If you notice bright orange, almost 'neon' spots on pear leaves in mid-summer—resembling drops of rusty metal—your garden has been attacked by the fungus Gymnosporangium sabinae. The unique feature of this disease is its need for two hosts: juniper (where the fungus overwinters) and pear (where it reproduces in summer). You cannot have one without the other.
1. Biological Detective: Where Does the Infection Come From?
Rust spores can travel 5–10 kilometers on the wind. In spring, after rain, orange jelly-like growths appear on juniper branches (especially Sabin and Virginian varieties). This is the 'launch pad.' Spores fly out and land on young pear leaves. By August, 'horns' containing spores grow on the underside of pear leaves, which then fly back to the juniper.
It's useless to cut down your own juniper if your neighbor has one. You must stay ahead of the curve and block spore germination on both plants simultaneously.
Laboratory Approach: When to Start Treatments?
The critical window is during bud break and pear flowering. This is when spores are most active. If you miss the spring window, fighting orange spots in summer becomes significantly harder as the mycelium will have already entered the leaf tissue.
2. Treatment Plan: Three Steps to a Healthy Garden
Defeating rust requires a systemic approach. A single application of 'Bordeaux mixture' is not enough.
Professional Spraying Schedule:
- 'Green Tip' Phase: Copper-based fungicides (Abiga-Peak, CHAMPION). This kills spores on the bark surface.
- 'Pink Bud' Phase (before flowering): Systemic fungicides like 'Score' (Difenoconazole). They penetrate the leaf and create a protective barrier.
- After Flowering (petal fall): Repeat treatment with 'Score' or 'Horus' (Cyprodinil). This prevents infection of the fruit set.
- Late Summer (August): Treatment with 'Falcon' or 'Topas' to prevent spores from flying back to the juniper.
3. What to Do with the Juniper?
Juniper acts as an 'incubator.' If you see bark swellings or orange coating, these branches must be pruned back to healthy wood and burned. In mid-April, be sure to treat conifers with the same systemic fungicide ('Score') used for the pear trees.
Prevention Measures:
Rust is not a death sentence for your harvest, but it is a call to action. By keeping the disease under control for 2-3 consecutive years, the infection level in the garden will decrease significantly, allowing you to return to standard preventive care.
