When troubleshooting crop problems, pay close attention to the difference between old and new leaf growth.
Plants deficient in macronutrients like nitrogen, phosphorus, and potassium will relocate those nutrients from old leaves to feed new growth at the plant tips.
If the old leaves look funny, but the new growth is okay, be suspicious of a macronutrient deficiency.
Follow up this visual inspection with a soil analysis and a leaf tissue or sap analysis.
These tests will indicate if the soil is deficient in the suspected nutrients, and the leaf or sap analysis should confirm that these nutrients are at critically low levels within the plant.
Compare the test results to the optimal parameters for plant nutrients and adjust your fertilizer strength accordingly.