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Is anyone able to identify this disease / pest? We have this on several citrus of different types including an orange, grapefruit, lime, and a mandarin orange.

image 1 image 2

My first thought was that it could be snails as some of the blighted area almost looks like it has snail tracks around it. I don't see snails around the trees, but they might be attacking at night.

This is in San Jose, CA.

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You'd see snail trails over the leaves were it those. Are they in pots or in the ground these plants, and have you looked on the underside of the leaves for signs of aphid infestation? –  Bamboo Sep 4 '12 at 11:55
    
they are in the ground. i can't see any aphids or other pest. –  Jeffrey Blattman Sep 4 '12 at 12:42
    
Is it just as bad on the orange as it is on the other citrus trees? And are all leaves affected, old and new? Any faint yellow (chlorotic) spots or blobs/marbled effect anywhere? Otherwise, I can see what looks like leaf miner damage in the right hand picture - is that what it is? –  Bamboo Sep 4 '12 at 13:13
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This is likely the work of the citrus leaf miner. More details can be found here. The key identifier is the squiggly tracks on the leaves. This moth and it's larvae can be found on all ages of citrus trees but the damage is more noticeable on younger trees.

Citrus leafminer rarely causes serious damage and management is normally limited to practices that limit succulent growth and protect natural enemies

If you feel that you have to try and control this pest these are the recommendations:

  • they are attracted to the new growth. Avoid pruning live branches more than once a year, so that the cycles of growth are uniform and short. Once the leaves harden, the pest will not be able to mine the leaves.
  • Do not prune damaged leaves since undamaged areas of leaves continue to produce food.
  • Do not apply nitrogen fertilizer in summer and fall when leafminer populations are high and growth will be severely damaged.
  • Remove water sprouts that develop on branches and above the graft union on the trunk and might act as a site for the moths to lay eggs . Suckers should always be removed. Chemical Control
  • Many insecticides registered for residential use do not effectively control citrus leafminer, because they have difficulty reaching the larvae inside the mines.
  • Pheromone traps are available

For the homeowner I would recommend cultural practices and pheromone traps or no action over chemical controls.

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Kevinsky: you don't think there's a chance of Citrus Cholorotic Dwarf virus as well? There don't seem to be any major signs of holes or raggedness to the leaves, which citrus larvae usually make before leaving... –  Bamboo Sep 4 '12 at 15:10
    
Well, sure, there is a chance. But that virus is still relatively unknown in the States. Closeup pictures might reveal the frass of the larvae which would be a definitive ID. –  kevinsky Sep 5 '12 at 1:24
    
That's what I was wondering, whether it had got to the States yet - it was on the 'Incipient Threat' list over ten years ago for the USA. –  Bamboo Sep 5 '12 at 10:29
    
after looking at other pics of CLM, i think this answer is spot-on. further research shows that there's no "cure", and the recommendations above seem to be the best we can do. –  Jeffrey Blattman Sep 9 '12 at 16:18
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