come on man! How do you expect us to answer that when you provide no info!
fill this out:
SOIL:
How long has this problem been going on?
What STRAIN are you growing?
What was the establishing technique? (seed or clone?)
What is the age of your plants?
How Tall are the plants?
What PHASE (seedling, vegetative or flower) are the plants in?
What Technique are you using? (SOG, SCROG etc)
What size pots are you using? (Include how many subjects to pot)
What substrate/medium are you using? What brand of soil mixture are you using?(percentage of perlite, vermiculite...etc?)
What Nutrient's are you using?How much of each with how much water? How Often? *Knowing the brand is very helpful*
What is the TDS/EC/PPM of your nutrients used?
What is the pH of the "RUN-OFF"?
What method of pH test was administered? Using Strips? pH pen?
How often are you watering?
When was your last feeding and how often are you feeding?
What size bulb are you using?
What is the distance to the canopy?
What is your RH Factor? (Relative Humidity)
What is the canopy temperature?
What is the Day/Night Temp? (Include fluctuation range)
What is the current Air Flow? (cfm etc.)
Is the fan blowing directly at plants?
Is the grow substrate constantly wet or moist?
Is your water HARD or SOFT?
What water are you using? Reverse Osmosis (RO)? Tap? Bottled? Well water? Distilled? Mineral Water?
Has plant been recently pruned, cloned or pinched?
Have any pest chemicals been used? If so what and when?
Are plant's infected with pest's?