Marigold and Radish Against Root-Knot Nematodes

ISEF Category: Plant Sciences

Subcategory: Agriculture and Agronomy  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

Root-knot nematodes can turn healthy roots into bumpy, damaged messes. That matters because those bumps can stunt plants and cut yields fast. If one companion plant lowers that damage, you have a real pest-management story you can test with simple scoring and careful controls.

What Is It?

Root-knot nematodes are tiny soil worms that invade roots and trigger galling, which means swollen knots on the root system. You can think of the root like a pipe. The nematode blocks the pipe and makes the plant spend energy on damage instead of growth.

Intercropping means growing two crops together in the same space. In this project, you are testing whether marigold changes the soil or the local root environment enough to reduce nematode attack on radish. Marigolds have a long reputation in garden pest control, but your job is to measure whether that belief holds up in your soil and with your setup.

Why This Is a Good Topic

This topic works well for science fair research because you can test a clear cause-and-effect question with visible results. Root galling gives you a concrete measurement, and intercropping connects to real farming and garden pest control. You can learn how to compare treatment groups, score plant damage, and think about confounding factors like soil history and plant density.

Research Questions

• How does intercropping marigold with radish affect the severity of root-knot galling compared with radish grown alone?
• What is the effect of different marigold-to-radish planting ratios on radish gall scores?
• Does soil from infested community-garden samples change the galling response more than sterilized or noninfested soil?
• To what extent does marigold intercropping change radish root mass, shoot height, or leaf count along with galling?
• Which visual galling scale gives the most consistent scoring between repeated observations?
• What is the effect of row spacing on the protective effect of marigold in mixed planting beds?

Basic Materials

• Radish seeds.
• Marigold seeds or seedlings.
• Pots or planting trays with drainage.
• Potting soil from the same batch for all groups.
• Soil from infested community-garden samples, handled with local safety rules.
• Labels and a waterproof marker.
• Digital kitchen scale with 0.1 g accuracy.
• Ruler or measuring tape.
• Phone camera for root photos.
• Disposable gloves.
• Small trowel or scoop.
• Tray for washing roots.
• Printed root galling score sheet.

Advanced Materials

• Dissecting microscope or stereo microscope.
• Stereomicroscope camera or digital microscope camera.
• Soil sieves for root and soil prep.
• Centrifuge tubes for soil subsampling.
• Micropipettes for optional soil extract work.
• PCR access for nematode confirmation, if available.
• Image analysis software for root area and gall coverage.
• Balance with 0.01 g accuracy.
• Soil moisture probe.
• Controlled-growth chamber or greenhouse bench with documented light conditions.

Software & Tools

• ImageJ: Measures root size, gall area, and image-based scoring from standardized photos.
• Google Sheets: Organizes treatment groups, score data, and summary statistics.
• R: Runs nonparametric tests or mixed models for galling data.
• LibreOffice Calc: Gives you a free backup for data entry and quick graphs.
• PubMed: Helps you find review articles on root-knot nematodes and companion planting.

Experiment Steps

1. Define the exact comparison you will test, including your control group, planting pattern, and the outcome score you will measure.
2. Decide how you will standardize the soil source, pot size, watering, and plant density so treatment differences come from intercropping, not setup noise.
3. Choose a galling scale and a photo method that lets you score roots the same way every time.
4. Plan replication and randomization so each treatment has enough pots and no group always sits in the same spot.
5. Build a data table before planting so you know which measurements you will collect at each checkpoint.
6. Decide in advance how you will compare groups with statistics that fit scored damage data, not just averages.

Common Pitfalls

• Using soil from different spots without mixing it well, which makes nematode pressure uneven between pots.
• Letting marigold and radish compete too unevenly, which can hide whether any gall reduction comes from pest effects or crowding stress.
• Scoring roots before washing off all soil, which makes gall counts unreliable.
• Taking root photos under changing light, which changes the appearance of bumps and size.
• Running too few replicate pots, which makes a real effect look random.

What Makes This Competitive

A stronger version of this project goes beyond a simple yes-or-no result. You can compare planting ratios, spacing, or multiple radish varieties, then use a scoring method that stays consistent across observers. If you add careful randomization, blind scoring, and a statistical test for ordinal data, your project starts to look like real plant pathology research. The best entries explain not just whether marigold helps, but when, how much, and under what soil conditions.

Project Variations

• Test whether marigold intercropping changes nematode galling on lettuce or tomato instead of radish.
• Compare marigold with another companion plant, such as basil or mustard, to see which suppresses galling best.
• Add a root biomass or shoot growth analysis so you can compare plant health and galling in the same experiment.

Learn More

• USDA National Agricultural Library: Search for review articles on root-knot nematodes, marigold suppression, and crop rotation research.
• PubMed: Search for papers on Meloidogyne, intercropping, and botanical nematode control.
• USDA ARS publications: Find practical studies on nematode management in crops and gardens.
• University extension plant pathology guides: Search land-grant extension pages for root-knot nematode identification and galling photos.
• OpenStax Biology 2e: Review plant roots, parasites, and experimental design basics in a free textbook.