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It could be fulsarium wilt.It could be fulsarium wilt. This can be caused by high humidity, poor circulation, splashing of water on leaves. It may have been on the plant before you got it. See here for pics of leaf disease: http://plantclinic.cornell.edu/factsheets/basildiseases.pdf If possible remove the infected leaves, adjust conditions accordingly and see if it gets stronger. I've grown several seedlings from the same set in one pot and often at least one fails to thrive: exact same soil, conditions, etc. I don't ever know why.

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