Cedar-apple rust is a common disease in our area. If you have apple trees you have probably seen it before. It is an unusual disease in that it alternates between hosts. Let’s talk about this disease so that you can learn how to identify and control it.
Cedar-apple rust (CAR) is from the Pucciniaceae family. Members of this family need more than one host to complete their life cycle. Some other diseases in this family are black stem rust of wheat, soybean rust, and white pine blister rust. Apple trees that are heavily infested with CAR will see reduced yield and blemished fruit. It can also weaken and kill eastern red cedar is the infection is severe enough, but this is rare. Red cedar is the primary alternate host for CAR. On apple trees, the infection creates orange-yellowish spots. On cedar trees, you will see gelatinous masses form that are bright orange. You also see what are called telial horns that grow out of the mass. To me these masses look like they are not from this earth. If the tree is heavily infected it might look like little Christmas decorations hanging from the limbs.
These masses will start to appear on cedar and juniper trees about seven months after infection and turn gelatinous 18 months after infection. The horns begin to appear, but if the weather is dry, they will dry out quickly, and then swell back out after a rain. The horns release spores to infect apple trees. The masses on the cedar tree will dry up and remain on the tree for about another year, but they are dead at that point. The spores released can only infect plants from the rose family, like apple trees, not more cedar trees. The spores released can travel 2 or 3 miles on the wind to infect an apple tree. When they land on an apple tree, they germinate and create the orange spot. These spots release a sticky liquid to attract insects. As the insects pick up the sticky liquid they transport spermatia from one infection to the next, which fertilizes the disease. The disease then grows through the leaf and creates new spores on the bottom side that are blown back to a cedar tree to start the cycle all over again.
Spraying to control this disease on cedar trees is not recommended. CAR will rarely kill cedar trees. You can prune out galls to slow its spread. If you see this disease on a cedar tree, it may be best simply to co-exist with the disease, as it does little damage to the cedar. On apple trees there are some other management options. It will cause apple trees to lose their leaves and infects the fruit. Captan and myclobutanil can both be used preventatively against CAR. If you have cedar trees right next to your apple trees you might consider removing them. But keep in mind that the spores can travel a couple of miles from cedar trees, so even if you remove all the cedar trees that you can see, you can still get this disease because we have a lot of cedar trees around.
Cedar-quince rust looks similar to CAR, but it infects pear trees instead of apple. This disease also forms gelatinous masses along its branches instead of balls like CAR.
If you have questions about cedar-apple rust contact your County Extension Office or email me at [email protected].