While some varieties have a tendency to develop a lighter edge, others are experiencing a culture problem. Usually you will find haloing appears more prominently on the mature leaves. It takes on a yellow, cream or light green appearance.
Deficiency: The problem is most often due to a nitrogen deficiency which causes the outer leaves to yellow at the tips, followed by yellowing that moves evenly up the leaf. If the soil pH drops below 6.5 or goes above 7.0, a nitrogen lock may occur. The plant is unable to utilise the nutrition being provided. If you see this pattern appearing on any of your other plants, have the soil pH tested. Ultimately a nitrogen lock will produce yellow leaves and tight centres. Both soil and water pH plays an important role in growing healthy African violets. When other culture problems have been ruled out, investigate pH.
Haloing can also occur when a plant has been on a show schedule of low nitrogen fertilisers. If this is the case, try applying a balanced fertiliser (ideally 20-20-20 however, in WA you may have to accept a ratio close to that) for at least one month and monitor the plant's progress.
Preventative: If you do not see a change, you may be experiencing a soil or water pH problem. Another condition that produces a haloing-like effect is transplant shock. The result can be a blotchy yellow-brown edge on the outer rows of the leaves. To avoid this unsightly condition, pot down necks before they get too long. A regular potting schedule (twice annually) can be a preventative for many plant ailments.
Drenching: First, drench the soil with lukewarm water. Water from the top until about a cup (200mils) of water has drained from the pot. Allow any excess water to drain. This will help leach out any of the substances that may have contributed to a pH imbalance. If these substances were the cause of the problem, they may still be present in the old soil that clung to your African violet's roots.
After you have drenched the soil, wait one week. During this time, do not use a fertiliser. The nutrients in the soil will be sufficient until you begin fertilising again.
Fertilising: After one week, you may notice that your Violet has already begun to recover. If it has, it was most likely suffering from a pH imbalance in the soil. In either case, you can again start fertilising. When selecting a fertiliser, be sure that it contains approximately equal amounts of the primary nutrients, nitrogen (N), phosphorus (P) and potassium (K). For standard-sized African Violets (in 70mm pots or larger), an NPK of 14-12-14 is recommended. For Miniature Violets, you will need a fertiliser specifically formulated for miniature plants. In addition to being fully-dissolving, make sure that the nitrogen is not derived from urea, since urea will cause Root Burn on African Violets.
Fresh: Repot your Violet with the fresh potting soil. After separating the rootball from its container, shake as much of the old soil from the roots as you can without disturbing the roots too much. Repot the plant in a potting soil that is light, porous and guarantees a pH of 5.8 to 6.2 and the changes could take up to six weeks so be patient.