'Notebook' Nicholas Sparks Reacts To Gena Rowlands Alzheimer's, 'Cruel Disease'

"The Notebook" author Nicholas Sparks is lamenting Gena Rowlands' battle with Alzheimer's -- feeling for her and her family, and thinking about the sad parallel to the classic book and movie.

Nicholas says ... "Alzheimer's is such a cruel disease. On days like today, I'm reminded of Gena's powerful portrayal of Allie and the depth she brought to the character. Her performance touched millions of hearts and helped bring my story to life in ways I never imagined."

Of course, Gena's character in the big screen adaptation of Nicholas' famous romance novel battles the same disease she's now coping with in real life.

On social media, Nicholas shared an old photo of Gena on the 'Notebook' set along with his heartfelt tribute ... saying he's truly honored to have worked with "such a talented artist."

He's also sending thoughts and prayers to Gena and her family, including her son, Nick Cassavetes, who directed "The Notebook."

Remember ... the movie stars James Garner as Duke, -- Noah's alias -- who visits Allie and recounts their love story ... which, of course, also starred Rachel McAdams and Ryan Gosling as Noah and Allie when they were younger

As we reported ... Gena and Nick announced her diagnosis Tuesday via Entertainment Weekly ... as part of the 20th anniversary of the movie.

Gena's mother, actress Lady Rowlands, was also diagnosed with Alzheimer's before she died.