Can be caused by:
• Improper pH
• Nutrient Lockout
Preventative: Boron is an element requiring active water movement in the plant for uptake, so understanding transpiration and how to optimize it is key (heat, air, humidity). If there is not enough transpiration then boron will not be taken up as much as necessary.
When working with hydroponic solutions, be sure to check the pH to make sure that it is within a range that will allow plants to absorb the boron. If the pH is too far above 6.5 or approaching 7.0 then there may be issues with boron.
In the case of boron deficiency in soil solutions use a small amount of fertilizer such as Borax or Solubor will help remedy issues. Be VERY CAREFUL with the amount of boron being applied because a toxicity of boron can also occur very easily.
1/4 to 1/5 tbsp of Boric Acid per 4L of water. This will be absorbed very quickly however if applied too much the plant will burn very quickly.