Yellowish spots on some (but not all) leaves

So I noticed tonight that some areas of my canopy are showing the yellowish spots in the photos below. About 1/10th of all canopy-level leaves seem to be affected, mostly the large fan leaves just below the major colas.

As far as the setup background, this is a single plant in a ScrOG setup, in about 5gal of organic soil with dry ferts (Espoma Plant-tone and Happy Frog Fruit & Flower) mixed in and occasionally top-dressed. I'm using about 300w worth of CFLs and I water about once every 10 days.

The box it's growing in has an average low temp of 66F and an average high temp of 84F. RH ranges from mid-30s percent to around 60%. There could be spots in the canopy that get poor air circulation (thus hot or humid spots) because I only have one fan blowing air around, and it's not blowing directly across the canopy under the lights. Perhaps hot spots might be causing this issue...?

Unfortunately I don't have a functioning pH meter right now, so that's one measurement I can't give you. Here are the photos, pictures (1) and (2) are closeups of affected leaves, while (3) shows some unaffected leaves (with the leaf from (2) in the background):







Finally, here's a shot of my whole canopy, just to give you an idea of the overall health of this plant:

have you been spraying them ? you need to get the P H sorted m8 , how good is the drainage? needs a bigger or more smaller fans imo,,,, yer plant looks good m8 hope it gets sorted out

Haven't been spraying. Using the guides, I was thinking this looks like calcium or manganese deficiency, though there could be a lockout occurring since I don't have a pH meter. Any other suggestions?

Oh and drainage is good.

How many weeks into flowering are u?

mikmike said:

How many weeks into flowering are u?

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Around day 50 of 12/12 now, this strain goes something like 100 days or more usually.

mikmike said:

How many weeks into flowering are u?

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Around day 50 of 12/12 now, this strain goes something like 100 days or more usually.

some early Mg def, methinks. a shot of epsom salts, imo

looks good, though. i always get some of that peripheral yellowing in flower. MgSO4 seems to fix it.

scroglodyte said:

looks good, though. i always get some of that peripheral yellowing in flower. MgSO4 seems to fix it.

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Thanks! I've got some Epsom salts, actually, so I'll try that out right away.

It seems like most of the affected leaves are limited to the center-back part of the canopy (incidentally the area I suspect gets the worst air flow), so I'm suspecting heat is at least a factor here. I've watered with a little epsom salts (about 1 tsp/gal), and it looks like the issues aren't spreading beyond that area of the canopy.

That being said, the affected area does seem to be getting at least a little worse. Now, there are small, raised spots appearing on the fan leaves in that part of the canopy, like these:



Most of the rest of the canopy looks fine, even healthy, so it is really strange that this seems to be so restricted. Just to show what I mean, here is a photo from the edge of the canopy and then a wide shot showing the whole plant:





At this point I'm hoping these are just residual effects, and the epsom salts will cure the ailment over the next couple days. If not, anyone have any further suggestions?

So it doesn't seem to be improving noticeably, and might be getting slightly worse. For example, this is the first leaf with the spots I've seen that's this close to the buds:



Also, this is a leaf I just clipped from deep within the canopy (possibly a hot spot with minimal air flow?):



The plant still seems fairly healthy overall, but I'd like to figure out what I can do to address this issue. Lemme guess... go buy a pH meter? lol...