What was the number of seasons that was directed by Joanna Kerns?
CREATE TABLE "table1_17356106_1" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text
);
SELECT COUNT("season_num") FROM "table1_17356106_1" WHERE "directed_by"='Joanna Kerns';