Callus is tissue formed as plant cells divide in response to injury or certain propagation conditions. At a mango graft it contributes to healing around the joined surfaces, but a visible swelling is not proof of a complete vascular connection. A scion can open buds briefly using stored reserves before failing. Judge graft success by sustained healthy growth and a sound union over time, while preventing wrapping material from becoming a constricting band.
Graft compatibility is the ability of a scion and rootstock to form and maintain a functioning, durable union. It is different from the initial appearance of callus or bud growth: a join can begin growing yet later develop weakness or poor transport between the parts. For mango propagation, use suitable, healthy material and established combinations where information is available. Compatibility does not mean that the rootstock guarantees dwarfing, disease resistance, or cold tolerance for every scion and location.