Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
I love hollyhocks, but I’ve never been able to grow them successfully. Every year, no matter how careful I am, no matter which cultivar I plant, they always succumb to rust.
What is Rust?
Rust is caused by fungi that belong to the order Pucciniales. There are about 168 rust genera with approximately 7,000 species. Why so many? Because each fungi species is unique to a specific plant family. Which means that the rust that infects my hollyhocks will not infect anything else in my garden. It only grows on hollyhocks. It will also only infect a certain part of its host. So in the case of my hollyhocks, it will only infect the leaves. Other species of fungus will infect only the leaves or petioles (leaf stems) or the plant stems or fruits. No other part of the plant is infected.
How did it get its name rust? Because the spores of the fungi are brown or rust colored. The fungi doesn’t just cause rusty patches on plant surfaces, they can also cause cankers, galls or what is known as witches brooms which is when a mass of shoots grows out of a single spot on a tree. They look like giant birds nests or brooms, hence their name.
Rust fungi are obligate parasites which means they need a living host. Although they can stunt a host plant’s growth or severely damage the leaves, they will not kill their host plant.
Rust is a huge problem in agriculture where they can ruin entire crops of grain or fruit. It is also a big problem for the home gardener where it can attack fruit and nut trees, ornamental trees and shrubs, roses and other flowering plants.
How to Spot Rust
Rust is easy to spot. It will usually present as rusty spots like a rash on the leaves or stems of your plants. If you see cankers, galls or witches brooms, they may or may not be rust fungi. They can also be caused by bacteria or viruses.
Most gardeners will see rust on the leaves of their plants which will curl, wither or even drop off the plant completely.
How to Prevent Rust
Don’t Introduce it to Your Garden
When you are purchasing plants, look at them very closely. Make sure that you don’t see any signs of the fungus on the leaves, stems, or fruit. If you do, put it back and either purchase another one or, since rust can spread, buy plants elsewhere.
Plant Rust Resistant Varieties
Plant hybridizers are constantly developing new varieties of garden favorites. They breed for size, color and disease resistance. Planting rust resistant flowers will ensure that rust won’t have a chance to take hold in your garden.
Water at the Roots
Rust thrives in a moist environment so your watering technique can make all the difference. Never water from overhead so that the water splashes on to the foliage of your plants. Rust can also be spread via water droplets. If you water from overhead with a watering can or hose, you can spread the spores from infected leaves to healthy leaves on other plants in your garden.
Use drip irrigation to deliver water to the roots or use a watering wand so that you can water at soil level.
A thick layer of mulch will help keep the soil moist so you will need to water less often. It will also prevent weeds from growing which will compete with your plants for water, sunlight and nutrients.
Space Your Plants Correctly
It’s very tempting to squeeze plants in when you have more plants than space, but crowding plants only invites disease and insect infestation. Rust spores pass easily from plant to plant via water or wind so if your plants are too close together, they can all be rapidly infected.
Divide your perennials when they get too crowded as well.
Use a Preventive Fungicide
You can purchase fungicides from your local nursery or you can make your own organic fungicide. If you have aspirin in the house, dissolve 2 (uncoated) aspirin in a quart of water or 8 (uncoated) aspirin in a gallon of water. Spray on your plants in the early spring. Be sure to spray the undersides of the leaves also.
Another fungicide that is easily made at home uses baking soda. Mix 1 ounce of horticultural oil and 4 teaspoons of baking soda in a gallon of water. Use it in the spring and make certain that you are spraying the undersides of the leaves as well as the rest of the plants.
Do a Thorough Garden Clean-up in the Fall
In the fall, remove all dead plant material from your garden so that the spores will have no place to over winter and reinfect your garden in the spring.
How to Get Rid of Rust in Your Garden
Get Rid of Infected Leaves or Plants
Keep a close eye on your garden and the minute you see rust infecting a leaf or stem, get rid of it immediately. Break it off and discard it in the trash. Do not compost infected plant parts. The spores can survive the composting process and infect your garden again when you use the compost.
If a plant has too many leaves or stems infected with rust, remove and discard the entire plant so that it can’t infect the rest of your plants. Discard the plant in your trash. Don’t put it in your composter where the spores can survive to re-infect your garden next year.
Use Neem Oil
Neem oil is effective in getting rid of rust. Remove any infected leaves or stems and then spray the plant weekly with neem oil to prevent the infection from spreading to the rest of your plant.
© 2020 Caren White
Danny from India on July 29, 2020:
Yes, Caren, you are right. Its growing plants in nutrient-based water. What I meant was that the hydroponic companies and dealers have products which can control pests, fungi & rusts.
Caren White (author) on July 29, 2020:
Abby, I can understand why you don't want to be outside in the garden lately. I live right next door in NJ and have been enduring the same heatwaves that you are going through. It's a good time to be indoors in the air conditioning.
Caren White (author) on July 29, 2020:
To be honest, I'm not sure what hydroponics has to do with killing rust. Hydroponics is a technique for growing plants in water, not for controlling disease or pests.
Danny from India on July 28, 2020:
Hi Caren. Indian farmers use organic manures and pesticides nowadays. Neem oil is also preferred but sometimes the pests are very stubborn.
I have heard about hydroponics and found it very effective in dealing with such rust bugs.
They have nutrient solutions and pest repelling products which is quite good to invest. Though it's expensive, it will just do what it is meant for, killing pests, fungi, rusts, etc..
Abby Slutsky from America on July 28, 2020:
I think I may have this on one of my plants outside. I will need to take a more careful look. Feeling lazy at the moment.
Caren White (author) on July 28, 2020:
You are so fortunate! Thanks for reading and commenting.
Peggy Woods from Houston, Texas on July 27, 2020:
I do not seem to have any problem with rust on our plants, but this is excellent information to know in case it would crop up.