Disfigured leaves are upsetting on any plant, but they’re especially unwelcome on houseplants kept for their beautiful foliage. Fungal rust is a common cause of leaf spotting, though there are other reasons, too. Let’s look at why rust spots on leaves happen, how to treat the problem, and the best ways to prevent it from recurring.
Causes of rust spots on leaves: Rust is caused by a group of fungi from the Pucciniales order. It shows up as a scattering of orange, yellow or red spots on foliage. High humidity and prolonged leaf wetness encourage these pathogens. Removing affected leaves, drier conditions, soil adjustment, or chemical sprays can resolve the problem.
Overview Of Rust Spots On Leaves
Colored spots popping up on your plant’s prized leaves can be alarming, but don’t panic. The problem can usually be mitigated with treatment or improvements to the plant’s environment. As long as you correctly identify the issue and address it promptly, you can often minimize your losses to just a few leaves.
Fungal rust disease is a common cause of alien-looking, brightly colored spots. What you see on the rust-stricken leaves is actually caches of reproductive spores. Once these spots mature, the spores can spread throughout the plant or surrounding vegetation. It’s important to resolve the problem before it grows worse.
Although Rust fungus is a classic culprit, there are other causes of leaf spotting that can look similar, so it is important to identify the cause before treating:
- A number of Leaf Spot diseases produce muted spots that resemble those of fungal rust.
- Brown spots and speckling on leaves may be due to incorrect pH and the lack, or overdose, of specific nutrients.
- Residual salts in the soil from excess fertilizer can affect roots and lead to spotting symptoms.
- Pest infestations may cause mottled spotting. Spider mites are a particular risk, and their small size means they can be missed in a routine inspection.
Rust Disease On Plants
Rust is caused by a group of fungi from the Pucciniales order. It shows up as multiple orange, yellow or red spots on foliage. Their size varies but is typically less than a ½ inch in diameter. As a whole, these organisms attack plants of every description, but each rust subtype can typically attack only a very narrow range of plants.
The spots created by fungal rust are actually repositories of powdery spores. These can be partially rubbed off with a tissue or cloth, so the good news about fungal rust is that it’s easy to diagnose. Just wipe a spot and see if the color transfers to the material.
Spores that cause rust spots can be carried by air currents. They stay dormant until conditions are favorable. The spots occur randomly – literally where the wind blows them – and can form on either side of any leaf. Because rust spores require consistent moisture to grow, however, they typically form first on lower, inner foliage where there is less light and ventilation.
Treating And Preventing Rust Spots On Leaves
Rust fungus tends to attack weakened plants: healthy specimens are much more resistant. A rust infection can be a sign the plant isn’t receiving optimal care.
Once you’ve determined your plant’s problem is fungal rust, immediately remove the affected leaves and safely discard them. Don’t put them on the compost pile! You can cut away spots from a leaf you’d like to save, but carefully dispose of the spores so that they can’t contaminate other plants.
In order to grow and “blossom,” fungal rust needs the underlying foliage to remain wet for at least 12-24 hours. Good airflow can keep the disease at bay. Double-check your plant’s husbandry and make sure it’s getting adequate ventilation – especially if you have a moisture-loving plant in a humid environment.
In other words, the basic treatment for rust is also the way to prevent it from taking hold: Avoid wet conditions.
Here are some tips:
- When watering, try to wet the soil and not the foliage.
- Space your plants apart so air can circulate around them.
- Water early to give the plant time to dry during the day.
- Hold off on misting.
- Use rust-resistant cultivars of your favorite plants if they are available.
- Try to ensure the plants you buy are free of disease, and quarantine them for a few weeks after purchase.
If you want to treat more aggressively, either a chemical or natural treatment can be effective:
Fungicidal Sprays
All-purpose, copper-based formulas can limit spore propagation, but they don’t completely eradicate the disease. Organic sulphur and fungicidal soap sprays are similarly effective. Follow the label’s instructions and repeat every week to 10 days until spotting stops.
Home Treatments
Spray the plant with a solution of a teaspoon of baking soda to a quart of water. Another popular spray recipe is to let a ½-cup of minced garlic sit in a quart of water for 24 hours before application. Repeat every five days until the symptoms cease.
Other Leaf Spotting That Can Look Like Rust
Leaf Spot Diseases
Leaf Spot pathogens comprise a large group of organisms that can cause foliage spotting. Some fungal infections act much like rust; others are caused by bacteria or even a virus. Some causes of leaf spot are often limited to certain plant families, but most plants are vulnerable to at least one type.
These pathogens can live on leaves without becoming visible until wet conditions allow their populations to explode. They are normally non-lethal and result in mostly cosmetic damage; of course, foliage disfigurement is a serious issue for an ornamental houseplant!
Leaf Spot infection produces circular lesions of various sizes. These can be either raised or depressed and may have clearly defined or fuzzy edges. The spot colors vary from tan to reddish-brown or black. Sometimes the center of a spot will dry and fall out, leaving holes in the foliage.
Isolate the plant to help prevent further contamination, and give them the treatment recommended for rust disease: Spray with a copper-based or sulphur fungicide or use a baking soda or garlic mix weekly upon appearance of the disease. These solutions don’t eliminate Leaf Spot infection completely, but they help prevent further spore germination.
Nutrient Deficiency
The lack of certain nutrients can cause leaf spotting along with foliage wilting or yellowing – but be aware this isn’t the most common reason for spotting. Plants aren’t particularly susceptible to shortfalls in fertilization; they produce most of their “food” from photosynthesis.
These common deficiencies may cause leaf spots:
Calcium
A shortage of this vital element results in the withering of new growth, curled tips, and yellow or brown leaf spots. The spots are typically outlined by a dark brown edge. These show up on older, mature vegetation near the bottom of the plant.
The problem can develop quickly; the spots start out small and increase in size as the syndrome progresses. With proper care, you can stop but not reverse the damage.
Treatment depends upon why the plant isn’t accessing sufficient calcium. An acidic pH of 6.4 or lower can cause calcium absorption difficulties. If the problem is truly a lack of calcium in the soil, a lime-based amendment can correct the issue. Quality soil mixes that use peat or other naturally acid ingredients often include lime for adequate calcium availability.
Magnesium
A lack of this macronutrient can show up as yellowing leaves with a marbling of small brown patches or rust spots on leaves. A boost of magnesium can remedy the situation. Some fertilizing compounds include both calcium and magnesium.
Both of these deficiencies should be fairly easy to distinguish from fungi causing rust spots on leaves. However, determining the right cause will make the world of difference in nursing your plant back to health.
Overfeeding And Nutrient Burn
As mentioned, undernutrition isn’t the most common issue for houseplants. In fact, too much sodium, potassium, magnesium, or other basic elements can cause problems. Don’t simply add more fertilizer if your plant looks poorly – overfertilization can burn sensitive roots and cause brown or rust spots on leaves and crisped edges.
Leaf damage from overfeeding can happen all over the plant or on just part of it. Wilting and stunted growth are other symptoms.
The treatment for overfertilization is to flush the soil. This washes away excess elements from around your plant’s roots. Remove any fertilizer stakes you might have and run extra water through the soil. Let the container drain completely. Repeat one or more times.
Leaves damaged by nutrient burn won’t recover, but new growth should be healthy. Even if you’re not sure whether your plant has been overfertilized, flushing the soil isn’t going to hurt your plant. It’s good to do every month or so in the growing season as regular maintenance.
Read my guide to fertilizing houseplants for more information about how to avoid deficiencies and over-fertilization problems.
pH Extremes
Issues with nutrient deficiency and oversupply can often be traced to your soil’s pH level. A reading of six to seven pH generally provides the greatest availability of nutrients to plants. If the soil is out of your plant’s preferred pH range, the roots may be “locked out” of absorbing the elements it needs.
If pH is the issue, adding more nutrients can actually make the problem worse. Excess fertilizers may lead to salt buildup that alters soil pH. It seems counter-intuitive, but overfertilization can cause undernutrition.
If you find rust spots on leaves, or leaf spots of any color for that matter and aren’t sure of the cause, find out your plant’s required pH and test your soil to see if it’s within the correct range.
Pro Tip: pH meters are a good option, but using a pH dye kit is also accurate enough for basic soil assessment.
Soil amendments can bring pH to the correct level: a lime compound makes the soil less acidic (raises pH), while sulphur and organic materials lower alkalinity (lowers pH). Be aware that soil with peat or other organic matter tends to be more acidic—and this effect can increase over time as the materials decompose.
Pest Damage
Spider mites can cause leaf spotting, too. If you see small white, yellow or even brown/rust-colored spots scattered over the leaf surface, you may be facing a pest infestation.
It’s important to diagnose a spider mite problem is as early as possible; these little terrors can weaken and eventually kill your plant if not dealt with.
Look closely at your leaves and check for tiny dark specks that may (or may not) be moving. A magnifying glass is helpful. You might see tiny webs if the infestation is well established.
Treatment requires perseverance. Rubbing alcohol kills on contact if dabbed directly on the pests. Both horticultural oil and insecticidal soap are effective for treating larger colonies. Coat all the leaves on both sides—these substances must contact the mites to work. They don’t have a residual effect, so reapply every few days until the mites are gone.
Effective Treatment For Rust Spots On Leaves
If you’ve found rust-colored spots on the leaves of your houseplants, the main priority is to identify the cause. Determine whether it is fungal rust disease, or one of the other leaf spot conditions causing similar lesions. Then you can choose the correct treatment and nurse your plant back to health.
