Plant Doctor

Plant Doctor 🌱

A crop is failing: yellowing leaves, dark spots, stunted stems. Is it a fungus? A virus? A mineral shortage? Guess wrong and the whole harvest could be lost. This module is the plant pathologist's method — reading symptoms, confirming a diagnosis, and understanding how plants defend themselves. (All of 4.3.3 is **Biology-only**.)

Reading the symptoms 🔍

Diseased plants show visible signs. Learn to recognise them: • **Stunted growth** and **malformed** stems or leaves • **Discolouration** — often yellowing • **Spots** on leaves and **areas of decay** (rot) • Abnormal **growths** on the plant A symptom on its own only **suggests** disease — the same yellowing could be a mineral deficiency. That is why diagnosis needs a proper process, not a snap judgement.

Sign of disease?

A gardener notices a plant with dark spots on the leaves, patches of decay and unusually stunted growth. These are examples of what?

• Symptoms that suggest the plant may be diseased
• The plant's defence responses
• Normal signs of a healthy plant
• Evidence of fast photosynthesis

Identifying the culprit 🧪 [Higher tier]

Spotting symptoms is one thing; **naming the pathogen** is another. There are three ways to identify a plant disease (this detail is **Higher tier only**): • Refer to a **gardening manual** or **website** to match the symptoms. • Take the diseased plant to a **laboratory** to identify the pathogen. • Use a **testing kit** that contains **monoclonal antibodies** — these bind to a specific pathogen, so a positive result confirms exactly which one is present.

Make the diagnosis

An interactive activity.

How the test works [Higher tier]

A rapid field kit gives a positive result for one specific plant pathogen and nothing else. Which component makes it that specific?

• Monoclonal antibodies that bind to that one pathogen
• Antibiotics that kill the pathogen
• Chlorophyll extracted from the leaf
• Digestive enzymes

How plants fight back 🛡️

Plants can't run, so they defend themselves in **three** ways: • **Physical** barriers — a waxy **cuticle**, **cellulose** cell walls, and layers of dead cells (**bark**) that pathogens must get through. • **Chemical** defences — **antibacterial** chemicals, and **poisons** that deter herbivores from eating the plant. • **Mechanical** defences — **thorns** and **hairs** that deter animals, leaves that **curl or droop** when touched, and **mimicry** (e.g. looking like an unhealthy plant, or having markings that look like insect eggs).

Sort the defences

Name the category

A cactus is covered in sharp spines that deter animals from eating it. Which type of plant defence is this?

• Mechanical defence
• Chemical defence
• Physical defence
• Not a defence at all

Fill the defence gaps

A tough waxy cuticle is a _____ defence; poisons that deter herbivores are a _____ defence; thorns and leaf curling are a _____ defence.

In the exam 🎓

Clinic closed. Grade-9 habits for plant disease: • Treat diagnosis as a **process**: symptom → compare against a **reference** → confirm with a **lab or monoclonal-antibody kit**. Never diagnose from one symptom. (ID methods are **HT-only**.) • Sort every defence into **physical** (cuticle, cell walls, bark), **chemical** (antibacterial chemicals, poisons) or **mechanical** (thorns, hairs, leaf curl, mimicry). • Always give a **named example**, not a vague "it protects itself".