What Is Geranium Rust – Learn About Treating Geranium Leaf Rust

By: Liz Baessler

Geraniums are some of the most popular and easy to care for garden and potted plants. But while they are usually low maintenance, they are prone to some problems that can be a real issue if left untreated. Geranium rust is one such problem. It’s a very serious and relatively new disease that can completely defoliate and even kill a plant. Keep reading to learn more about recognizing geranium leaf rust symptoms and managing and treating geraniums with leaf rust.

What is Geranium Rust?

Geranium rust is a disease caused by the fungus Puccinia Pelargonii-zonalis. It originated in South Africa, but over the course of the 20th century it spread throughout the world, reaching the continental United States in 1967. It is now a serious problem on geraniums worldwide, particularly in greenhouses where quarters are close and humidity is high.

Geranium Leaf Rust Symptoms

Rust on a geranium begins as small, pale yellow circles on the underside of the leaves. These spots quickly grow in size and darken to brown or “rusty” colored spores. Rings of pustules will surround these spots, and pale yellow circles will appear opposite them on the upper sides of the leaves.

Heavily infected leaves will drop. Untreated geraniums with leaf rust will eventually become completely defoliated.

Treating Geranium Leaf Rust

The best method of geranium leaf rust treatment is prevention. Only buy plants from reputable sources, and thoroughly inspect leaves before purchasing. Spores thrive in cool, damp conditions, and are especially prevalent in greenhouses.

Keep your plants warm, space them well for good airflow, and keep water from splashing on leaves during irrigation.

If you see signs of rust, immediately remove and destroy infected leaves, and treat the rest of the leaves with fungicide. If a plant is heavily infected, it may have to be destroyed.

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Read more about Geraniums

What is Rust?

Here’s the deal, there are thousands of species of rust that can affect your garden or indoor plants. Different types of rust attack different varieties of plants.

Common rust, however, is well… a common species that affects a wide variety of plants, including edibles like beans and tomatoes. It’s more likely to affect mature plants than it is younger ones.

Rust is an unattractive fungal disease that can stunt growth and reduce the healthfulness of your plants, but it rarely kills crops outright.

However, if you leave the disease to run its course, your plants will decline in health and may not produce as expected – edible plants in particular. On top of that, if you sell your plants at the market, no one wants to buy your leafy greens if they’re covered in rust.

That’s why it’s best if you address a rust outbreak straight away.

MSU Extension

Kristin Getter, Michigan State University Extension, Department of Horticulture, and Jan Byrne, MSU Diagnostic Services, Department of Plant Pathology - April 17, 2012

Be proactive in managing Xanthomonas in your greenhouse to prevent geranium losses.

Bacterial blight of geraniums is caused by a specific pathovar of the Xanthomonas hortorum (formerly known as Xanthomonas campestris) pathogen and is sometimes called bacterial stem rot, bacterial wilt, or bacterial leaf spot. Xanthomonas hortorum pv. pelargonii is specific to Pelargonium and Geranium plant species. This disease is of special concern for growers because it is easily spread (by propagation, irrigation, or whiteflies), symptoms may not be visible to the naked eye until months after an infection, and once plants are infected there is no effective chemical or biological control.

Symptoms of bacterial blight may vary depending on the plant species and cultivar. When the bacteria are spread via irrigation water, or splashing, small (2-3 mm), water-soaked spots will develop first, usually beginning on the underside of the leaves and then showing through to the upper leaf surface. These spots will then turn tan to brown and will be slightly sunken with well-defined margins. Spots may then spread into wedge-shaped areas of chlorosis followed by necrosis (Photo 1). Eventually, the bacteria infect the vascular system and cause wilt of the entire plant and, finally, stem rot and plant death.

Photo 1. Xanthomonas on geranium with wedge-shaped leaf necrosis. Photo credit: Nancy Gregory, University of Delaware, Bugwood.org

If the geranium was infected through the roots or was taken as a cutting from an infected mother plant, the first symptoms may be wilting of the lower leaves. Ivy geraniums may not exhibit the wilting symptoms, but are certainly carriers of the pathogen and symptoms may mimic nutrient deficiencies or a mite infestation. As such, it is not advisable to hang ivy geraniums above other geraniums in the greenhouse.

So, how do you manage such a disease? Your best bet is to make sure you have excellent sanitation and clean cuttings. If you use your own stock plants to propagate via cuttings, consider sending stock plant samples to a reputable lab for culture-indexing of these plants to verify that they are not carrying the disease. If you purchase cuttings, ask your supplier about their culture-indexing program and prevention practices. In addition, keeping different sources of geraniums separate from each other is also advisable. This will prevent the spread of an infection should one source discover an infection. If any plants become infected in your greenhouse, place those plants and the ones around them directly into garbage bags before carrying them out of the greenhouse and disposing of them promptly.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

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