I have P. infestans isolates, 3 years old, maintained at room temperature as agar plugs in sterile water containing rice/wheat grains (see attached photo). The isolates are able to grow through tuber slices of a susceptible potato cultivar with a lot of mycelia. Inoculum from these isolates has very little sporangia and on infecting potato leaves, lesions appear after almost two weeks.

From the article below, it seems variation is only between isolates in culture and those from infected leaf tissue.

Article Phytophthora infestans Sporangia Produced in Culture and on ...

Any idea why the isolates passed through tuber slices will not sporulate as before?