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Admittedly I haven't paid a close attention to this tree for a few months but the other day I spotted several of these spots on my pear tree. So what gives?

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I'm afraid its Pear Rust, sometimes called European Pear Rust - its one of the gymnangosporangium rusts, and as such, needs two different plant species to complete its life cycle, in this case, Pear trees and Juniper plants. If you have a Juniper in your garden, it might be worth removing it, but usually, this plant is widely planted and if your neighbours have a juniper, then that's not going to work.

Unfortunately, in the UK, there are no effective fungicidal treatments suitable for use on food plants which will control or kill it, so control is usually based around pruning out visible growths on any Juniper plants nearby; Junipers with this infection produce orange cankers or swellings on the stems, with horn like projections in spring if the weather is damp, so pruning those out does reduce the problem.

Rust may be unsightly, but it doesn't usually kill the pear tree - more information here https://www.rhs.org.uk/advice/profile?PID=236

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    I've never seen this rust! Wow. Aren't there just the most incredible creatures that have adapted so well to their own little niche.
    – stormy
    Oct 20 '16 at 19:49
  • @stormy although I thoroughly commend your open attitude, from a gardener's point of view, this infection is one of many on a long list of recently acquired pests in the UK - with no effective fungicides to treat with because they've almost all been withdrawn. For very good reasons, admittedly, but effectively it means there's no treatment. The Sporangias are an interesting group from a scientific point of view though!
    – Bamboo
    Oct 20 '16 at 21:26
  • They've banned ALL fungicides? How about dormant oils? The only fungus I know that can be treated AFTER the fungus is amungus is powdery mildew. Otherwise, fungicides are like 'raincoats' to prevent infection. Here is one for you, have you heard of White Mold or Schlerotinia scherotiorum? Truly a bad fungus, leaves 'bunny poop pellets' IN THE STEMS. They are the same size of the true bunny poops. Very, insidious stuff. One spore and a susceptible plant is d e a d, and now one has to deal with the spread to other plants.
    – stormy
    Oct 21 '16 at 5:19
  • @stormy - there are one or two available, but none is suitable for use on food plants - and for some reason, oil based treatments have never been used much. Neem is now available, but at great expense - its this shrinking of the fungicide market which has encouraged trialing milk in various solutions for mildews and black spot.
    – Bamboo
    Oct 21 '16 at 10:05
  • Really? Shall I send you some NEEM by snail mail? Serenade seems to have been a good fungicide but I won't use it for food plants. This is bothersome. Have there been any publicized results for milk as a 'fungicide'? Why it works? Fungal spores are not only prolific in soil but in the air? yes? Fungus is amazingly more detrimental to plants than insects or virus, from what I know. And you have little access to fungicide? You should be on the board of pesticides for England, Ms. Bamboo!!
    – stormy
    Oct 21 '16 at 22:00

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