Detection & Identification of Plant Diseases | Biology for Grade 10 PDF Download

Pathogens as causes of plant diseases

A pathogen is an organism that causes a disease. There are four main types of pathogen:

• virus
• bacteria
• fungi
• protists

All types of pathogen have a simple life cycle. They infect a host, reproduce (or replicate themselves if a virus), spread from their host and then infect other organisms.
Diseases caused by pathogens are called communicable diseases. This means they can be caught. The spread of communicable diseases is called transmission.
Plant pathogens kill or reduce the growth of many plants, which in turn can reduce biodiversity. Plant pathogens of food crops are especially serious, such as the Irish potato famine around 1850, which caused deaths of large numbers of people through starvation. The pathogen caused 'blight' - an infestation that discolours the leaves but also rotted the tubers making them inedible.

Viruses

Tobacco mosaic virus (TMV) chlorosis seen in a backlit tobacco leaf (Nicotiana tabacum)

The tobacco mosaic virus infects tobacco and lots of other closely related species like tomatoes and peppers. It is transmitted by contact between plants, either naturally or on the hands of farmers. It infects the chloroplasts of plant leaves and changes their colour from green to yellow or white in a mosaic pattern. It can also make leaves crinkled or curled up. This reduces the plant's ability to photosynthesise and grow properly, which can reduce farmers' crop yields.
There is no cure, thus farmers' only option is to stop their crops from being infected or reduce the spread if they have.

Fungi

Rose black spot fungus causes black and purple spots on leaves

Rose black spot is caused by a fungus which infects roses. It infects leaves and causes black or purple spots on the leaves. The rest of the leaves often turn yellow and can drop off the plant. This reduces the plant's ability to photosynthesise and reduces growth. Black rose spot can be transmitted in air or water as well as direct contact by gardeners. It is treatable using fungicides and by removing and destroying infected leaves.

Insects

As well as microbial pathogens, insects can infest a plant. Aphids are a common example of these seen in UK gardens and greenhouses. In particular, they infest roses and tomatoes. They are found in large numbers on new shoots, from which they suck sap. This reduces that growth rate of plants and can eventually kill them. Aphids are eaten by ladybirds, so gardeners often try to cultivate these in their gardens as a natural way of reducing aphids.

Mineral ion deficiencies

Deficiency diseases are diseases caused by a lack of essential vitamins or mineral ions. Scurvy is a deficiency disease historically found in sailors who did not have enough fresh fruit or vegetables, so were deficient in Vitamin C. Plants suffer from deficiency diseases when they do not have enough mineral ions.

Nitrate deficiency

Chlorosis in orange leaves caused by magnesium deficiencyPlants use nitrates as a supply of nitrogen, which is needed to make proteins for healthy growth. Plants absorb nitrates in water through their roots. Nitrates are present in high levels in plant fertilisers.

Without nitrates, the amount of chlorophyll in leaves reduces. This means leaves turn a pale green or yellow colour. This reduces the plant's ability to photosynthesise and grow properly, which reduces the farmers' crop yield. Farmers or gardeners can add chemical or natural fertilisers, such as manure to increase nitrate levels.

Magnesium ion deficiency

Plants use magnesium ions to make chlorophyll in their leaves. Like in nitrate deficiency, the plant is limited in terms of its photosynthetic ability and the plant growth is compromised. Magnesium is a limiting factor in healthy plant growth.

Growth of plants in different nutrient solutions

Plants suffer from deficiency diseases when they do not have enough mineral ions. This can be investigated in science experiments by measuring the growth of barley seedlings.

Method

• Sow ten barley seeds onto cotton wool in a Petri dish. Add 10 cm3 of distilled water, seal using tape to avoid water loss through evaporation and place on a sunny windowsill to germinate. Repeat with three other Petri dishes.
• About one week after germination, open the each Petri dish, pour out any distilled water and add 10 cm³ of the following solutions:
  • Petri dish 1 - Distilled water
  • Petri dish 2 - Culture solution containing magnesium sulfate and potassium nitrate
  • Petri dish 3 - Culture solution containing magnesium sulfate (deficient in nitrogen)
  • Petri dish 4 - Culture solution containing potassium nitrate (deficient in magnesium)
• Replace all Petri dishes on the window sill and leave for two weeks.
• Open each Petri dish and carefully remove each seedling from the cotton wool.
• Dry in an oven and record the total dry mass of each set of ten seedlings.

Results

Example: What conclusions can you draw from this data? Use numbers in your answer. 

The average dry mass of seedlings increases from 2.1 to 3.4 grams when water is replaced by a culture solution.
When this solution does not have nitrogen the dry mass reduces to 2.3 grams .
When this solution does not have magnesium the dry mass reduces to 2.6 grams.

Observations

Seedlings grown without nitrogen are smaller than others. Nitrogen is used to make proteins, which are used for growth and repair. Seedlings grown without magnesium will appear yellow and will be smaller. Magnesium is used to make chlorophyll, which is essential for photosynthesis.

Symptoms of plant diseases and identification of pathogens - Higher

Symptoms

Plant pathogens cause diseases with a range of different symptoms. These symptoms can be used to identify the pathogen and then treat the disease, or limit its effects.
The symptoms of common plant infections are shown below.

Symptom and example of disease

Stunted growth from mealybugs

Symptom and example of disease

Spots on leaves caused by the rose black spot fungus

Identification

Farmers and gardeners often use books and the internet to identify plant diseases. They can also take a small cutting of an infected plant (or a photograph of it) to a local garden centre, which have staff that can often help identify and treat the disease. For diseases that are more difficult to identify, cuttings of the plant may be analysed by scientists in laboratories using testing kits containing monoclonal antibodies.