Rust on pear — symptoms and treatment

Severity: medium

What is Rust

Pear rust is caused by the fungus Gymnosporangium sabinae. It is unusual among pear diseases because it cannot complete its life cycle on pear alone — it requires juniper as an alternate host, spending part of the year on each. That dependency is the key to managing it: no juniper in the neighbourhood means no pear rust, and a juniper nearby means the problem returns every season however carefully you treat the pear. Severity is medium — the tree is not killed, but repeated defoliation weakens it and the fruit is marked.

Symptoms

The disease announces itself with bright orange spots on the upper leaf surface — vivid, almost luminous, rounded and often with a darker or reddish rim. Turn an affected leaf over and the picture completes: on the underside, directly beneath the orange spots, the tissue swells and produces horn-like projections, small tubular outgrowths clustered on a raised, thickened patch. Heavily spotted leaves yellow and drop, and premature defoliation weakens the tree and its crop. Fruit spots also occur, disfiguring and sometimes deforming the pears.

• Early signs: small bright orange spots on the upper surface of the leaves
• Advanced signs: thickened patches with horn-like projections beneath, premature defoliation, spotting on fruit
• Confusable with: little else on pear — the orange upper spot paired with horn-like growths below is diagnostic

Causes and conditions

The fungus overwinters on juniper, where it forms galls and swellings on the branches. In spring, in wet weather, these swell into gelatinous masses and release spores that blow to nearby pear trees and infect the young leaves. The infection develops through summer on the pear, and the horn-like structures on the leaf undersides release spores late in the season — but those spores cannot infect other pears. They travel back to juniper, and the cycle restarts. Pear rust therefore never spreads pear-to-pear: every infection arrives from a juniper. Wet spring weather during spore release drives the severity of the year, and trees closest and most exposed to an infected juniper suffer most.

Treatment

Myclobutanil — chemical

Timing: from bud break. Apply fungicide from bud break. Remove nearby juniper hosts if possible. The spray is protective — it defends young, expanding leaves during the spring window when spores are blowing in from juniper, so it needs to be on the leaves before wet weather rather than after the orange spots appear. Removing the alternate host is the more permanent solution where that is an option. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Remove nearby junipers where you can — without the alternate host the fungus cannot complete its cycle
• If junipers must stay, inspect them in late winter and cut out branch galls before they release spores
• Protect young leaves from bud break, when they are most susceptible and spores are arriving
• Avoid planting new pears close to established junipers

Frequently asked questions

Will it spread to my other pear trees? Not directly. Pear rust cannot pass from one pear to another — spores produced on pear leaves can only infect juniper. If several of your pears have it, each was infected from the same juniper source, not from one another.

Can I eat pears from an affected tree? Yes. Rust marks the skin and can deform the fruit, but the pears are safe to eat. Observe the pre-harvest interval for any fungicide you have applied.

Do I really have to remove the juniper? It is the only permanent fix, but not the only option. If the juniper is not yours or cannot go, protective sprays from bud break and cutting galls out of the juniper's branches in late winter both reduce the pressure considerably.