I'm no expert but I'd guess a nutrient deficiency caused by either a ph imbalance, or a lack of micronutrients probably sulfur which works best with a ph of 6.0 to 9.0, maybe get a micronutrient solution like Earth Juice Microblast. Basically a safe ph for all nutrients would be 6.5 to 7.0. I'd say your best bet would be to raise your ph. Dolomite lime, hardwood ash, bone meal, crushed marble, or crushed oyster shells, potassium hydroxide will raise your ph.