PlantLife Online

Answer

Even though it is hot in Florida right know over watering the viburnum could be the culprit. When plants do not take up all the water they are given they can end up sitting in stagnant water causing a lack of oxogen in the soil known as an anaerobic environment. The anaerobic conditions are a prime breading ground for bad pathogens that can cause root rot and other disease. To check for over watering use a soil probe or shovel to investigate the rootball. If over watering is not the case and the rootball is on the dry side perhaps the amount of water you have been giving each day has not been enough or the water is simply running over and around the actual rootball of the plant. If neither of these seem to fit the cause consider the planting, were the plants planted too high, were the plants planted to low? Too high and the rootball could be getting baked too low and the roots could not be getting enough air. The ideal planting would be to make the surface of the rootball 1-2" above the grade of the soil you are planting in.
You can check the plants vitality by simply scratching the limb of the plant with your thumb nail, if its green then there's hope, if its brown it may be too late.