Super-compost is the first thing to get; make with pineapples bought for 2 gp each at Catherby, you are allowed to buy 40 per day. Super-compost makes your seeds/plants less likely to disease.
Fairy Tale part 1 and 2 get you the magic (greem ) secateurs which help your seeds to grow with less chance of getting disease.
The shield from the Falador diary gives extra xp while farming at the plots south of Falador.
The Lumbridge diary explorer ring 3 gives you a cabbage-port teleport to the Falador plot.
These are the most direct activities to give you more efficient farming but you may have to do a number of quests and gain levels of other skills to get them.
The best way to get more efficient farming is just to do more farming until you have a high level. Like real life there are not many shortcuts.

good luck from a level 80 farmer
