Plant Diseases

Types of Plant Disease

Plants can be infected with viruses, bacteria, fungi and insects:

• Tobacco mosaic virus (TMV) is a viral infection that affects a lot of different plants. It causes a distinctive mosaic pattern on the leaves with reduces the amount of photosynthesis that can take place and therefore limits growth of the plant.
• Black spot disease is a fungal infection that affects rose plants. It causes black or purple dots to form on the leaves which limits photosynthesis and plant growth. 
• Aphids are insects that pierce the stem of the plant and feed on the sap from the phloem. When large numbers of aphids feed from the same plant, they become weakened. Aphids can also spread other diseases around (vector). Aphid infestations can be controlled using pesticides or by introducing a natural predator of the aphid, such as the ladybird.

Detection and Identification

Plant diseases can be detected and identified using symptoms of the disease, just like in animals.

Observable symptoms of plant diseases include:

• Stunted growth
• Discolouration or spots on leaves
• Rotting
• Growths
• Malformed plant structures
• Visible pests

Specific diseases can be identified by researching the physical symptoms in gardening manuals and websites. 

Plant diseases can also be identified using specific laboratory techniques or testing kits made using monoclonal antibodies.  

Defence Responses

To prevent disease and being eaten by herbivores, plants have physical, chemical and mechanical defence systems. 

Physical:

• Cell walls made of cellulose provide protection against microorganisms to individual cells.
• Waxy cuticle on the surface of leaves that blocks pathogens from entering.
• Layers of bark (dead cells) protect the stem and prevent pathogens entering.  

Chemical:

• Plants produce lots of different antibacterial chemicals that help fight infections from pathogens. Many of these chemicals can be adapted and used to treat human diseases.
• Plants may also produce poisons to deter herbivores from eating them. 

Mechanical:

• Some plants have thorns which make them painful to eat and deter herbivores. 
• Hairy plant stems deter insects from feeding and laying their eggs on the plant.
• Some plants have leaves that droop when touched to remove insects that land on them.
• Plants can use mimicry to trick animals into thinking the plant is diseased or occupied by a large insect and therefore prevent feeding from the plant.

Mineral Deficiencies

Plants require certain ions to be able to function properly. If a plant is lacking these important ions they can become damaged.

Nitrate deficiency causes stunted growth because nitrogen is a key component of amino acids and proteins. Protein synthesis is required for growth. 

Magnesium deficiency causes chlorosis (yellow leaves) and stunted growth because magnesium is needed to make chlorophyll. Chlorophyll gives leaves their green colour and absorbs the light for photosynthesis. 

To produce lots of healthy plants, optimum conditions must be provided. This includes providing the optimum ion levels for each individual plant.