Lettuce growing very slowly; brown spots on leaves

[Dennis Jay]

My system has been fully cycled for six months, but still not getting good growth out of most of my plants. Any suggestions would be welcome.

The system:

50 juvenile largemouth bass in 330 gal tote.

12' x 2' x 12" grow bed with hydroton, cycling every 18 minutes.

System is inside greenhouse with grow lights.

Water tests are pretty consistent with pH at 6.5; ammonia at .25 ppm; 0 nitrites and nitrates at 160 ppm

Temperature stays between 60 and 70. Higher on sunny days.

Humidity is high at 80 to 85%

I add a bit of liquid iron and potassium about every two weeks.

The problem:

30 days ago I planted lettuce seedlings. They grew for a couple of weeks and then very little growth since then. Some leaves died off. Others have brown and black spots (see photo). At the same time, I also planted a few seedlings in dirt. They are now about double the size of the lettuce in the grow bed.

Thinking that the lights might be too harsh on the plants, I moved them up to about 18" above the plants. I'm also thinking that there may be too many solids entering the grow beds (even though I have several hundred red worms), so I began filtering the water.

The fish are doing great, and they've about doubled in size in the last 60 days.

Anyone have any ideas? Thanks.

[Vlad Jovanovic]

Hey Dennis. Did you buy those seedlings from a garden store/nursery by any chance?

[Alex Veidel]

Just out of curiosity, what are you using for grow lights?

[Vlad Jovanovic]

Yeah, I was wondering that myself Alex?...At any rate the reason I asked about the nursery is that those black spots (it's pretty hard to be sure from just that photo) look like the beginning of some sort of septoria (leaf spot). Crap like that is why I'll never (for years now) ever, never purchase any kind of seedlings from anybody/anyplace. Every time I've done so, I've regretted it. If it does turn out to be a certain type of leaf spot, you can forget about growing any lettuce in the system for about the next two years. More 'macro' pics would be nice...and get yourself a magnifying glass...it's a really helpful too to have for all kinds of reasons.

Those spots have nothing to do with plant essential element deficiencies. They are either bacterial, or fungal. (Again, can't tell which from just a pic). You can take a sample down to your local extension and have them check it out...

[Dennis Jay]

Grow lights are full-spectrum.

Alex Veidel said:

Just out of curiosity, what are you using for grow lights?

[Dennis Jay]

I sprouted the seedlings from seeds I bought at local hardware store. Might have been old seeds, if that matters.

Vlad Jovanovic said:

Hey Dennis. Did you buy those seedlings from a garden store/nursery by any chance?

[Vlad Jovanovic]

By full spectrum grow lights I take it you mean CMH (ceramic metal halide)? And how many Watts...each bulb... (since that will kinda determine the height/distance you need to keep from the seedlings...still is not the cause of those spots though...)

Leaf spot is no joke...I take it its been cold-ish and humid in the GH? Get a magnifying glass and see if the brown/black spots are sunken in...

Old seeds just won't germinate, so it's it's not that. Seeds cultivated from infected plants are however a problem... Or, you could have picked up the leaf spot from environmental conditions...

[Jonathan Kadish NYC AA Chair]

I had leaf spot when I started my system and did a fair amount of research in to it. I think I remember that it is possible to get leaf spot from tainted seeds. But it can more often be from seedlings or transplants. Once you have it your stuck with it for a while... it will effect your leafy greens (basil lettuce ect). The way I dealt with it was to just grow tomatoes for 6 months, then the next crop of basil did great - no more leaf spot. If there is nothing for the bacteria to infect then it will go away.

[Dennis Jay]

Lights are T5HO

Vlad Jovanovic said:

By full spectrum grow lights I take it you mean CMH (ceramic metal halide)? And how many Watts...each bulb... (since that will kinda determine the height/distance you need to keep from the seedlings...still is not the cause of those spots though...)

 

Leaf spot is no joke...I take it its been cold-ish and humid in the GH? Get a magnifying glass and see if the brown/black spots are sunken in...

Old seeds just won't germinate, so it's it's not that. Seeds cultivated from infected plants are however a problem... Or, you could have picked up the leaf spot from environmental conditions...

[Roger Baldwin]

Hopefully, the spots are a minor fungal issue that is only affecting old growth that has been too wet for too long and the lack of vigor is caused by some unrelated problem that is easier to solve. Is the lettuce grown in "dirt" under the same lights as the AP lettuce? IMO no grow light can compete with sunlight regardless of the growing method or type of light. Maybe there is a lack micronutrients in your AP system. You could give it a shot of seaweed extract (Maxicrop) or some worm castings tea, which may also help with your fungal issues.

[Dennis Jay]

The plants in dirt are growing under the same light. One of those plants also is developing brown spots.

Roger Baldwin said:

Hopefully, the spots are a minor fungal issue that is only affecting old growth that has been too wet for too long and the lack of vigor is caused by some unrelated problem that is easier to solve. Is the lettuce grown in "dirt" under the same lights as the AP lettuce? IMO no grow light can compete with sunlight regardless of the growing method or type of light. Maybe there is a lack micronutrients in your AP system. You could give it a shot of seaweed extract (Maxicrop) or some worm castings tea, which may also help with your fungal issues.

[Jon Parr]

Yank them plants in both soil and AP, burn them, plant fresh seeds. It's not worth the two weeks it takes to replace those little things. The spots look like mold, probably sooty mold. Since both soil and AP are infected, then chances are the seeds were carriers, or your conditions are too wet and/or air is too stagnant. Keep your lights to within a few inches, and buy a fan. If you had a more valuable crop, and farther along in the grow, I'd suggest that you spray with a good bacterial tea brew of worm castings. Converse has some recipes on this site.

I would personally mist the plants and surrounding area lightly with hydrogen peroxide to kill spores while handling, and to help prevent any spores from going airborne. No big deal, but H2O2 is a relatively safe disinfectant, and leaves nothing behind but water and oxygen.

[Dennis Jay]

Thanks for all of the advice. I've pulled the lettuce out — about 100 plants and replaced with some tomato plants that have been growing in dirt. I've also installed a fan, which has helped to lower the humidity by 5 to 10%. Some of the other plants — mint, okra and peppers — are doing fine. But some basil seedings I've been sprouting in dirt have developed dark spots (see photo). Is this anything to be concern about? I've also attached a better photo of the infected lettuce. The fungus has a dark and silver color to it.