Detecting and identifying plant diseases
When plants are suffering from a disease they express symptoms. We can observe these symptoms and identify the disease.
These symptoms can include:
1.Stunted growth, normally due to low nitrate ion concentration in the soil (mineral deficiency).
2.Spots on leaves, if this occurs on rose leaves it is commonly caused by a fungal infection called rose black spot.
3. Areas of decay (rot) it typically caused by a fungal infection.
4.Growths on plants are caused by a disease called crown gall. This disease is caused by a type of bacteria.
5. Malformed stems and leaves can be caused by insects called aphids.
6. Discolouration of leaves can either be due to mineral ion deficiency such as magnesium, which causes yellowing of the leaves, or it can be caused by tobacco mosaic virus which leaves a mottled pattern on the leaves.
7. Presence of insect pests on plants can be visibly seen, such as aphids.
Diagnosing Plant diseases
After noticing plant symptoms, there are three ways that we can identify the exact cause of the disease:
1.A gardener can use a gardening manual or internet website to research the symptoms to find the cause.
2. Infected plants or a sample of an infected plant can be taken to a laboratory where they can identify the specific pathogen that is causing the disease.
3. There are testing kits, which use monoclonal antibodies that can help to identify a specific disease.
Plant Pathogens.
A pathogen is a disease causing microorganism.
Bacteria, viruses and fungi can all infect plants, where they act as pathogens.
Tobacco mosaic virus
Tobacco mosaic virus is a virus that infects a wide range of plants, not just tobacco plants.
Tobacco Mosaic Virus got its name because it was first discovered in tobacco plants
| Symptoms | Mosaic pattern of discolouration on leaves. Stunted growth |
| How it affects the plant | Tobacco mosaic virus damages chloroplasts, leading to less chlorophyll and reduced photosynthesis. This means that growth slows down and crop yield will decrease |
| Transmission | How Tobacco mosaic virus spreads: Direct contact between plants (infected and non infected) Contaminated tools, hands, clothing (from infected plants to uninfected plants) Pests (e.g., insects) that carry infected sap to unifected plants Can remain infectious in soil and dead plant material |
| Prevention or control | Remove and destroy infected plants Disinfect tools Control pests using pesticide Grow TMV-resistant plant varieties |
Rose Black Spot
Rose black spot is caused by a fungus (Diplocarpon rosae).
Rose black spot (Diplocarpon rosae) is very host-specific, which means it only affects roses and does not infect other plants.
| Symptoms | Purple or black spots on leaves Leaves may turn yellow and drop early |
| How it affects the plant | Less leaf surface,so less photosynthesis and slower growth Can reduce the plant’s flower yield |
| Transmission | Spores spread in water or by wind Can survive on fallen leaves and soil and infect new plants |
| Prevention/control | Remove and burn affected leaves Use fungicides Grow disease-resistant varieties Avoid overhead watering (reduces spore spread) |
Aphids
Aphids are small insects that feed on plant sap in phloem vessels. They are not a virus, bacterium, or fungus.
Aphids pierce phloem with their sharp mouthparts and suck sap from phloem.
This can cause:
1.Stunted growth of plant
2.Curling or malformed leaves and stems
3.Reduced yield of crop
Aphids can spread viral diseases between plants (like Tobacco mosaic virus)
Aphids can be controlled by:
1.Biological control: ladybirds or parasitic wasps that eat aphids
2.Chemical control: insecticides
3.Cultural control: removing infested leaves, using barriers
Mineral ion deficiency
If the soil is mineral ion deficient it can cause damage to plants.
Common examples of mineral ion deficiency include nitrate ions or magnesium ions.
Nitrate ion deficiency
Nitrate ions are actively transported from the soil into the root hair cells.
Nitrate ions are combined with glucose produced in photosynthesis to make amino acids. These amino acids are joined together to form proteins for growth.
If the soil is nitrate ion deficient, then less proteins will be produced for growth, this will lead to stunted growth.
Magnesium ion deficiency
The plant uses magnesium ions to make chlorophyll which is a green pigment found in leaf cells.
If the soil is magnesium ion deficient, then the leaves will turn yellow due to a lack of chlorophyll. This is known as chlorosis.
As a result of less chlorophyll, less photosynthesis will occur.
Practice Questions
1. State the names of two common plant diseases
2. Describe how a soil that is deficient in nitrate ions could affect cabbage plants
3. Describe how aphids can cause harm to plants.