streaks on the leaves of plants infected by Black Sigatoka, appear as a result of the cells inside the leaf being digested by enzymes from the pathogen. 

The fungi release the enzymes extracellularly (outside of their cells) and then absorb the nutrients from the digested plant cells. 

Why is it possible for fungi to digest their food extracellularly instead of having an internal digestive system? 

List some methods that can be taken by people that own banana plantations in order to prevent the spread of Black Sigatoka amongst their plants. 

Name three methods.

When plants are infected with Black Sigatoka they have stunted growth and 50% lower fruit yield. 

Explain why this occurs. 

A student is planning an experiment to investigate the effect of mineral deficiency on the growth of plants.

This is the method used.

1. Nine broad bean plants were grown for two weeks from seeds. 

2. Each of the nine plants were grown in different types of soil, three in soil high in nitrates, three in medium levels of nitrate, and three in low levels of nitrate. 

3. After the two week growth time the height of each of the plants was measured using a rule. 

Suggest two ways the student could improve their experiment.

The results of the investigation are shown in Table 1. 

Table 1

Using Table 1, calculate the mean height of the plants in the high soil nitrate concentration after two weeks. 

The student made the following conclusion:

"The higher the nitrate concentration in soil, the taller plants will grow."

Using evidence from Table 1 and your own knowledge, evaluate this student's conclusion.

Suggest why it is not always the best option for growers to increase the amount of nitrate in the soil if their plants aren't growing as tall as expected. 

A farmer identified that a number of his plants were showing signs of disease. 

The farmer researched the symptoms of one of his plants and the research suggested that a possible cause of the symptoms was a magnesium deficiency. 

Magnesium deficiency symptoms include:

• Yellow leaves

• Stunted growth

Explain how a deficiency of magnesium could cause these symptoms.

Higher Tier Only

The farmer also thinks their strawberry crop is infected with a fungal pathogen Botrytis cinerea.

The farmer obtains a monoclonal antibody test kit for Botrytis cinerea.

To make the monoclonal antibodies a scientist first isolates the Botrytis cinerea protein from the fungi.

Describe how the scientist would use the protein to produce the Botrytis cinerea monoclonal antibody.

After the farmer lost a large number of his plants to disease they decided to try breeding some resistant strains on the farm. 

The farmer will selectively breed some of the surviving plants in order to achieve this. 

Describe how the farmer would do this. 

Another strategy that the farmer could use would be to take cuttings from resistant plants and clone them. 

Evaluate whether cloning is a better strategy for the farmer to use to get resistant crops than selective breeding.