Correct identification of a disease problem can usually only be made by a plant pathologist because of the microscopic size of bacteria and fungi and their amazing diversity of form. Once disease has set in, it can spread rapidly through the plant. The micro-organisms are carried on air currents, by insects, on tools or through handling. They can even be splashed from leaf to leaf if the plant is watered carelessly. Isolate plants you suspect are diseased to reduce the risk of spreading infection, and always wash and tools after touching diseased plants.
Treatment of disease is a different problem.
Evidence of plant disease usually indicates a susceptibility of the individual plant, and removal of damaged parts or chemical treatment may not lead to recovery. If damage is severe, it is best to discard the plant and start over with a new plant and fresh, sterile soil. It appears on any above-ground part of the plant as a white, powdery growth. In the early stages it spreads slowly and will not cause much harm to the plant. Move the plant to a drier location with proper ventilation, and remove damaged parts.