Garlic Spray And Tomato Blight Lesions

ISEF Category: Plant Sciences

Subcategory: Pathology  ·  Difficulty: Advanced  ·  Setup: University Lab  ·  Time: Full Year

The Hook

Tomato blight can wipe out plants fast enough to ruin a whole garden bed. That makes disease control a real problem, not just a lab exercise. If a simple garlic spray changes lesion spread, you could measure a plant defense effect with real-world value.

What Is It?

This project asks whether garlic extract helps tomatoes resist disease-like damage after exposure to a suspicious soil slurry. Think of the spray as a shield you test before the hit, not after. You are not proving garlic is magic. You are testing whether treated plants show fewer, smaller, or slower-growing lesions than untreated plants.

Late blight is a disease caused by water mold pathogens in the genus Phytophthora, which are not true fungi. They can spread fast under wet conditions and leave dark, spreading lesions on leaves and stems. In a school project, you would not try to identify the exact pathogen from a backyard sample unless a supervised lab has the right biosafety setup. You would study the plant response, compare treatments, and track visible disease scores with care.

The main idea is simple. If garlic extract contains compounds that stress or slow the pathogen, the treated plants should hold up better. If not, the spray may do nothing, or it may damage the leaves itself. That means you can test both protection and plant safety in the same project.

Why This Is a Good Topic

This is a strong science fair topic because you can ask a clear yes-or-no question, then turn it into measurable data. Disease incidence, lesion size, leaf color, and plant growth all give you numbers to compare. The project connects to crop loss, home gardening, and plant disease management, so the real-world stakes are easy to explain. A student can learn experimental design, biosafety, controls, and basic statistics while working with a topic that feels current and useful.

Research Questions

• How does garlic-extract foliar spray change the percentage of tomato leaves that develop lesions after exposure to a suspect slurry?
• What is the effect of garlic-extract concentration on lesion size in tomato plants?
• Does garlic-extract spray change the time until first visible lesion appears on tomato leaves?
• To what extent does garlic-extract spray affect healthy control plants that are not exposed to the slurry?
• Which tomato cultivar shows the largest difference in lesion incidence between treated and untreated groups?
• What is the effect of repeated garlic-extract spray on leaf damage scores compared with water spray?

Basic Materials

• Tomato seedlings of the same cultivar or age
• Garlic extract or fresh garlic for extract preparation
• Clean spray bottles, one per treatment
• Distilled or dechlorinated water
• Digital camera or smartphone with fixed settings
• White background or light box for photos
• Measuring ruler or caliper for lesion size
• Disposable gloves
• Labels and waterproof marker
• Notebook or spreadsheet for daily scoring
• Access to a supervised greenhouse, school lab, or approved plant pathology space.

Advanced Materials

• University-approved plant growth space with biosafety oversight
• Tomato seedlings from a single cultivar and age class
• Sterile containers for slurry preparation under supervision
• Micropipettes and sterile tips
• Laminar flow hood or clean bench, if required by the supervisor
• Image-analysis setup with a calibration target
• Chlorophyll meter or SPAD meter, if available
• Gas exchange or fluorescence tools, if available
• PCR access for pathogen confirmation, only if the supervising lab approves it
• Autoclave or approved decontamination method for plant waste.

Software & Tools

• ImageJ: Measures lesion area, leaf damage, and color changes from standardized photos.
• R: Runs group comparisons, plots disease trends, and checks whether treatment effects are real.
• Google Sheets: Organizes plant scores, treatment groups, and daily observations in one place.
• JASP: Gives you clean t-tests, ANOVA, and effect size outputs without paid software.
• QGIS: Helps if you expand the project to map where backyard soil samples came from.

Experiment Steps

1. Define the plant disease outcome you will score, such as lesion count, lesion area, or a simple severity index.
2. Choose one garlic-extract treatment range and one untreated control so your question stays narrow.
3. Plan how you will randomize plants into groups and keep growing conditions as similar as possible.
4. Design a photo and scoring system that lets you compare leaves in the same way each day.
5. Build a control plan that separates spray effects from exposure effects, leaf damage from handling, and normal growth from disease response.
6. Map out the statistics you will use before you collect data, so you know how many plants and repeats you need.

Common Pitfalls

• Using backyard slurry with no supervision, which creates biosafety and identification problems.
• Making the garlic spray too strong, which can injure leaves and fake a disease effect.
• Comparing plants from different seed lots or ages, which adds growth differences that hide treatment effects.
• Photographing leaves under changing light, which makes lesion measurements inconsistent across days.
• Calling every leaf spot late blight, which confuses stress damage, contamination, and true disease signs.

What Makes This Competitive

A stronger version of this project goes past a simple treated-versus-untreated comparison. You would test dose response, include careful negative controls, and quantify lesions with image analysis instead of rough visual guesses. You could also compare garlic with another safe plant defense treatment, then use statistics that separate small trends from real effects. That kind of design shows that you can think like a researcher, not just run a classroom demo.

Project Variations

• Test garlic extract on one tomato cultivar versus another to see whether genetic background changes disease response.
• Compare garlic spray with a commercially available organic foliar treatment to see which gives better lesion reduction.
• Measure whether garlic spray changes lesion development on pepper plants instead of tomatoes, using the same scoring system.

Learn More

• USDA Agricultural Research Service: Search for plant disease management, garlic extract studies, and tomato pathogen resources on the ARS site.
• National Center for Biotechnology Information Bookshelf: Look for free chapters on plant pathology and host-pathogen interactions.
• PubMed: Search review articles on garlic compounds, antifungal activity, and Phytophthora control.
• APS Net: Read extension-style articles from the American Phytopathological Society on late blight and disease scoring.
• NIH PubChem: Search garlic-derived compounds such as allicin to learn about their chemistry and biological activity.
• MIT OpenCourseWare: Look for free biology and statistics material that helps with experimental design and data analysis.