Of all the problems plants can have, fungal diseases are by far the most widespread and harmful. Many of the world's most notorious famines can be traced back to fungi and fungal organisms. The famine in Ireland, for example, was caused by a fungal organism, Phytophthora infestans, which can infect an entire field of plants overnight. In the United States, the wheat stem rust fungus Puccinia graminis tritici has rendered millions of bushels of wheat crops unusable.
As gardeners, we have a complicated relationship with mushrooms. On the one hand, we need beneficial fungi and bacteria to promote the healthy growth of our plants. On the other hand, we have to deal with fungi, which can adversely affect our plants. In this blog post, we're going to dig deeper into the group of fungi classified as rust diseases and how we can identify and treat rust problems before they overtake our gardens.
Good products for the treatment of rust diseases:
What is rust disease?
Very dramatic rust pustules on point vetch. Source: The Bone Collector II
In the beginning, rust occurs on plants in a completely different process than rust occurs on metals. What you see as brown or red spots 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 environment through thread-like tendrils called hyphae. One group of hyphae is called a mycelium. As a gardener, you may have heard of the term mycelium, which is 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 mushroom kingdom is huge and ancient. Rust fungi alone comprise around 7,000 species, and scientists believe that they have been on the planet a long time in order to have the opportunity to evolve with their plant hosts. Researchers speculate that 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 such as corn, grain, coffee and sugar cane, some types of rust fungus have been extensively studied. While rust diseases are usually not fatal to the plant, they can severely affect the plant's growth and production.
Types of rust fungi
Broad bean rust with pustules and spores. Source: glyn_nelson
Rust fungi are obligate parasites, which means they depend on live host plants to live. These mushrooms have had hundreds of millions of years to specialize highly, meaning that there may be a particular rust fungus that is only adapted to specific host plant species. Ornamental plants and food plants are equally susceptible. Rust fungi of the genera Puccinia, Uromyces, Gymnosporangium, and Cronartium have been well studied for their impact on human society.
Rust fungus life cycle
Rust fungi have very complex life cycles. Many species of rust fungus have five different spore stages on two unrelated hosts and are reproduced both sexually and asexually.
For example, if we take the wheat stem rust fungus, rust spores land on healthy host plants, germinate and penetrate the plant tissue. Then the pathogen absorbs nutrients from the host plant. Over time, the fungi will grow and produce a new generation of urediniospores in abundance, causing the tissue of the host plant to break out and release these new spores into the air. This is the red-brown powder you see on plants infected with rust fungus. Each spore can repeat this life cycle every two weeks, which can lead to high infection rates. During this part of the life cycle, urediniospores are all clones of each other and reproduce asexually.
At the end of the growing season for the first host plant, the rust fungus produces a different type of spore called teliospores. These are black, hibernating spores that survive in a dormant state and begin a cycle of sexual reproduction on an alternate host plant to introduce new genetic variations in 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 another type of spore, aeciospores. Aeciospores break out from pustules on the underside of the leaves and then re-infect the first host.
An important aspect when thinking about rust diseases in the home garden is that fungal spores can overwinter and stimulate the following year. They not only reproduce and complete their life cycle like some other fungal diseases. Remove blackened leaves or stems from host plants and don't add them to your compost!
Symptoms of rust
Blackberry leaves are also prone to rust. Source: John Tann
Rust is a very visible fungal disease that typically occurs in late summer through fall. The main symptom is the appearance of orange, red, yellow or brown pustules or circular spots on the leaves and stems of infected plants. In some cases, you can also see rust pustules on flowers and even fruits.
On plants like roses, the rust fungus can cause the leaves to fall prematurely and work their way up from the lower to the upper leaves. Other ornamental flowers that are attacked by rust fungi are carnations, asters, geraniums and snapdragons. Rust is also a common problem on lawn grass and lawns and appears as powdery yellow spots on blades of grass.
Combating rust disease
Rust fungi, like many other mushrooms, thrive in hot and humid conditions. With the right combination of rain, persistent humidity, and heat, more visible signs of rust can appear in late summer. You can use a combination of cultural and chemical practices to control rust in your garden.
Rust treatment
Rust pustules on a raspberry leaf. Source: Aidehua2013
If you notice symptoms of rust on your plants, remove the infected leaves, stems, or flowers as soon as possible to prevent the spores from spreading further. If your growing area has had rust problems in the past, you can start preventive fungicide treatment at the beginning of the growing season, apply it every seven to ten days, and do five to six applications. While there are many fungicides on the market, not many are approved for organic use. The active ingredient for organic fungicides is typically sulfur, but copper fungicides can also control rust. Read the labels carefully and follow the instructions for use.
Prevent rust
Rust fungus prevention starts with choosing resistant varieties whenever possible. When choosing a lawn grass for your lawn, there are now several varieties of bluegrass and perennial ryegrass that are rust resistant.
Many gardeners love growing beans. If you are concerned that rust will be a problem for your beans, you can try strains like the Kentucky Blue Wonder Pole Bean or the Golden Wax Early Bush Bean, to name a few rust-resistant options. Contact your state's expansion office for a list of the plants that can grow best in your local conditions, especially if fungal diseases are a major concern.
Other proven cultural methods of preventing rust include leaving enough space between your plants to encourage good airflow. Try not to water from above and keep your leaves as dry as possible. Keep a consistent deep watering schedule, such as setting up drip irrigation to encourage overall healthy roots so that your plants are generally better protected against plant diseases. Once a plant is infected with rust, that plant can become more susceptible to viral infections and other pests due to its reduced vitality.
frequently asked Questions
Cedar quince rust shows up as a crab on the leaf knot of this hawthorn. Source: outburst
Q: is rust fungus dangerous to humans?
A: While rust spores can be carried by humans if they land on our clothes or shoes, it is not dangerous for humans or pets.
Q: Will the rust go away on its own?
A: Lawn rust can go away on its own without the application of fungicides under the right cultural conditions such as deep irrigation, fertilization, and elevation of mowing height.
Q: Does the rust fungus die in winter?
A: Rust fungi have a hibernating stage during their life cycle in which they are dormant, but reactivate in the following year under the right temperature and humidity conditions.
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