Please consult the following literature:

Plant Pathogen Detection and Disease Diagnosis (Second Edition)

By P. Narayanasamy

Dear Krithika Krishnamoorthy

Usually, we maintain the obligate parasite on a selected host plants by serial inoculation using Wright stage for inoculums and Wright Phenological phases of host plant, and renewal the inoculation in a time stages of infection, all depend on the fungi and the host plant

Pls. find the attached files

https://www.apsnet.org/publications/phytopathology/backissues/Documents/1983Articles/Phyto73n02_241.pdf

Regards

Prof. Houda Kawas

Thanks to each other persons helped me in suggesting the links. that was really meaningful. 

In aseptic condition on the same host. or on the same tissue by plant tissue culture.

Similar to root organ culture (ROC) for Mycorrhiza can also be tried.

You can do for maintaning them on host itself the care u must take of contamination of different pathotypes and favourable conditions for the growth of the same.

again you must adopt the detached leaf technique as for any experiment.

dont use the fresh brought diseased sample each time for inoculation as it may have other contaminnats as well as pesticide residues which can alter your studies