Wouldn't you set your Crop dropdown to only display unique items?
Then, you'd use FilterOwssvr qRules command to return only the Events that are associated with the selected crop, binding your Events drop down to the data source you are applying FilterOwssvr to?
So, you'd create a regular SharePoint list connection to your list, and only select the "Crop" column. Use that data source for your Crop drop down.
Then, on the field bound to the Crop drop down, you'd set a rule. That rule would run the qRules FilterOwssvr command, setting the filter field to your Crop column in your ShP list, and the value to the selected value from your field bound to the Crop drop down.