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".