Rust fungus doesn't usually kill a plant but besides becoming an eyesore, it can quickly spread to cause significant damage to the foliage, fruits, or vegetables. Severe outbreaks of rust fungus can turn leaves yellow and cause leaf drop.
What Is Rust Fungus?
Rust fungus is a group of fungal diseases that produce spores affecting mostly leaves but also stems, flowers, and fruits. Rust fungus spore pustules on a plant appear in various colors such as orange, yellow, rusty brown, black, or white.
The rusts are a large group of fungal plant diseases under the umbrella family name of Pucciniales that includes over 7,000 species, including Uromyces, Gymnosporangium, and Cronartium. Rust disease does not typically affect a plant's general health when caught early but you should learn how to get rid of rust fungus once and for all.
Rust Fungus Identification
Rust fungus is found on the surfaces of plant leaves in speckled masses. Many types of rust fungus disease affect specific annual and perennial flowering plants. Like the namesake, many of these pustules are rust-colored, but gardeners should also suspect rust if they see brown, orange, purple, red, or yellow dry spots on foliage.
Rust Fungus Life Cycle
Rust fungus tends to develop during rainy, damp summers when leaves are wet for prolonged periods. For example, if plants are wet all night and morning temperatures are very warm, rust infections could proliferate. The spores are spread by wind or water to infect healthy plants.
Commonly Affected Plants
Rust disease often affects Alcea rosea (common hollyhock), but hollyhocks aren’t the only potential victims of this fungus. Different fungi will attack different plants and it can also attack turfgrass. Pay close attention to the signs of rust fungus in the following plants during the summer months:
Symptoms of Rust Fungus on Plants
Plants with a heavy rust infestation might experience the following:
- Curling, distorted, or withering of leaves
- Spotted needles and blades
- Leaf drop leading to stunted plant growth
- Pale spots that turn colors and contain spores
- Orange or yellow streaks on the underside of a leaf
- Stunted plant growth due to yellowing leaves and reduced photosynthesis
How to Treat Rust Fungus
How to Water Plants to Prevent Rust Fungus
Rust fungi, like many fungal plant diseases, flourish in wet conditions. The most important step you can take to reduce rust in your flower garden is to evaluate your watering practices and above all, stop overhead watering. Instead, use a drip irrigation system or a watering wand to deliver water at ground level. If this isn’t possible, water your flower garden early in the morning when the sun’s rays will quickly dry your plants’ foliage.
Control by Organic Methods
A weekly dusting of sulfur can prevent and treat rust fungus. Neem oil, a botanical fungicide and pesticide, also controls rust. Some organic gardeners swear by baking soda to control garden fungus. The efficacy of baking soda spray might be enhanced by mixing it with light horticultural oil.
Control with Commercial Fungicides
You can choose several commercial fungicides to control rust fungus. Apply the sprays when you see the first signs of disease and continue applications according to package directions through July. Look for commercial fungicides that contain these active ingredients:
- Chlorothalonil: Acts as a preventative treatment for rust
- Mancozeb: Acts as a protectant
- Myclobutanil: Eradicates most types of rust
- Trifloxystrobin: Inhibits fungal respiration and growth
Not all ingredients are effective on all rusts. If you are unsure about the active ingredients that would work best to control a rust issue in your garden, contact your local Cooperative Extension Service for recommendations.
How to Prevent Rust Fungus
Choose Rust-Resistant Varieties
Hybridizers are continuously developing new flower varieties resistant to rust fungus. If you’ve experienced rust for multiple growing seasons, it might be time to remove susceptible plants and replace them with these proven varieties:
- New England Asters ‘Honeysong Pink’ and ‘Purple Dome’
- Hollyhock 'Happy Lights’
- Snapdragons 'White Monarch’ and the Rocket series
Space Plants Properly
Even if you plant rust-resistant varieties, you must give them enough room to grow. Proper plant spacing increases air circulation which enables plants to dry more quickly. Overcrowding plants and dampness are the perfect conditions for many fungal diseases.
Purchase Healthy Plants
Ensure newly purchased plants do not have rust fungus before bringing them home from the garden center or nursery.
Keep Plants Separate
Keep new plants separate from others for a few weeks before planting them in the garden with your other species.
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Is rust fungus harmful to humans?
Rust fungus spores are not harmful to humans or animals.
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Where does rust fungus come from?
The spores of rust fungus spread in the air and need a film of water to germinate.
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Is rust fungus airborne?
Yes, it is an airborne fungus spread by water and wind.