Plant disease
Rust
Orange, rusty pustules on leaf undersides that rub off on your finger.
Rust is in its active season now — scout your plants this week.
How to identify rust
Rust shows up as small orange, rusty, or reddish-brown raised pustules, usually clustered on the undersides of leaves. Run a finger across them and they smear a rusty powder on your skin — that rub-off test is the surest way to know it's rust.
On beans the pustules are orange spots ringed with yellow; on onions and garlic (allium rust) they're orange streaks on the leaves; on asparagus and corn they show as reddish-brown blisters on the stalks and leaves. Heavy rust yellows and withers the foliage and saps the plant's vigor.
It is driven by humidity and lingering leaf moisture, so it tends to build in warm, muggy weather and in crowded, still plantings.
Attacks: Beans, Onions, Garlic, Asparagus, Corn
Life cycle: Rust spreads by airborne spores that germinate on damp leaves in humid, warm weather. It overwinters on infected debris (and on perennial hosts like asparagus and garlic), then flares as humidity rises through the season.
Signs of rust
What you actually see on the plant — usually before you spot the pest itself.
- Orange or reddish-brown powdery pustules on leaf undersides
- Rusty powder that rubs off on your finger
- Yellowing, withering leaves as the pustules spread
- Weakened, less productive plants in a heavy infection
Organic control, least-toxic first
Start at the top and only move down if you need to. Physical and cultural fixes come before any spray.
- Remove infected leaves
At the first orange pustules, pick off the affected leaves and bag them for the trash, not the compost. Getting the spores out early is the most effective step.
- Space and thin for airflow
Wide spacing and thinning let leaves dry and drop the humidity that rust needs. Crowded, still plantings are where it takes off.
- Water at the base, not overhead
Water the soil in the morning so leaves aren't sitting wet. Overhead watering keeps foliage damp and splashes spores from leaf to leaf.
- Plant resistant varieties
Rust-resistant bean varieties in particular are widely available and worth choosing where rust is a yearly problem. The catalog will name the resistance.
- Apply a sulfur fungicide only where it's severe
Where rust is heavy and cultural steps aren't holding it, a registered sulfur fungicide can protect healthy leaves. Sulfur cautions: don't use it in high heat over 90F or within about two weeks of an oil spray, it can irritate skin and eyes, and always follow the product label.
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One rule for any product you spray: follow the label. The label is the law, and it is the tested, safe rate for your plants — homemade mixes and dish-soap sprays are not, and can scorch foliage.
Prevent it next season
- Space and thin plantings so leaves dry and humidity stays low
- Water at the base in the morning, never over the leaves
- Choose rust-resistant varieties, especially for beans
- Remove and trash infected leaves as soon as they appear
- Clear crop debris at season's end so rust can't overwinter on it
Questions about rust
What are the orange spots on the underside of my bean leaves?+
That's bean rust, a fungal disease. If a rusty powder rubs off on your finger, you've confirmed it. Remove the affected leaves, improve airflow, and water only at the base.
How do I get rid of rust on my plants?+
Pick off and trash infected leaves, space and thin for airflow, water at the base in the morning, and plant resistant varieties next season. Use a registered sulfur fungicide only where it's severe, and follow the label.
Is it safe to spray sulfur for rust?+
Sulfur is a registered organic fungicide, but don't apply it in high heat over 90F or within about two weeks of an oil spray, and it can irritate skin and eyes. Follow the product label every time.
Plan a garden that fights back
Healthy, well-spaced plants shrug off pests that flatten a crowded bed. PlotToTable sizes your beds, spaces every crop, and flags the pests that hit what you grow.