Answer: It sounds like your plants are suffering from a root or crown rot. Infected plants lose foliage quickly and the entire plant usually dies. Shasta daisies have a big problem with root and crown rot in the south. Excellent drainage is a must. It helps to build a raised bed with added compost and sand to facilitate drainage. Replanting every 2 or three years is often a part of growing Shastas in the south. I recommend dividing clumps annually to keep plenty of new plants coming along.
Keep your plants moist, but avoid overwatering or wetting them every day. This aggravates their disease problems. Most of the disease problems can be controlled by these cultural means. The problem will not necessarily spread throughout your entire bed, but if the cultural factors are not alleviated, more plants will succumb to the fungal or bacterial organisms involved.
Dahlberg daisies are especially sensitive to overwatering. Make them think they are in West Texas and they'll respond well.
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