Brown spots on phal leaves

[turock]

Hi all, time to play Name That Spot!

I have a large phal hybrid that is one of my healthiest orchids--or was, I should say, until a mealy bug mob moved in. I recently repotted the plant and used a systemic pesticide (imidacloprid) to try and control the bugs.

Now I'm noticing many small brownish spots on the underside of the leaves. They look kind of watery but don't ooze anything when I poke them.


In some cases the spots are also visible on the top side of the leaves. They also cause the plant's tissue to be translucent in the sunlight.


I cut off the first few blemished areas I found, about three days ago. But when I checked the plant again today, two entire leaves were dotted with them. I cut off both leaves and am still seeing some spots in random places on the remaining leaves.

I've seen sunburn, insect damage, cold damage and edema, but this doesn't look like those. Could this be a reaction to the insecticide, or could I have an infection on my hands? Should I continue to hack and slash at my once-lovely plant or is there a better way to move forward?


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[estación seca]

Are those in the places mealy bugs once existed?

[cjm3fl]

I've never, personally, seen that with my Phals but I'm going to say it's an insect issue, still.

Systemic pesticide take time to get up into the plant and begin working. Sometimes weeks before we see the full effects.
I'd spray the exterior with something to assist the imidacloprid until it's had time to work.

It's a kind of damage I've never seen on my Phal's but that's not like any kind of damage from sun spots/burn that I've experienced.


If that showed up on one of my Phal's I'd use an exterior pesticide, remove the plant from any direct sun light, stop feeding it, and I'd reduce amount of water (not frequency) for now.

When applying the exterior pesticide...spray any and all plants in the area you normally keep this Phal.

Best of luck.

[Orchid Whisperer]

Before any more cutting, determine if those spots are growing or not. Take some photos of leaves where you see this, daily under similar conditions, and compare photos day-to-day.

Are the spots growing (indicating an active growing infection)? If yes, then cutting may be justified. If the spots are not growing, I would stay the knife and stick with watchful waiting, and maybe more daily photos, at least for a week or two.

[FourN6]

I have similar spots on an oncidium that showed up over a few days. Turns out I had aphids... I have no idea which plant the pests came from but I have my suspicions it was a neighboring Zygo. I hit all plants in the space with insecticidal soap and havent seen any new spots (or bugs) since. This weekend the plants will get a second spraying but I'm hoping the first spray in addition to repotting took care of the worst of the aphids.

[turock]

Thank you, guys, for the insight. I cut off the worst leaf and decided to watch it before cutting anymore. The spots did not grow.

The mealies were mostly tucked into the crevices between leaves, where they always like to hide. I followed your advice, cjm, and am seeing far less mealies and no new spots. I'm also wondering if they could be a strange form of edema, since I soaked my plant in the evening before repotting it.

Interestingly, I ordered some phals from a big wholesale nursery and noticed the exact same spotting on some of the leaves! Chocking it up to insect damage seems plausible. Also, I know those large nurseries use all kinds of chemicals and fertilizers, so I'm still suspicious that it could be a sensitivity to the insecticide.