how much do you feed that plant ,when you feed,,just saying this,,and dont take my word,,this is a highly debatable issue these day,,if you feed them 3 gallons,,take a tbsp of agriculture lime and disolve it in you water,,feed that one plant a shot,,it will balance your soil to a nuetral 7.0 ph,,im still thinking it just a finiky bitch,,every strain is difrent,,some hog some not,,other option is to swing that one plant,,by that i have had plants that i would have to swing ph higher or lower,,now this i had to do in coco,,i always feed them 6.5,,and i swing it up to 6.7,,didnt work so swing it to 6.0 and plant loved it,,in soil the soil buffers that and really you can ph water but soil will still balance it ,,so just as my boxes outside,,i take ag lime and spread just a fine layer on top of soil,,just enough to see you did and then water it in,it then buffers as it goes to bottom of box,,in your case,,i strongly think that plant is just special and need extra attention,,but as i said yesterday,,i would just feed it water,,because that sure looks like a nutrient burn,and if it continue on the same path,,i would add a teasponn or tablespoon of ag lime and water it in,just that one time,,kinda like taking a asprin for headache,,if you dig what im saying,,i had same thing ,once,,never did find out why ,just that one limb,but it turned out great and after i just gave up it still produced great buds,,just had funky leaves on that branch,,if it was threw out your garden ,i would say you have a problem ,but just one plant on one limb,,it something you did to cause it,,just saying.. put a teaspoon of lime in 3 gallon water and see what happens,,