I don't think it is a disease, especially with that particular species of plant and those symptoms.
New transplants in summer are in for a stressful time. It takes months for a plant to get a root sytem established that extends far and wide enabling the plant to withstand the demands and stresses of summer with some resilience. In the meantime is difficult to keep the roots adequately moist without overwatering. Drought kills roots but when the soil is kept too saturated the roots cannot get oxygen and die. The goal is to keep the new plant's roots moist with regular watering until it has a few months to get some roots out into the surrounding soil so it is more resilient.
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