N Deficiency could be one of the culprits.....But im leaning more toward a K deficiency. Note that potassium starved plants initially appear healthy. And deficient plants are susceptible to disease. Symptoms include the following: older leaves (first tips and margins, followed by whole leaves) develop spots, turn dark yellow, and die. Stems often become weak and sometimes brittle. Potassium is usually present in the soil, but it is locked in by high salinity. First, leach the toxic salt out of the soil and then apply a complete N-P-K fertilizer. Potassium deficiency causes the internal temperature of the foliage to climb and the protein cells to burn or degrade. Evaporation is normally highest on leaf edges, and that's where the burning takes place.
The progression of the deficiency symptoms at a glance: Plants appear healthy with dark green foliage. The leaves lose their luster. Branching may increase, but the branches are weak and scrawny. Leaf margins turn grey and progress to a rusty brown color, and then curl up and dry. Yellowing of the older leaves is accompanied by rust-colored blotches.