Greetings Manjiri:
Without any further information or pictures, I suspect it is fungal white powdery mildew. HOWEVER, I strongly suggest you contact your local Ohio State University Extension office for potential on-site assistance and identification.
Here is some more information and control measures for white powdery mildew:
http://www.ces.ncsu.edu/depts/pp/notes/Ornamental/odin004/odin004.htm
Best of luck!