The hard diary rewards where you get double cactus (total of 6) in all places. I only get 3 dragonfruit
So I checked this during the development of 120 farming and this isn't actually true. The reward states "Cactus patches in the desert will be protected from disease and you'll get double the spines". Spines are only received from cacti and you get 3 without the diary completed and 6 with it completed. Potato cacti do not give spines and are therefore not effected by this buff. They always give 6 fruit no matter if you have the diary completed or not. For this reason the new cacti are also unaffected by the diary reward.