Vegetative form of Mango Malformation, a Fusarium fungus in the veins. This infection spreads easily in graftwood and on clippers and pruning saws, etc.
Cut those branches off, about a foot below the affected spots, then put the visibly infected wood in the city trash bin, so that it doesn't get composted; put the normal-looking pruned-off branches under the same tree. Thoroughly sanitize saw and gloves--- such as with 91% rubbing alcohol, then hydrogen peroxide, then mid-day sun.