Cedar-Apple Rust

After a spring rain, those orange jelly horns on a redcedar gall are already sending spores to the apple, so cut the galls you can reach before they swell, and if you still want clean eating apples from a susceptible tree, start a labeled protectant at pink, not in July, because by then the spots are already on the leaf. Cedar-apple rust is the fungus Gymnosporangium juniperi-virginianae. It cannot finish its life on apple alone, and it cannot finish on juniper alone, so it spends about a year and a half in a woody gall on eastern redcedar or another juniper, then a few wet weeks on apple or crabapple leaves.

People call the swollen orange mass jelly horns once they see it after rain. Those horns are still this rust. On the apple the useful sign is a bright yellow-orange leaf spot, often with a red border and tiny black dots in the center, and later a ring of short tubes on the underside. One ugly June rarely kills the tree. Olive-green velvety spots belong to a different disease, and a blackened shoot bent like a shepherd’s crook is fire blight, not this fungus.

Cut the galls you can reach. Spray the apple at pink, not in July

Once the leaf is painted orange, that infection is already done, so a bottle in July cannot rewind the spot. The useful job this season is to cut next spring’s spore load on the juniper you can actually reach, and to coat new apple leaves while the galls are still wet and orange, because that is the only window the fungus uses to jump from cedar to apple. Spores from the apple leaf do not infect the next apple leaf. They blow back to a juniper, so raking apple leaves the way you would for apple scab does not break this cycle.

Walk eastern redcedar and other junipers in late winter, before the first warm rain, and prune the brown, dimpled, golf-ball galls while they are still dry. Bag them. A gall left in the grass can still swell when it wets up. Cutting every gall on your own tree helps the apple in the same yard. It will not save that apple if the neighbor’s fence row is full of galled redcedar, because the wind can carry spores a mile or more, and most backyard infections still start within a few hundred feet. Do not take down a healthy screen of cedars hoping the woods two miles off will go quiet.

If the tree is a susceptible eater and you want fruit that is not orange-spotted, a labeled protectant has to start when the flower buds show color but are not yet open, the pink stage, and keep going through petal fall and first cover, while the juniper galls are still throwing jelly after rain. Most home products coat the surface rather than move inside the leaf, so rain washes them and you spray again on the label interval, shorter when the week stays wet. Stop when the galls stay dry and shriveled for good, because the cedar has finished sending spores for the year. Skip an all-purpose fruit mix during bloom if the insecticide in that bottle will hit bees you still need. Read the apple line on the label, and the days to harvest, because some myclobutanil mixes are sold for ornamentals only and those are not an eating-apple program. A kitchen baking-soda mix is not a labeled apple program, and it can burn the leaf.

1. Confirm. After rain, orange jelly horns swell from a brown dimpled gall on juniper. On apple or crabapple, look for yellow-orange spots with a red border and, later, short tubes under the leaf. Olive velvet, or a shepherd’s-crook twig, is a different job.
2. Prune reachable galls in late winter, before they swell, and destroy them. Do not count on that cut if galled redcedar stands upwind for a mile.
3. Do not plant a susceptible apple or crabapple against an eastern redcedar. A few hundred feet helps. Clearing every cedar in the county is not a backyard job.
4. On a susceptible eater, a labeled apple protectant from pink through first cover, while the galls are still gelatinous after rain. Stop when the galls dry for the season. Skip a July rescue spray on spots that are already there. Liberty, Freedom, Redfree, and William’s Pride skip most of this calendar; Jonathan, Rome, Golden Delicious, York Imperial, and Wealthy do not.

You do not have to cut down every cedar in the neighborhood

The fungus needs a juniper to survive the winter, so gardeners keep hearing that the only cure is to fell every eastern redcedar within two or three miles. Commercial orchards have tried that, and even they cannot keep a landscape full of ornamental junipers off the wind. Spores travel several miles. Most infections still start within a few hundred feet of a galled tree, so moving a new apple well away from your own redcedar is worth doing, and leaving the neighbor’s windbreak alone is the honest backyard finish.

On the juniper itself the gall is usually looks, not a death sentence. A twig beyond a large gall can die back to the gall, and a young plant covered in galls can look decorated after rain, but a mature redcedar almost always lives. Prune the galls you can reach if the orange week bothers you, or if you are trying to spare an apple in the same yard. A spray program on the cedar from June to September is an orchard leftover, not the numbered job on a home screen planting. Do not replace a healthy redcedar with another juniper that still hosts this fungus and then call the apple cured.

A spotted apple tree can still live

Cedar-apple rust is not a lethal disease on a healthy apple or crabapple, even when a wet spring paints the canopy orange and some leaves drop in a dry July. Live with an ornamental if the ugly month does not bother you, water it through drought so the wood does not go into winter weak, and spend the pruners on the juniper galls rather than fogging a canopy of spots that are already finished. Several years of heavy leaf drop on a very susceptible eater can shrink next year’s bloom and leave the tree more open to winter injury, so a Jonathan or Rome that yellows every wet May is a candidate to replace if you want fruit you will actually eat.

Orange spots on the fruit are usually shallow. Wash the apple and cut or peel the marked skin if the flesh is firm. A fruit that cracked and then went soft or moldy is not worth saving, because a rot fungus can enter the split. Fruit stems can pick up the infection on a very susceptible variety and drop early, and that crop is already done for the year. Planting a resistant eater is the long job that skips most of the spray calendar. Liberty, Freedom, Redfree, and William’s Pride are the names garden centers actually stock for this fungus. Resistant is not a promise against apple scab or fire blight, and a galled redcedar next to the new tree still sheds spores, so the spacing still matters.

Orange jelly horns on the gall, and tubes under the leaf, confirm this rust

Look at the juniper first after a warm spring rain, because that is when the disease is obvious from the street. A mature gall is brown to dull red, round or a little kidney-shaped, often pea-sized up to about two inches, with dimples like a golf ball. Wet weather makes bright orange, gelatinous horns push out of those dimples. The horns shrink to rusty spines when they dry, then swell again the next time it rains, sometimes several times in one spring. After that season the gall dies. It may hang on the twig for another year, but it does not throw a second crop of horns.

On apple and crabapple, tiny yellow spots show on the upper leaf about ten to fourteen days after infection, usually after bloom. They turn yellow-orange, often with a red border, and pick up a stipple of tiny black dots. By midsummer the underside under each spot grows a ring of short, finger-like tubes, and those tubes fray at the tip when they shed powdery orange spores toward the juniper. Fruit lesions start as slightly raised orange patches. They can crack and brown as the apple swells, and they do not run deep into sound flesh the way a maggot trail does. There is no second round of apple-to-apple infection. New galls on juniper take until the next year to show, and they do not throw horns until the spring after that, so the whole loop is about two years.

Olive velvet is apple scab. Short stubby horns are a different rust

Olive-green velvety spots with a fringed edge, often along a vein, then corky brown patches on the fruit, are apple scab. That fungus winters in last year’s apple and crabapple leaves, not in a juniper gall, so the job there is rake and a green-tip spray, not cutting orange horns. Bright yellow-orange with tubes on the underside is this rust. Do not spend July raking for a tree whose actual spots are rust, and do not paste that scab calendar onto this job.

A smaller, less regular gall on juniper, with short, stubby, darker horns instead of long noodles, is cedar-hawthorn rust. That fungus hits hawthorn hardest, and it can also spot apple and crabapple. Confirm the gall, then see that disease. Do not paste its hawthorn-fruit method here. Cedar-quince rust does not make a round golf-ball gall. It swells a juniper twig into a spindle and oozes orange from cracks in the bark, and on hawthorn it can spike the fruit and twigs. There is no separate quince-rust URL here, so prune the swollen juniper wood and do not copy this apple-at-pink calendar onto a hawthorn fruit that is already spiny.

A young shoot that wilts, blackens, and bends over like a shepherd’s crook, with leaves that cling as if scorched, is fire blight. That is a bacterium. It can kill limbs. Rust spots a leaf and blemishes fruit; it does not torch the twig. Do not run a bloom antibiotic program for orange spots. White powder on new shoots in dry heat is powdery mildew, a different leftover.

Related reading

• Apple scab — olive-green velvet and corky fruit that winter in fallen apple leaves, not in a juniper gall.
• Cedar-hawthorn rust — smaller galls with short stubby horns; hawthorn takes the worst of it.
• Fire blight — a blackened shepherd’s-crook shoot, not an orange leaf spot.
• Powdery mildew — white film on new shoots in dry heat, not wet-weather rust.