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. 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, and that process is what comes next. (All of 4.3.3 is **Biology-only**.)

Order the diagnosis

A grower brings you a sickly plant. Put the steps of a proper diagnosis into the order you would carry them out.

• Observe and record the visible symptoms
• Rule out a mineral deficiency, which can look the same
• Match the symptoms against a gardening manual or website
• Confirm the exact pathogen with a lab or a monoclonal antibody test kit
• Only then decide on a treatment

Identifying the culprit [Higher tier]

Spotting symptoms is one thing; **naming the pathogen** is another. Three ways to do it, and this detail is **Higher tier only**:

Name the method

Matching a plant's symptoms against recorded ones in a book or on a website means using a _____. Sending the plant away so the pathogen itself can be identified means using a _____. Using antibodies that bind to one pathogen and no other means using a _____, and those antibodies are described as _____.

Make the diagnosis

A tomato plant has yellow blotches on its leaves. Work through the diagnosis like a scientist. No guessing from a single symptom.

• You see the yellow blotches. What is the best FIRST step?
• The reference lists several pathogens with similar yellow blotches. How do you confirm exactly which one it is?
• The kit confirms a specific fungal pathogen. What does that let you do?

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 cannot run, so they defend themselves in **three** ways. The one that costs marks is telling **physical** from **mechanical**, because both sound like they mean "not chemical". The difference is what the defence acts on:

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".