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Thursday, July 23, 2015
In the past couple of weeks, I've seen a lot of apple tree leaves showing the below symptoms:
Source: Grant McCarty
This copper/red/yellow leaf spot appears on the leaves of apple and crab apple trees. It can also sometimes infect the fruit. This is what is known as Cedar Apple Rust. Caused by a fungus, it overwinters on the galls of cedar trees and will then spread into your apple orchard.
Source: Grant McCarty
The reason we are seeing a lot of Cedar Apple Rust has to do with the weather we've had recently. Apple trees become infected with the disease during wet periods and when the temperature is 46-75F. Both of these conditions we have seen in the last couple of weeks.
Cedar apple rust can be a large problem for apple trees as they can defoliate the trees and blemish the fruit. With the trees I've seen this year, the infections are not wide spread to cause any decline. The fruit should still bear and the tree should be productive. In most cases, cedar apple rust does not lead to long term problems.
If infection is very widespread season after season, you might even consider removing any nearby cedar trees that are hosts to the fungus.
Not all apple cultivars are targeted by cedar apple rust. Certain apple tree cultivars are more susceptible than others. For instance, Golden Delicious, Fuji, and Gala are all highly susceptible to cedar apple rust while Jonamac and Granny Smith are not. This guide from Arkansas provides a good list on varieties' susceptibility: http://www.uaex.edu/publications/pdf/FSA-7538.pdf
There are some fungicides that are effective against the rust disease that should be applied periodically from the pink stage of bud development through third cover to protect the emerging leaves and developing fruit. This depends on the spread of the disease. Consult home fruit spray guides to determine your best option.
As with any disease, it is important to make sure what you have is what you have.