I'd need more information to begin to answer your question. Potting mix, fertilization techniques, amount of sunlight and water could all play a part. We do have a plant diagnostics service at the University. If you can bring in a representative sample of the problem (an entire plant would be great) to the most convenient County office then Master Gardeners, agents, and specialists can better help you. Here is a direct link to the clinic's website and the form that you'll need to complete when you submit the sample http://extension.udel.edu/ag/plant-diseases/ud-plant-diagnostic-clinic/ .