Out of all the problems that plants can experience, fungal diseases are by far the most pervasive and damaging. Many of the world’s most notorious famines can trace their origins back to fungi and fungal-like organisms. The Irish potato famine, for example, was caused by a fungal-like organism, Phytophthora infestans, which can infect a whole field of crops overnight. In the United States, the wheat stem rust fungus, Puccinia graminis tritici, has caused millions of bushels of wheat crops to be unusable.
As gardeners, we have a complicated relationship with fungi. On one hand, we need beneficial fungi and bacteria to help promote the healthy growth of our plants. On the other, we have to manage fungi that can adversely impact our plants. In this blog post, we’ll do a deeper dive into the group of fungi classified as rust diseases and how we can spot and treat rust issues before they overtake our gardens.
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What Is Rust Disease?
To start out, rust on plants occurs through a totally different process than rust on metals. What you see as brown or red patches on leaves or stems are actually spores. Fungi are not plants and they do not have the ability to produce food through photosynthesis. Instead, fungi are organisms that absorb nutrients from their surroundings through thread-like tendrils called hyphae. A cluster of hyphae is called a mycelium. As gardeners, you might have heard of the term mycelium as something beneficial to your soil and can help your plants absorb nutrients. When you see mold, another type of fungus, on food, you are actually seeing their mycelia.
The fungi kingdom is vast and ancient. Rust fungi alone encompass some 7,000 species and scientists hypothesize that they’ve been around the planet for a long time to have the opportunity to co-evolve with their plant hosts. Researchers speculate there may be many more undiscovered species in tropical and subtropical regions of the world.
Because they attack some of the world’s most economically important crops like corn, grains, coffee, and sugar cane, a few types of rust fungi have been extensively studied. Although rust diseases are typically not fatal to the plant, they may severely stunt the plant’s growth and production.
Types Of Rust Fungi
Rust fungi are obligate parasites which means that they depend on living host plants to live. These fungi have had hundreds of millions of years to become highly specialized, which means that there might be one particular rust fungus that’s only adapted to specific host plant species. Ornamental plants and food crops alike are susceptible. Rust fungus from the genus Puccinia, Uromyces, Gymnosporangium, and Cronartium have been well studied because of their impacts on human societies.
Life Cycle Of The Rust Fungus
Rust fungi have very complex life cycles. Many species of rust fungi have five distinct spore stages on two unrelated hosts and undergo both sexual and asexual reproduction.
If we take the wheat stem rust fungus, for example, rust spores land on healthy host plants, germinate and penetrate the plant tissue. Then, the pathogen will start absorbing nutrients from the host plant. Over time, the fungi will grow and produce a new generation of urediniospores in abundance so that the host plant’s tissue erupts and releases these new spores into the air. This is the reddish-brown powder that you see on plants infected by rust fungi. Each spore can repeat this lifecycle every two weeks which can lead to high levels of infections. During this part of the life cycle, urediniospores are all clones of one another and reproduce asexually.
As the growing season for the first host plant ends, the rust fungus produces another type of spores called teliospores. These are black, overwintering spores that survive in a dormant stage and start a cycle of sexual reproduction on an alternate host plant to introduce new genetic variations the following year. For the wheat stem rust fungus, the second host is the barberry plant. Sexual recombination occurs in the leaves of the barberry plants and results in yet another type of spores, aeciospores. Aeciospores erupt from pustules on the underside of leaves and then reinfect the first host.
A key takeaway when thinking about rust diseases in the home garden is that fungal spores can overwinter and kickstart the following year. They don’t just reproduce and complete their life cycle like some other fungal plant diseases. Remove any blackened leaves or stems from host plants and do not add them to your compost!
Symptoms Of Rust
Rust is a very visible fungal condition that can typically be seen in late summer through autumn. The main symptom is the appearance of orange, red, yellow, or brown pustules or circular spots, on the leaves and stems of infected plants. On some occasions, you may also see rust pustules on flowers and even fruits.
For plants like roses, the rust fungus can cause the leaves to fall prematurely and work its way up from lower to upper leaves. Other ornamental flowers that are affected by rust fungi include carnations, asters, geraniums, and snapdragons. Rust is also a common problem for turfgrass and lawns and appears as powdery yellow spots on the blades of grasses.
Controlling Rust Disease
Rust fungi, like many other fungi, thrive in hot and humid conditions. You may start to see more visible signs of rust in late summer with the right combination of rain, sustained humidity, and heat. You can use a combination of cultural and chemical practices to control rust in your garden.
When you identify symptoms of rust on your plants, remove the infected leaves, stems, or flowers as soon as possible to prevent the further spread of spores. If your growing area has a history of rust problems, you can start a preventative fungicide treatment at the beginning of the growing season and apply every seven to ten days and continue for five to six applications. Although there are many fungicides on the market, not many of them are approved for organic use. The active ingredient for organic fungicides is typically sulfur, but copper fungicides can also control rust. Make sure to carefully read the labels and follow the direction for use.
Prevention of rust fungi starts with selecting resistant varieties when possible. If you are selecting a turfgrass for your lawn, there are now several cultivars of bluegrass and perennial ryegrass that are rust-resistant.
Many gardeners love to grow beans. If you are worried that rust fungi will be a problem for your beans, you can try out cultivars like the Kentucky Blue Wonder pole bean or the Golden Wax early bush bean just to name a few rust-resistant options. Check with your state’s extension office for a list of plants that may grow best in your local conditions, especially if fungal diseases are a big issue.
Other best cultural practices for preventing rust include leaving enough space between your plants to promote good airflow. Try not to water from above and keep your leaves as dry as possible. Have a consistent deep watering schedule such as by setting up drip irrigation to promote overall healthy roots so your plants may have a better defense against plant diseases in general. Once a plant is infected by rust, that plant may become more susceptible to viral infections and other pests because of its reduced vigor.
Frequently Asked Questions
Q: Is rust fungus dangerous to humans?
A: While rust spores can be transported by humans if they land on our clothes or shoes, it is not dangerous to humans or pets.
Q: Will lawn rust go away on its own?
A: Lawn rust may go away on its own without the application of fungicides under the right cultural conditions such as deep watering, fertilization and an increase in mowing height.
Q: Does rust fungus die in winter?
A: Rust fungi have an overwintering stage during their life cycle where they are dormant but will reactivate the following year under the right temperature and humidity conditions.
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