It's not disease, it's just weather damage, always a risk with magnolia flowers, especially if they are early flowering. Frost is the common cause, but battering from rain and wind can also cause it. If you think the weather you have been experiencing is untypical for the time of year, then just pray for better weather in future years. If you think the weather is not untypical, then perhaps plant a later flowering one instead before its too late, or if you have put it in an exposed location, perhaps you can find a more sheltered location for it (though it won't appreciate being dug up and moved, better buy another). Putting my M. stellata in my east facing front garden was likewise a mistake... I believe that M. x soulangeana comes in a number of varieties which flower at different times, and with different shades of flower.