This looks like it could be powdery mildew. Are the yellow patches because of infections on the underside of the leaf? If powdery mildew is the cause of the issue, there are some steps you can take to reduce further infections. Avoid watering with overhead sprinklers, as this can increase humidity. Also, make sure the plant is receiving adequate nutrition, but avoid overuse of nitrogen, because this can promote growth of highly susceptible plant tissue. You can remove and destroy heavily infected leaves, which will both reduce the number of spores available to reinfect the plant and also open up the canopy, reducing humidity. Sulfur fungicides can be used if absolutely necessary, but cultural controls are often adequate to control this disease. The following link has some more info, if you are interested:

http://www.ctahr.hawaii.edu/oc/freepu...