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Tomato Plant with White Spots

I grew this seedling and five others by starting a slice of tomato in a pot. It's about 6" tall. It's been growing perhaps about a month, and I embarrassingly just found out seed starting mix doesn't come with nutrients, and fertilized it yesterday. It and its siblings all have purple stems near the base, and leaves beginning to yellow. This evening, I found this and a couple others had white spots on them.

Could this be due to the nutrient deficiency? Or is this some type of fungus that needs a fungicide?

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Yes I would say that nutrients are the issue. Not one in particular, the presence of yellow leaves at the base of the plant indicates that as the top extends further upwards since there is little nutrient in the soil the plant steals what it needs from the oldest leaves. Your plants seem to be way past the stage of a seeding mix which is really only intended to get the plants past seedling stage. Once germinated and having reached a size where they can be reasonably handled they should move into larger pots with a standard soil mix preferably containing a good proportion of regular garden soil which provides a surprising amount of nutrient to allow the lower leaves to remain on the plant much longer.

No fungicide needed, just a weak feed in solution or top dress with a bit of rich soil or if the pots are filled with roots move on to a good mix of potting mix and good earth. The plants will thank you for it.

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  • Thanks - the purple on the stems and leaves looked like phosphorous deficiency, and the yellowing leaves looked like nitrogen deficiency. The white spots on the leaves were what I was concerned about wrt to fungus. As far as soil goes, is it really necessary to switch to a potting soil? Does watering w/ water soluble fertilizer not provide enough nutrients to stick to seed starting mix? – John M. Apr 28 '20 at 14:27
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    Purple/red on the stems lower down usually means exposure to cool air and means little since the plants outgrow it. As for fertilizer, it's a matter of choice; sure, use soluble fert if you like, my pref. is to just mix up garden soil and compost and then use nothing but pure rain water for the entire season. – Colin Beckingham Apr 28 '20 at 14:54

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