There are many diseases that affect apple trees, but today we’ll focus on one very common disease, cedar-apple rust, and some ways of controlling it. If you have apple or flowering crabapple trees, it is likely you have seen cedar-apple rust and its symptoms in the past. It is a fungal disease that regularly affects the leaves of apple and crabapple trees during early summer months.
It is called cedar-apple rust because it overwinters in galls on juniper (cedar) trees. These galls expand during April, typically after a rain. They then appear as golf ball-sized orange balls of gelatin hanging in the cedars. Some of you may have noticed these and not known what they were. The orange galls are spore-producing structures in the disease’s life cycle. These orange galls begin to release spores soon after they expand in early April. The spores can float through the air and can potentially infect apples and crabapples up to 2 miles away.
The apple-affecting part of the disease’s life cycle is identified on apples and crabapples as 1/8- to 1/4-inch spots that appear on the leaves in late spring or early summer. The spots will be a bright, yellow-orange color soon after they appear and may have small black fruiting bodies (fungal reproductive structures) in the middle of the spot.
Cedar-apple rust can cause leaves to drop prematurely and will weaken infected trees. Premature leaf drop can cause decreased yield, and less fruit to set the following year on apple trees, and will cause a reduction of aesthetic qualities of ornamentals. It is rare for cedar-apple rust to directly kill an infected tree, but a tree already weakened by disease can more easily fall prey to other diseases and pests.
Luckily there are several options for control and management of cedar-apple rust that don’t involve cutting down your neighbor’s cedar trees. Two fungicides are available to homeowners for control of cedar-apple rust, as well as apple scab, another fungal disease of apples. The two fungicides, Ferbam and Immunox, control rust on both apples and flowering crabapples.
For best control, you need to start spraying as soon as you see the orange galls swelling on junipers. If you don’t want to scout for the activity of these galls, you can simply schedule your spray applications by the calendar. Do your first application in early April and then spray again every 10-14 days until the end of May.
If you are interested in a more organic approach to controlling rust, you could use a copper compound, such as Bordeaux, to get some control of fungal diseases. However, the control will not be as good as it is with the other fungicides. If you choose to use copper compounds, read the directions before application, as there are several formulations that may vary in application practices.
Another option you have in the control of cedar-apple rust is to choose and plant varieties of apple and crabapple that have resistance to the disease. Enterprise, Freedom, Liberty, Redfree, and Williams Pride are all apple varieties that show good levels of resistance to cedar-apple rust. Most of these varieties are also resistant to apple scab. At the Leavenworth County Extension Office we have an extensive list of crabapple varieties that are also resistant to rust. For more information on resistant crabapples, or for any other questions, you can contact me at the Leavenworth County Extension Office at Hughes and Eisenhower roads in Leavenworth, or call (913) 250-2300. I can also be reached via email at firstname.lastname@example.org.