It's hard to conceive of shastas blooming in March!! Mine won't be in bloom until mid/late May at the earliest :-)

Without any more information than that provided, it sounds as though the plants may be suffering from a fungal problem. These are typically brough on by warm, moist conditions. Avoid any overhead watering, make sure the plants have decent air circulation and mulch to prevent reinfecting the plant with fungal spores from the soil surface. If the infection is severe, you might be best served by cutting off any affected parts....your season is long enough for the plants to regrow and rebloom.