Powdery mildew. Take off the lower leaves of the plant and open it up by removing any leaves you don't need. Make sure you discard of infected leaves and debris to get rid of the spores. Make sure the plant gets plenty of air circulation. Powdery mildew thrives in humid conditions. It is best to start a fungicide program before you have a problem as soon as environmental conditions conducive to fungal growth occurs. Like long periods of continuous rain will be folllowed by humid conditions after the rain stops. Spray within 24 hours of the rain stopping. Reapply if it rains again. It is easier to control fungal infectiions before they start.