Bright orange spots on pear leaves are hard to miss, and they tend to show up all at once in early summer. In most of North America the cause is pear trellis rust, also called European pear rust. It is a fungal disease with an unusual habit: it cannot complete its life cycle without a juniper nearby. That one fact explains most of what you can do about it.

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The short answer: yellow to orange spots on the upper side of pear leaves, followed later by brown, acorn-shaped growths on the underside, are pear trellis rust. It is usually cosmetic but can weaken a tree in wet years. Remove and dispose of infected leaves, look for galls on nearby junipers, and choose rust-resistant junipers if you are planting near a pear.

What pear trellis rust looks like

The disease is caused by the fungus Gymnosporangium sabinae. According to Washington State University’s Hortsense and UConn’s Home and Garden Education Center, the signs on the pear follow a clear sequence:

In spring, small yellow spots appear on the upper surface of young leaves.
Over summer the spots enlarge and turn bright orange to reddish orange, up to about an inch across.
By midsummer, tiny black dots form in the center of each spot.
By late summer, brown, blister-like swellings grow on the underside of the leaf, directly below the spots. These develop into pointed, acorn-shaped structures with slotted, lattice-like sides. That trellis pattern gives the disease its name.

Fruit and twigs can also be infected, and heavily spotted leaves may twist and drop early.

Where	What you see	When
Pear leaf, top	Yellow, then bright orange spots with black dots	Late spring to summer
Pear leaf, underside	Brown, acorn-shaped growths with slotted sides	Late summer to fall
Juniper twig	Spindle-shaped swelling	All year
Juniper twig in wet spring weather	Orange, jelly-like spore horns	Spring

The juniper connection

Like the related cedar-apple rust, this fungus needs two hosts. Spores made on infected junipers in spring infect only pears. Spores made on the pear leaves in late summer and fall infect only junipers. The fungus then lives in the juniper twig, and UConn notes that those galls can keep producing spores for several years.

The Connecticut Agricultural Experiment Station reports that pear leaves are most susceptible when they are young, less than about ten days old, and that infection is favored by cool, wet spring weather. That is why a rainy spring often leads to a bad rust year.

Spores can travel a long way on the wind, so the source juniper is not always in your own yard. Still, the closest junipers matter most.

How serious is it?

On a healthy, established pear, pear trellis rust is mostly a cosmetic problem. The Connecticut fact sheet describes the disease as generally cosmetic, but heavy, repeated infections in wet years can cause early leaf drop, dieback and reduced vigor. Young trees and trees already stressed by drought or poor soil are more at risk.

What you can do

Clean up infected leaves. Remove and dispose of infected leaves as you see them and rake fallen leaves in summer and fall. That reduces the number of spores reaching junipers.

Check your junipers. In late winter and spring, look for swollen, spindle-shaped areas on juniper twigs, and for orange gelatinous masses after rain. Prune out infected twigs in fall or winter and get them out of the yard.

Mind the distance. The Connecticut fact sheet suggests separating pears and junipers by more than 1,000 feet where possible. That is not realistic in most neighborhoods, but it is a reason not to plant a new juniper hedge right next to a pear.

Choose plants carefully. All pears can be infected, but some varieties are less affected than others, and some junipers, including common juniper (Juniperus communis), are reported to be more resistant. Ask a local nursery or your extension office what works locally.

About fungicides. Products labeled for rust on pears can protect new leaves in spring if they are applied before infection, but they do not cure existing spots. For most home trees, cleanup and juniper management are the more practical steps. Always follow the label exactly.

Could it be something else?

Black, curled shoot tips that look scorched are fire blight, a bacterial disease that needs pruning. Brown blotches without orange color have other causes, covered in why pear leaves turn brown.

When to get help

Call your county extension office if you are unsure of the diagnosis; a leaf sample is often enough to confirm rust. Call a certified arborist if the tree is losing most of its leaves early year after year or shows branch dieback. The ISA Find an Arborist directory is a good place to start. The juniper side of the story is in why junipers turn brown.

Sources

Pear: Pear Trellis Rust, Washington State University Hortsense
Pear Trellis Rust, UConn Home and Garden Education Center
Pear Trellis Rust (fact sheet), Connecticut Agricultural Experiment Station

Top photo: Björn S..., CC BY-SA 2.0, via Wikimedia Commons

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