Help!!! Rust-brown spots, some yellowing

I'm growing two autoflowering ladies, both of which were fine up until a couple of days ago. I noticed it first on the fan leaf of one of the plants, the third node from the top of the plant. Around a day and a half later, I noticed it on the same leaf on the other plant, and now it has spread to others. Additionally, I've noticed some slight yellowing on the lowest leaves.

The spots are rust-colored to dark brown, opaque from the underside, one of the leaves with the spots look sort of dry,with darker patches filling in the interveinal areas, more noticeable from the underside as well. It's looking like it might be a pH problem, but I've also been having issues with airflow, since my plants have grown too big for their case and the two intake CPU fans I have at the bottom don't seem to be doing as good of a job pushing air up through the canopy to the exhaust fan. Some spotting on the tips of the newest leaves, as well, and other smaller leaves. Also, I noticed that when I've been watering, a few little dark flies have flown up from the soil, but never more than one or two, and I haven't seen any larvae, nor any pests on the leaves themselves.

I don't know what to do, they seemed so healthy.

Looks like they just need some nitrogen

I thought nitrogen was just yellowing? I didn't know it could cause spotting like that, I've seen that with pH issues and cal/mag deficiencies... you think it might be N? Their first preflowers came out on the 4th of this month, I wouldn't have expected a deficiency so soon, I fed with Mexican bat guano (high nitrogen) last on the third, and before then fairly regularly, plus there was some in the soil, and added earthworm castings originally to the potting soil when I first planted the seeds.

hhhhhmmmm I see that on my moms lower leaves all the time. Do you get alot of runoff when you water? And how long has the plant been in that soil?

also what's the temps in your room?

I transplanted them from their solo cups when they were 8 days from sprout, so the 24th of May (they're 27 days old now). I don't water until the top inch of the soil is dry, and when I do I typically just let a bit of water drip through the drainage holes, and stop there. I added around 1TB of molasses to my water on the 3rd and on the the 7th, and I also started adding a bit more water (their growth just exploded recently), so that I think a couple of root tips (there are a few root tips coming out the bottom of my pots) were sitting in the water that had filled up the water tray a bit. I'm wondering if there was residue on the bottom of the water tray that might have mixed in with the additional water and overloaded the plants... ?

Lately the temp. has been btw. 78-84F, but I'm worried there's a temp. difference going on in my case, like the leaves have grown in so much heat's getting trapped at the top.

The soil is composed largely of sphagnum peat moss and composted bark, and the only things I've added to it so far have been earthworm castings, Indonesian, Jamaican, and Mexican bat guano, and organic unsulphured blackstrap molasses.

to me she doesn't look bad at all and remember she will never be spotless, If possible get some lime to topdress with when using something that is peat based or your gonna end up with problems

I hope so, I wouldn't be so worried if it didn't seem like the problem was spreading, but I trust your judgment over my own inexperience. : /

I read that H202 can help with issues like this, I use a brita filter and let my water sit out 24-48 hrs. before using it, but maybe there's still too much chlorine/impurities. Think it's worth a try to add it? I believe it's recommended to add 2 tsp. per gallon of water, supposed to help with various root issues as well.

Also, my pH meter broke a while back, so I'm definitely still worried about pH issues - I ought to go out and buy another one today, I was hoping I had a good enough feel for my plants to go the rest of the grow without it, but it's not worth it if my plants unexpectedly start spotting.

go buy one NOW lol don't wait.... get off your computer at this very moment and go get one lolol very important

Found some pH test strips I had lying around - 5.5! Too acidic for soil, right?

that was from runoff i collected this morning, + the residue from the water trays

are you using ro water?

No, unfortunately not. All I have right now is a Brita filter (which doesn't really filter much out anyway, besides smell and taste ), and I let the water sit out for 24-48 hrs., which only takes care of the chlorine. Should I look into getting a reverse osmosis filter?

Every time I've mixed my nutes in, the pH has been between 6.5-7.0. That's higher than my plain water, which without anything added is 6.0.

Plus, turns bat guano is acidic, along with the aforementioned peat moss (both have been featured heavily in my grow, and I noticed that the soil mix didn't have any dolomite lime or anything similar to buffer the acidity of the peat). I have some bone meal on hand, which is a bit basic and could potentially be a replacement for the guano as far as the girls' phosphorus goes, but I don't know. I won't add anything until my girls are stable and I'm sure the pH is at a desirable level.

When I watered this morning, I used a solution of water + H202 that (surprisingly, as H202 is supposed to be a bit acidic...) had a pH of around 6.8-7.0, used it to spray the leaves as well, and the spots have stopped progressing, though it doesn't seem like the damage already done will recede. At this point I'm still waiting, and planning on doing a more thorough flush, as I didn't do the recommended ratio of water to pot volume. Thank you for all your help, guys, +rep, I think I'd be nervous wreck right now without your support - my babies!

One more question - if my plants had fungus gnats, what sort of damage would they be showing??

feelingreen said:

One more question - if my plants had fungus gnats, what sort of damage would they be showing??

Click to expand...

Fungus gnats can be tricky to tell when it comes to damage. At times they can cause phantom cal-mag deficiencies. First, set up a sticky trap, or a small glass of clear soda near you plants. Rake the top inch of your medium and look very closely. Or, let the soil dry out and give it a quick watering. Watch as the water pools up. If there are any gnats you will see them. If you find yourself searching and searching for 10 minutes and not really sure if you see anything, you are fine. You will know if you have them for sure Check the traps periodically.