The leaves on my backyard pear tree get orange spots; the leaves then turn brown, then black and fall off. What is this? It’s happening to my neighbour’s tree, too.
Pear rust is caused by the rust fungus Gymnosporangium sabinae – it causes bright orange spots on the upper surfaces of pear leaves in summer as well as early fall. The fungus needs both pears and junipers to complete its life cycle (it attacks both – the juniper is its winter host). It overwinters in swellings (called galls) on infected twigs/branches of affected junipers. Eventually these galls produce spores that can travel up to 6 km. These spores reach pear leaves, and the rust spots are very noticeable in the early summer. These spots should also contain raised, pimple-like spots that contains spores and a sticky substance that attracts insects (which transfer the spores, resulting in fertilization).
Try and control it by removing and destroying any affected pear leaves as soon as you see the rust – this minimizes spore production. Also, keep the pear and juniper hosts at least 1 km apart from one another. Inspect the juniper plants regularly and prune out any suspicious swellings. If the pear tree develops cankers (sunken lesions in the bark), prune these out. Unfortunately, there are no fungicides available in Ontario to control the disease either on pears or juniper, but some fungicides used to control other diseases may have some impact.
I’d suggest that you take a sample of an affected pear leaf to your local garden centre (encase it in plastic wrap) – to ensure that what your tree has is indeed the rust discussed above.