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I recently noticed that a Malabar Java plum (Jamun) tree in our garden has most of its leaves covered in a strange white layer, like in the picture I've attached.

enter image description here

I'm curious to understand why this is occurring and whether it could potentially lead to allergies such as allergic rhinitis. What's causing this, and what can be done about it? Any help understanding this would be great. Thanks!

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    $\begingroup$ Please read about powdery mildew to see if that fits what's going on with your plum. Without handling the leaf, I can't be sure, but it's the most common cause of white, grey or brown spots on plants. $\endgroup$ Feb 18 at 10:30
  • $\begingroup$ You also have scale insects on there (along the mid-rib are the most obvious adults, but lots of life stages all over in patches) - these are sucking insects, often covered in white waxy or powdery substance. $\endgroup$
    – bob1
    Feb 18 at 21:49
  • $\begingroup$ @bob1 - Wow, I need new glasses; I completely missed that. Scale insects also exude a sweet substance that promotes fungal growth, but I think the primary problem there is scale. $\endgroup$ Feb 18 at 22:57
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    $\begingroup$ @anongoodnurse Yep, scale looks like the problem - you can even see where a couple of adults have come off mid-leaf, leaving the rings behind. Feel free to write it up. $\endgroup$
    – bob1
    Feb 18 at 23:01
  • $\begingroup$ Thanks, but I missed it, you caught it. :) $\endgroup$ Feb 19 at 1:54

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The major problem your plant has here is a very heavy scale insect (Coccomorpha) infestation. These are sucking insects that bite into the veins of plants and suck the sap. They are related to aphids, and feed in a similar manner. The females attach themselves to the plant and are covered by a shield and/or a waxy layer for protection from the environment and from predators. Like aphids, when feeding they exude some sap residue from their gut and this can result in growth of fungal species on the plant. The fungal species are often black sooty molds, but I would hazard a guess that not always.

You can see adults clustered along the main mid-rib of the leaf and smaller instars clustered along the lines of leaf veins. The adults look like ovals with a darker line running down the middle. The instars are particularly obvious about mid-leaf, near the two ovals outlined in white, where you can see 3 approximately regularly spaced veins with lines of them feeding. The two ovals outlined are where adults have fallen off and left behind some of the waxy coating they coat themselves with.

I don't know which of the many many species of scale insect within the grouping you have here, but one common and well known grouping within the scale insects is the mealybug (not what you have here, mealybugs are all unarmoured, yours is armoured). You should be able to treat your plant with a product that is designed to kill mealybugs. These products usually contain an oil delivery system to counteract the waxy water repellent layer the scale insects coat themselves in, and an insecticide to kill the insects.

The white residue you are seeing on you plant is probably not mold and is the waxy substance you can see on the insects. I can't find any reports in the scientific literature of allergy to scale insects, so these are unlikely to be causing a problem. Molds from the secreted sap might cause a problem as fungi from many types are known to cause allergies.

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