Well NL is solid, it can take a licking and keep on growing healthy. It's tough to say with that pic and limited information, but since your not adding nutes yet, and none of the others are having symptoms, I have to guess that it is pH in that particular pot, or that the soil is too hot with ferts. It's a bit unusual to have them be putting out secondary branches before you start adding nutes, so for them to all be that vigorous implies the soil is stacked.