What are pit fields?
Pit vs plasmodesmata

From wikipedia > pit


  Pits are thin portions of the cell wall that adjacent cells can communicate or exchange fluid through, utilizing plasmodesmata. 


The definition of "pit" is therefore very related to the one of "plasmodesmata". From wikipedia > plasmodesmata


  Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them.


I think the following quote (from wikipedia > plasmodesmata again) will help you to distinguish pits from plasmodesma


  Primary plasmodesmata are formed when portions of the endoplasmic reticulum are trapped across the middle lamella as new cell wall is laid down between two newly divided plant cells and these eventually become the cytoplasmic connections between cells. Here the wall is not thickened further, and depressions or thin areas known as pits are formed in the walls. Pits normally pair up between adjacent cells. Plasmodesmata can also be inserted into existing cell walls between non-dividing cells (secondary plasmodesmata)


Pit field

A pit field however is just an area where there are plenty of pits! From Merria-Webster


  [A pit field is] an area in the wall of a plant cell in which one or more pits develop