syntax error in postgres stored procedure with dynamic parameters

i'm trying to create a stored procedure accepting a variable number of input parameters, and according to this performing an insert or replace task. Since i'm working on version 9.0 of postgres i can't use an UPSERT as suggested here, instead, i'm using an if-then approach. The stored procedure is returning void.

I get error near last END of the stored procedure, right before $$, i think i'm missing a punctuation or something like this, i tried to add a semicolon between last two END but it didn't work.

The query is very long but the syntax error i get is probably trivial, i apologize but i'm a newcomer of postgres stored procedures world. I attach a "pseudocode" short version and the entire version.

Pseudocode version

CREATE OR REPLACE FUNCTION function(
    IN parameter,
    ...
    )
RETURNS void AS
$$
BEGIN   
    CASE 
    WHEN(parameters) IS NULL THEN
          IF condition
          THEN
            UPDATE ...;
          ELSE 
            INSERT ...;
          END IF;
    WHEN ...
    ELSE
        IF condition
          THEN
            UPDATE ...;
          ELSE 
            INSERT ...;
          END IF;
    END
END;
$$ 
LANGUAGE 'plpgsql';

Full version

CREATE OR REPLACE FUNCTION insert_peronospora (
    IN _id_stazione  int,
    IN _lw_threshold numeric,
    IN _starting_condition_time   timestamp,
    IN _phenologic_condition_time timestamp = NULL,
    IN _germination_time timestamp = NULL,
    IN _infestation_percentage numeric = NULL,
    IN _infestation_time timestamp = NULL,
    IN _secondary_infestation_time timestamp = NULL,
    IN _secondary_infestation_number int = NULL
    )
RETURNS void AS
$$
BEGIN   
    CASE 
    WHEN(_phenologic_condition_time, _germination_time, _infestation_percentage, _infestation_time, _secondary_infestation_time, _secondary_infestation_number) IS NULL THEN
          IF EXISTS (SELECT id_stazione FROM dss_vite.peronospora WHERE id_stazione = _id_stazione and lw_threshold = _lw_threshold)
          THEN
            update dss_vite.peronospora 
            set starting_condition_time = _starting_condition_time 
            where id_stazione= _id_stazione and lw_threshold = _lw_threshold;
          ELSE 
            INSERT INTO dss_vite.peronospora (id_stazione, lw_threshold, starting_condition_time) VALUES(_id_stazione, _lw_threshold, _starting_condition_time);
          END IF;
    WHEN(_germination_time, _infestation_percentage, _infestation_time, _secondary_infestation_time, _secondary_infestation_number) IS NULL THEN
          IF EXISTS (SELECT id_stazione FROM dss_vite.peronospora WHERE id_stazione = _id_stazione and lw_threshold = _lw_threshold)
          THEN
            update dss_vite.peronospora 
            set starting_condition_time = _starting_condition_time,
            phenologic_condition_time = _phenologic_condition_time
            where id_stazione= _id_stazione and lw_threshold = _lw_threshold;
          ELSE 
            INSERT INTO dss_vite.peronospora (id_stazione, lw_threshold, starting_condition_time, phenologic_condition_time) 
            VALUES(_id_stazione, _lw_threshold, _starting_condition_time, _phenologic_condition_time);
          END IF;
    WHEN(_infestation_percentage, _infestation_time, _secondary_infestation_time, _secondary_infestation_number) IS NULL THEN
          IF EXISTS (SELECT id_stazione FROM dss_vite.peronospora WHERE id_stazione = _id_stazione and lw_threshold = _lw_threshold)
          THEN
            update dss_vite.peronospora 
            set starting_condition_time = _starting_condition_time,
            phenologic_condition_time = _phenologic_condition_time,
            germination_time = _germination_time
            where id_stazione= _id_stazione and lw_threshold = _lw_threshold;
          ELSE 
            INSERT INTO dss_vite.peronospora (id_stazione, lw_threshold, starting_condition_time, phenologic_condition_time, germination_time) 
            VALUES(_id_stazione, _lw_threshold, _starting_condition_time, _phenologic_condition_time, _germination_time);
          END IF;
    WHEN(_infestation_time, _secondary_infestation_time, _secondary_infestation_number) IS NULL THEN
          IF EXISTS (SELECT id_stazione FROM dss_vite.peronospora WHERE id_stazione = _id_stazione and lw_threshold = _lw_threshold)
          THEN
            update dss_vite.peronospora 
            set starting_condition_time = _starting_condition_time,
            phenologic_condition_time = _phenologic_condition_time,
            germination_time = _germination_time,
            infestation_percentage = _infestation_percentage
            where id_stazione= _id_stazione and lw_threshold = _lw_threshold;
          ELSE 
            INSERT INTO dss_vite.peronospora (id_stazione, lw_threshold, starting_condition_time, phenologic_condition_time, germination_time, infestation_percentage) 
            VALUES(_id_stazione, _lw_threshold, _starting_condition_time, _phenologic_condition_time, _germination_time, _infestation_percentage);
          END IF;
    WHEN(_secondary_infestation_time, _secondary_infestation_number) IS NULL THEN
          IF EXISTS (SELECT id_stazione FROM dss_vite.peronospora WHERE id_stazione = _id_stazione and lw_threshold = _lw_threshold)
          THEN
            update dss_vite.peronospora 
            set starting_condition_time = _starting_condition_time,
            phenologic_condition_time = _phenologic_condition_time,
            germination_time = _germination_time,
            infestation_percentage = _infestation_percentage,
            infestation_time = _infestation_time
            where id_stazione= _id_stazione and lw_threshold = _lw_threshold;
          ELSE 
            INSERT INTO dss_vite.peronospora (id_stazione, lw_threshold, starting_condition_time, phenologic_condition_time, germination_time, infestation_percentage,infestation_time) 
            VALUES(_id_stazione, _lw_threshold, _starting_condition_time, _phenologic_condition_time, _germination_time, _infestation_percentage, _infestation_time);
          END IF;
    ELSE
        IF EXISTS (SELECT id_stazione FROM dss_vite.peronospora WHERE id_stazione = _id_stazione and lw_threshold = _lw_threshold)
          THEN
            update dss_vite.peronospora 
            set starting_condition_time = _starting_condition_time,
            phenologic_condition_time = _phenologic_condition_time,
            germination_time = _germination_time,
            infestation_percentage = _infestation_percentage,
            infestation_time = _infestation_time,
            secondary_infestation_time = _secondary_infestation_time,
            secondary_infestation_number = _secondary_infestation_number
            where id_stazione= _id_stazione and lw_threshold = _lw_threshold;
          ELSE 
            INSERT INTO dss_vite.peronospora (id_stazione, lw_threshold, starting_condition_time, phenologic_condition_time, germination_time, infestation_percentage,infestation_time, secondary_infestation_time,secondary_infestation_number) 
            VALUES(_id_stazione, _lw_threshold, _starting_condition_time, _phenologic_condition_time, _germination_time, _infestation_percentage, _infestation_time,_secondary_infestation_time, _secondary_infestation_number);
          END IF;
    END
END;
$$ 
LANGUAGE 'plpgsql';