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I was sprouting some French marigolds and noticed some spots on the leaves. They didn't look good, especially since one of the plants didn't get them. I looked them out on the internet and figured it was some kind of leaf spot (though I don't know anything beyond that). Figuring it might have been because of being overly damp, I planted a second set except I started them outside (also in a seeding tray). I just checked them today and it looks like the cotyledons are starting to show signs of the spots along the outer edges though I won't for sure for a couple days. None of the other plants I sprouted with the same potting mix are showing any problems like this. The variety is Disco if that helps at all.

So a couple questions:

  • What are these spots and are they a problem?
  • What can be done to prevent them?
  • Should I give up on them and root them up?
  • Will I be able to reuse the potting mix, or should I get rid of it too?

French marigold with first couple sets of true leaves coming out

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Generally, small black spots on seedlings aren't much to be worried about. The probable cause (or a catalyst when combined with nutrient overdose) is over-transpiration - the plant is evaporating too much water because its soil is too warm and too moist.

Hopefully, this is the result of over-watering and over-heating. Keep the roots cooler (>60F during the day is fine) and let the soil dry out more between watering (the time for constant "very moist" soil is over). Wait for the soil to dry out somewhat, then water until saturated. This stimulates root growth.

If it IS nutrient burn, then when you water, flush the soil (6x the container's size worth of water or more) to flush some of the readily-available nutrients away.

This is an issue seen with some organic starter mixes, especially since the move away from sphagnum. Too much organic material is ham-fisted into the mix, resulting in an overabundance of Nitrogen.

Good luck. (It looks like over-transpiration to me, but I've never grown Marigolds so I won't give a "it is absolutely THIS" diagnosis.)

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    Thanks, let the soil dry out between waterings and the spots eventually disappeared as they got bigger. Not sure if I'm mistaken, but I think I'm starting to see a flower bud come up now too so that's super exciting – Jon Jun 6 '16 at 14:49

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