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The blisters seem to be spreading only to the older (darker, interior) leaves, and the sap is dripping out of the blisters (literally raining down)

What is this?

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    Are they actually blisters, or are they a form of armoured scale? Cut a leaf off and try and scrape one of the 'blisters' away, if you're not sure... – Bamboo Oct 6 '15 at 17:17
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    This looks like an incredibly massive scale infestation. Too regular for blisters? My goodness! – stormy Oct 6 '15 at 18:29
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I don't think I'm taking much of a risk by confirming Bamboo and Stormy's comments about scale. It is scale and lots of it. Have a look at these questions here to see similar problems:

As you can see there are lots of types of scale and many attack a variety of hosts.

Scale are resistant to control due to their hard armoured shell and tendency to hide in the axils of leaves or other hard to get places.

Control depends on the size and location of the tree but each control agent will have the same secret of success: spray more than once, preferably at five to six day intervals to get the young scale who have hatched out since the last spray.

As this is a food plant I assume you would want to avoid soil drenches or systemic insecticides which are quite effective but their residual activity would be taken up by the mangos.

Physical removal by scrubbing the branches with a rough scrub brush is effective if followed by a spray of soap and water at 5ml/liter or neem oil. For a tree the issue is getting good coverage which often requires a pressurized sprayer.

Other control methods are:

  • dormant oil spray in the non fruiting season
  • light horticultural oil during the growing season
  • pruning to open up canopies
  • biological control
  • controlling/stopping ants going up the tree to "farm" the scale
  • The tree won't be fruiting until spring, so in the meantime I am using a systemic imidacloprid. So this should work, but it's also likely to kill off beneficial insects (ladybugs) that would otherwise eat the aphids off my herbs (not being treated with systemic poisons) I'll probably supplement by spraying with neem oil & scrubbing. – Justin Alexander Oct 7 '15 at 11:29

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