Our “rainy season” is a good time to talk about rust disease in plants. Some years we have huge outbreaks of this plant disease, and other years, not so much. While there’s no way to predict the impact of rust disease this year, we can help minimize it’s impact with a few preemptive actions on our part. But first, let’s talk about how rust disease works on our plants.

Rust disease in one of the more interesting plant diseases we have in that it’s a disease that requires two different plants to complete its lifecycle. There are a variety of rust diseases. There’s cedar-apple rust, Asian pear rust, cedar-quince rust among others. Since they all follow a similar life cycle, let’s talk about cedar-apple rust.

Right now, cedar-apple rust disease (as well as the others) can be found residing on cedars and junipers. You might say that “I don’t have any cedar or junipers in my landscape, but the reality is that the spores from this fungal disease can travel over a mile riding on the wind. So likely somewhere within a mile radius of your home, someone has one of these trees, or they are growing in the wild.

To determine if you have cedar-apple rust galls on any of your cedars or junipers, look for some round, gall like growths hanging on the branches. This is the form of the disease that overwinters. As the spring rains begin, these galls absorb water and begin to push out what are called telial horns. These gelatinous horns are typically orange in color, and some might say they look rather sinister. These galls, typically about the size of a golf ball, but can range in size from between 1/16^th of an inch to about two inches in diameter. However, when these telial horns are fully extended, they can grow to baseball size.

These horns are full of disease spores, just waiting for the wind to carry it to the partner plant in their lifecycle. Once the spores make their way to the other partner plant, it attaches and begins to grow a new set of spores which typically get blown back to the waiting cedar or juniper a little while later, and the life cycle begins again.

Your first line of defense against this disease would be to monitor you cedars and junipers for these galls. If you see them, and can get to them safely, just pull them off and throw them away. At that point there is no need for treatment.

To minimize the impact to your apple, pear, or quince (among others) you can treat these plants with a fungicide through the month of April. Yes, we are past April, but better late than never. Fungicides would include copper hydroxide, chlorothalonil, myclobutanil, or propiconazole. Typically, you will need to apply one of these products every 7 to 10 days but follow the directions on your particular product of choice.

Then in late June through July, to reduce the transference of the disease back to the cedars and junipers, you can begin the fungicide treatment program on these plants.

The good news is that rust disease is not typically fatal to your plants. But, if it’s a fruit tree you would like to gather fruit from later in the season, rust disease can inhibit production. With a larger outbreak of the disease on your plants, they might lose a lot of their leaves. But as the leaves die, they are replaced with new, healthy leaves. It will just look a little awkward for a while. Good luck!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701 or by emailing us at mg@tulsamastergardeners.org. Photo: Tom Ingram