Friday, September 25, 2009

No, oranges do not grow on pear trees.

Sunday, taking a breather during a very animated birthday party at my son's house, I wandered among the fruit trees behind the garden to see what was on offer. A large pear tree caught my attention; from a distance, it looked as if it were bearing small oranges.

No such luck. Not even orangey pears. This is what I found:

Gymnosporangium, the pear-juniper rust.

These mounds grew on the underside of the leaves. Out of each, one or more nipple-like, slightly hairy, pale growths sprouted. Where the sun shone through the leaves, the whole mound glowed a brilliant orange.

Closer view of a couple of mounds.

Topside of a leaf

Some kind of gall, I thought; some insect growing inside. I Googled "pear gall" and got nowhere. "Pear disease" turned up Gymnosporangium sabinae, the European pear rust. This one is probably Gymnosporangium fuscum, the pear trellis rust, introduced to BC around the 1960s.

It's not an insect, after all: it's a fungus. And it has a surprising life cycle. The fungus overwinters in junipers, in the form of orange, jelly-like swellings along the twigs. When the warm spring rains arrive, it produces spores, which float in the air to the nearest pear tree. (All varieties of pear seem to be susceptible.) The pear develops, first, orange spots on the leaves, then the mounds. At the end of the summer, the "nipples" appear, and produce new spores, different than the ones that attached themselves to the pear trees. These are carried back to the junipers on the wind, re-infect them, and wait for the spring.

The recommended remedy is to remove any junipers from the vicinity of the pear trees, up to 100 feet away. (This site suggests 100 metres.) Fairly simple, if the junipers are on the same property. In this case, the pear tree is near a wall that borders an apartment complex. The landscaping (we can't see it) is probably mostly junipers.

The other solution is to remove the pear tree. Or put up with the spots and the occasional damaged fruit.



  1. There's something about parasitism that gives me the shivers. Thanks for the investigation, as I've always wondered what it was myself.

  2. Wanderin' Weeta - Thanks for the article, and pointing it out to me. Your photos are hauntingly clear. What a creepy looking disease.


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