Most frequently this is cosmetic damage from overdosing with fertilizer. Give the tips time to thoroughly dry and then carefully clip close to healthy growth with sterile cutters, being sure you don't cut into firm flesh. If the tips do not just dry, you have another problem.

Possibly overwatering. Keep as close to the window as possible and let dry out partially between waterings.

I'd guess it is insufficiently acidic soils. SoCal tends towards alkaline soils and this will typically manifest itself in JM's by showing what looks like leaf scorch.....or pretty much the symptoms your tree is displaying. I'd attempt to correct this as much as possible, ideally using some sort of sulfur application (which takes time to take proper effect) or cottonseed meal (also will take some time) or watering thoroughly with a dilute solution of MirAcid - all of these are soil acidifiers. In the meantime you can water with a vinegar solution: 1-2 Tbsp of vinegar to a gallon of water.

That looks physiological to me, not pathogenic. There are a couple of possibilities, depending on what you do with your plants. Some nutrient deficiencies can look like that; magnesium, potassium, maybe molybdenum. If you haven't fed them recently, or if you have been feeding them a straight NPK food and your soil isn't the best, consider supplementing with a complete fertilizer or tonic, one which has trace minerals. (I'm fond of Superthrive for hard-to-pinpoint problems like this.) Low pH can also mimic nutrient deficiency. It could be fertilizer burn, though it's not perfectly typical. If you've been feeding them a lot, lay off for a bit and see whether it resolves. The last possibility I can think of is spray burn. Sulfur and some oils can burn foliage at high temperatures (over 85F) and even simple soap at high concentration can burn; it's common for the tips of the leaves to get higher concentrations of the spray as it runs off. You didn't mention any earlier sprays, but if you have done any, watch your temperatures and try to get an even spray without a lot of runoff. Don't mix different sprays together, either. I don't suspect a fungus because the yellowing pattern is so regular, and the dead patches are in the center of the yellow areas which speaks of it either being the center of the toxicity/deficiency, or being a victim of sunburn as the leaves get too pale to handle the sun. I hope some of that helps. :) --Alison